This specification describes the syntax and semantics of XProc 2.0: An XML Pipeline Language, a language for describing operations to be performed on documents.
An XML Pipeline specifies a sequence of operations to be performed on documents. Pipelines generally accept documents as input and produce documents as output. Pipelines are made up of simple steps which perform atomic operations on documents and constructs similar to conditionals, iteration, and exception handlers which control which steps are executed.
Status of this Document
This document is an editor's draft that has no official standing.
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.
Publication as a First Public Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress. The significant changes in this draft are summarized in Appendix J, Change Log.
This document is a product of the XML Processing Model Working Group as part of the W3C XML Activity. This draft is a first attempt to address some of the requirements of [XProc V2.0 Requirements]. It is in many ways substantially incomplete. The Working Group is publishing it in order to establish an intended direction and to provide an official opportunity for comment.
An XML Pipeline specifies a sequence of operations to be performed on a collection of input documents. Pipelines take documents as their input and produce documents as their output.
A pipeline consists of steps. Like pipelines, steps take documents as their inputs and produce documents as their outputs. The inputs of a step come from the web, from the pipeline document, from the inputs to the pipeline itself, or from the outputs of other steps in the pipeline. The outputs from a step are consumed by other steps, are outputs of the pipeline as a whole, or are discarded.
There are three kinds of steps: atomic steps, compound steps, and multi-container steps. Atomic steps carry out single operations and have no substructure as far as the pipeline is concerned. Compound steps and multi-container steps control the execution of other steps, which they include in the form of one or more subpipelines.
[XProc 2.0: Standard Step Library] defines a standard library of steps. Pipeline implementations may support additional types of steps as well.
Figure 1. A simple, linear XInclude/Validate pipeline
This is a pipeline that consists of two atomic steps, XInclude and Validate with XML Schema. The pipeline itself has two inputs, “source” (a source document) and “schemas” (a sequence of W3C XML Schemas). The XInclude step reads the pipeline input “source” and produces a result document. The Validate with XML Schema step reads the pipeline input “schemas” and the result of the XInclude step and produces its own result document. The result of the validation, “result”, is the result of the pipeline. (For consistency across the step vocabulary, the standard input is usually named “source” and the standard output is usually named “result”.)
The pipeline document determines how the steps are connected together inside the pipeline, that is, how the output of one step becomes the input of another.
The example in Example 1, “A simple, linear XInclude/Validate pipeline” is very verbose. It makes all of the connections seen in the figure explicit. In practice, pipelines do not have to be this verbose. XProc supports defaults for many common cases:
If you use p:pipeline instead of p:declare-step, the “source” input port and “result” output port are implicitly declared for you.
Where inputs and outputs are connected between sequential sibling steps, they do not have to be made explicit.
The heart of this example is the conditional. The “choose” step evaluates an XPath expression over a test document. Based on the result of that expression, one or another branch is run. In this example, each branch consists of a single validate step.
This example, like the preceding, relies on XProc defaults for simplicity. It is always valid to write the fully explicit form if you prefer.
The media type for pipeline documents is application/xml. Often, pipeline documents are identified by the extension .xpl.
In this specification the words must, must not, should, should not, may and recommended are to be interpreted as described in [RFC 2119].
2 Pipeline Concepts
[Definition: A pipeline is a set of connected steps, with outputs of one step flowing into inputs of another.] A pipeline is itself a step and must satisfy the constraints on steps. Connections between steps occur where the input of one step is connected to the output of another.
The result of evaluating a pipeline (or subpipeline) is the result of evaluating the steps that it contains, in an order consistent with the connections between them. A pipeline must behave as if it evaluated each step each time it is encountered. Unless otherwise indicated, implementations must not assume that steps are functional (that is, that their outputs depend only on their inputs and options) or side-effect free.
The pattern of connections between steps will not always completely determine their order of evaluation. The evaluation order of steps not connected to one another is implementation-dependent.
2.1 Steps
[Definition: A step is the basic computational unit of a pipeline.] A typical step has zero or more inputs, from which it receives documents to process, zero or more outputs, to which it sends document results, and can have options.
[Definition: An atomic step is a step that performs a unit of processing on its input, such as XInclude or transformation, and has no internal subpipeline. ] Atomic steps carry out fundamental operations and can perform arbitrary amounts of computation, but they are indivisible. An XSLT step, for example, performs XSLT processing; a Validate with XML Schema step validates one input with respect to some set of XML Schemas, etc.
There are many types of atomic steps. The standard library of atomic steps is described in [XProc 2.0: Standard Step Library], but implementations may provide others as well. It is implementation-defined what additional step types, if any, are provided. Each use, or instance, of an atomic step invokes the processing defined by that type of step. A pipeline may contain instances of many types of steps and many instances of the same type of step.
Compound steps, on the other hand, control and organize the flow of documents through a pipeline, reconstructing familiar programming language functionality such as conditionals, iterators and exception handling. They contain other steps, whose evaluation they control.
[Definition: A compound step is a step that contains a subpipeline.] That is, a compound step differs from an atomic step in that its semantics are at least partially determined by the steps that it contains.
Finally, there are two “multi-container steps”: p:choose and p:try. [Definition: A multi-container step is a step that contains several alternate subpipelines. ] Each subpipeline is identified by a non-step wrapper element: p:when and p:otherwise in the case of p:choose, p:group and p:catch in the case of p:try.
The output of a multi-container step is the output of exactly one of its subpipelines. In this sense, a multi-container step functions like a compound step. However, evaluating a multi-container step may involve evaluating, or partially evaluating, more than one of its subpipelines. It's possible for steps in a partially evaluated pipeline to have side effects that are visible outside the processor, even if the final output of the multi-container step is the result of some other subpipeline. For example, a web server might record that some interaction was performed, or a file on the local file system might have been modified.
[Definition: A compound step or multi-container step is a container for the steps directly within it or within non-step wrappers directly within it.][Definition: The steps that occur directly within, or within non-step wrappers directly within, a step are called that step's contained steps. In other words, “container” and “contained steps” are inverse relationships.][Definition: The ancestors of a step, if it has any, are its container and the ancestors of its container.]
[Definition: Sibling steps (and the connections between them) form a subpipeline.][Definition: The last step in a subpipeline is its last step in document order.]
User-defined pipelines (identified with pfx:user-pipeline in the preceding syntax summary) are atomic. A pipeline declaration may contain a subpipeline, but the invocation of that pipeline is atomic and does not contain a subpipeline.
Steps have “ports” into which inputs and outputs are connected. Each step has a number of input ports and a number of output ports; a step can have zero input ports and/or zero output ports. (All steps have an implicit output port for reporting errors that must not be declared.) The names of all ports on each step must be unique on that step (you can't have two input ports named “source”, nor can you have an input port named “schema” and an output port named “schema”).
A Step may have zero or more options, all with unique names.
All of the different instances of steps (atomic or compound) in a pipeline can be distinguished from one another by name. If the pipeline author does not provide a name for a step, a default name is manufactured automatically.
2.1.1 Step names
The name attribute on any step can be used to give it a name. The name must be unique within its scope, see Section 3.2, “Scoping of Names”.
If the pipeline author does not provide an explicit name, the processor manufactures a default name. All default names are of the form “!1.m.n…” where “m” is the position (in the sense of counting sibling elements) of the step's highest ancestor element within the pipeline document or library which contains it, “n” is the position of the next-highest ancestor, and so on, including both steps and non-step wrappers. For example, consider the pipeline in Example 3, “A validate and transform pipeline”. The p:pipeline step has no name, so it gets the default name “!1”; the p:choose gets the name “!1.1”; the first p:when gets the name “!1.1.1”; the p:otherwise gets the name “!1.1.2”, etc. If the p:choose had a name, it would not have received a default name, but it would still have been counted and its first p:when would still have been “!1.1.1”.
Providing every step in the pipeline with an interoperable name has several benefits:
It allows implementers to refer to all steps in an interoperable fashion, for example, in error messages.
Pragmatically, we say that readable ports are identified by a step name/port name pair. By manufacturing names for otherwise anonymous steps, we include implicit connections without changing our model.
In a valid pipeline that runs successfully to completion, the manufactured names aren't visible (except perhaps in debugging or logging output).
Note
The format for defaulted names does not conform to the requirements of an NCName. This is an explicit design decision; it prevents pipelines from using the defaulted names on p:pipe elements. If an explicit connection is required, the pipeline author must provide an explicit name for the step.
2.2 Documents
An XProc pipeline processes documents. [Definition: A document is a representation and its document properties.]. [Definition: A representation is a data structure used by an XProc processor to refer to the actual document content.]
Documents have associated with them a set of properties. The properties are key/value pairs. [Definition: The document properties are exposed to the XProc pipeline as a map (map(xs:string, xs:string)).] Several property keys are defined by this specification:
content-type
The value of the “content-type” key identifies the media type ([Media Types]) of the representation. The “content-type” must always be present.
base-uri
The value of the “base-uri” key identifies the base URI of the document. If no such key is present, the document has no base URI.
Other property keys may also be present, including user defined properties.
2.2.1 Document Types
From an XProc perspective, there are two kinds of documents: XML documents and non-XML documents. Non-XML documents can be further subdivided into text documents and binary documents. Text documents are called out specially because they can be easily represented inline within a pipeline.
2.2.1.1 XML Documents
Representations of XML documents are instances of the [XQuery 1.0 and XPath 2.0 Data Model (XDM)]. They are identified by an XML media type. [Definition: The “application/xml” and “text/xml media types and all media types of the form “application/something+xml” are XML media types. ]
In order to be consistent with the XPath data model, all general and external parsed entities must be fully expanded in XML documents; they must not contain any representation of [Infoset] [unexpanded entity reference information items].
The level of support for typed values in XDM instances in an XProc pipeline is implementation-defined.
2.2.1.2 Text Documents
Text documents are non-XML documents. Representations of text documents are are implementation-dependent.
Text documents are identified by a text media type. [Definition: Media types of the form “text/something” are text media types. ]
Implementors are free to optimize by storing them in convenient formats, caching them on disk, etc.
2.3 Inputs and Outputs
Most steps have one or more inputs and one or more outputs. Figure 3, “An atomic step” illustrates symbolically an atomic step with two inputs and one output.
Figure 3. An atomic step
All atomic steps are defined by a p:declare-step. The declaration of an atomic step type defines the input ports, output ports, and options of all steps of that type. For example, every p:validate-with-xml-schemaXPS step has two inputs, named “source” and “schema”, one output named “result”, and the same set of options.
Like atomic steps, top level, user-defined pipelines also have declarations. The situation is slightly more complicated for the other compound steps because they don't have separate declarations; each instance of the compound step serves as its own declaration. On these compound steps, the number and names of the outputs can be different on each instance of the step.
Figure 4, “A compound step” illustrates symbolically a compound step with one subpipeline and one output. As you can see from the diagram, the output from the compound step comes from one of the outputs of the subpipeline within the step.
Figure 4. A compound step
[Definition: The input ports declared on a step are its declared inputs.][Definition: The output ports declared on a step are its declared outputs.] When a step is used in a pipeline, it is connected to other steps through its inputs and outputs.
When a step is used, all of the declared inputs of the step must be connected. Each connection binds the input to a data source that may be from a variety of sources (see Section 2.5, “Connections”). It is a static error (err:XS0003) if any declared input is not connected.
The declared outputs of a step are only connected when they are used by another step or expression. Usually, this connection is made in reverse where the use of the output describes the connection (see Section 2.5, “Connections”).
The primary output port of a step must be connected to some consumer. It is a static error (err:XS0005) if the primary output port of any step is not connected. Other outputs can remain unconnected. Any documents produced on an unconnected output port are discarded.
Output ports on compound steps have a dual nature: from the perspective of the compound step's siblings, its outputs are just ordinary outputs and can be connected the sames other declared outputs. From the perspective of the subpipeline inside the compound step, they are inputs into which something may be connected to produce the output of the compound step.
Within a compound step, the declared outputs of the step can be connected to any of the various available outputs of contained steps as well as other data sources (see Section 2.5, “Connections”). If a (non-primary) output port of a compound step is left unconnected, it produces an empty sequence of documents from the perspective of its siblings.
Each input and output on a step is declared to accept or produce either a single document or a sequence of documents. It is not an error to connect a port that is declared to produce a sequence of documents to a port that is declared to accept only a single document. It is, however, an error if the former step actually produces more than one document at run time.
It is also not an error to connect a port that is declared to produce a single document to a port that is declared to accept a sequence. A single document is the same as a sequence of one document.
An output port may have more than one connection: it may be connected to more than one input port, more than one of its container's output ports, or both. At runtime this will result in distinct copies of the output.
[Definition: The signature of a step is the set of inputs, outputs, and options that it is declared to accept.] The declaration for a step provides a fixed signature which all its instances share.
[Definition: A step matches its signature if and only if it specifies an input for each declared input, it specifies no inputs that are not declared, it specifies an option for each option that is declared to be required, and it specifies no options that are not declared.] In other words, every input and required option must be specified and only inputs and options that are declared may be specified. Options that aren't required do not have to be specified.
Steps may also produce error, warning, and informative messages. These messages are captured and provided on the error port inside of a p:catch. Outside of a try/catch, the disposition of error messages is implementation-dependent.
How pipeline outputs are connected to documents outside the pipeline is implementation-defined.
Input ports may specify a content type, or list of content types, that they accept. If an input port provides a set of acceptable content types, it is a dynamic error (err:XD1003) if an input document that arrives on the port has a content type that does not match any content type in that set.
2.3.1 External Documents
It's common for some of the documents used in processing a pipeline to be read from URIs. Sometimes this occurs directly, for example with a p:document element. Sometimes it occurs indirectly, for example if an implementation allows the URI of a pipeline input to be specified on the command line or if an p:xsltXPS step encounters an xsl:import in the stylesheet that it is processing. It's also common for some of the documents produced in processing a pipeline to be written to locations which have, or at least could have, a URI.
The process of dereferencing a URI to retrieve a document is often more interesting than it seems at first. On the web, it may involve caches, proxies, and various forms of indirection. Resolving a URI locally may involve resolvers of various sorts and possibly appeal to implementation-dependent mechanisms such as catalog files.
In XProc, the situation is made even more interesting by the fact that many intermediate results produced by steps in the pipeline have base URIs. Whether (and when and how) or not the intermediate results that pass between steps are ever written to a filesystem is implementation-dependent.
In Version 2.0 of XProc, how (or if) implementers provide local resolution mechanisms and how (or if) they provide access to intermediate results by URI is implementation-defined.
Version 2.0 of XProc does not require implementations to guarantee that multiple attempts to dereference the same URI always produce consistent results.
Note
On the one hand, this is a somewhat unsatisfying state of affairs because it leaves room for interoperability problems. On the other, it is not expected to cause such problems very often in practice.
If these problems arise in practice, implementers are encouraged to use the existing extension mechanisms to give users the control needed to circumvent them. Should such mechanisms become widespread, a standard mechanism could be added in some future version of the language.
2.4 Primary Inputs and Outputs
As a convenience for pipeline authors, each step may have one input port designated as the primary input port and one output port designated as the primary output port.
[Definition: If a step has a document input port which is explicitly marked “primary='true'”, or if it has exactly one document input port and that port is not explicitly marked “primary='false'”, then that input port is the primary input port of the step.] If a step has a single input port and that port is explicitly marked “primary='false'”, or if a step has more than one input port and none is explicitly marked as the primary, then the primary input port of that step is undefined. A step can have at most one primary input port.
[Definition: If a step has a document output port which is explicitly marked “primary='true'”, or if it has exactly one document output port and that port is not explicitly marked “primary='false'”, then that output port is the primary output port of the step.] If a step has a single output port and that port is explicitly marked “primary='false'”, or if a step has more than one output port and none is explicitly marked as the primary, then the primary output port of that step is undefined. A step can have at most one primary output port.
The special significance of primary input and output ports is that they are connected automatically by the processor if no explicit connection is given. Generally speaking, if two steps appear sequentially in a subpipeline, then the primary output of the first step will automatically be connected to the primary input of the second.
Additionally, if a compound step has no declared outputs and the last step in its subpipeline has an unconnected primary output, then an implicit primary output port will be added to the compound step (and consequently the last step's primary output will be connected to it). This implicit output port has no name. It inherits the sequence property of the port connected to it. This rule does not apply to p:declare-step; step declarations must provide explicit names for all of their outputs.
2.5 Connections
Steps are connected together by their input ports and output ports. It is a static error (err:XS0001) if there are any loops in the connections between steps: no step can be connected to itself nor can there be any sequence of connections through other steps that leads back to itself.
[Definition: A connection associates an input or output port with some data source.] Such a connection represents a binding between the port's name and the data source as described by various locations, inline expressions, or readable ports.
An input port can be connected to:
The output port of some other step.
A fixed, inline document or sequence of documents.
A document read from a URI.
One of the inputs declared on one of its ancestors.
A special port provided by an ancestor compound step, for example, “current” in a p:for-each or p:viewport.
When an input accepts a sequence of documents, the documents can come from any combination of these locations.
In contrast, output ports are connected when they are referenced by another input port, declared output or other expression and may be connected to:
The input port or input context of some other step.
As with an input, the output can be a sequence of documents constructed from any combination of the above.
An output port may have multiple consumers and this results in multiple connections. A subset of these connections are the input port connections for various sibling or contained steps.
Within the context of a compound step, the declared outputs of the compound step must describe their connections. The set of possibilities for this connection is exactly the same set as for any other input port within the current environment.
2.5.1 Namespace Fixup on XML Outputs
XProc processors are expected, and sometimes required, to perform namespace fixup on XML outputs. Unless the semantics of a step explicitly says otherwise:
The in-scope namespaces associated with a node (even those that are inherited from namespace bindings that appear among its ancestors in the document in which it appears initially) are assumed to travel with that node.
Changes to one part of a tree (wrapping or unwrapping a node or renaming an element, for example) do not change the in-scope namespaces associated with the descendants of the node so changed.
As a result, some steps can produce XML documents which have no direct serialization (because they include nodes with conflicting or missing namespace declarations, for example). [Definition: To produce a serializable XML document, the XProc processor must sometimes add additional namespace nodes, perhaps even renaming prefixes, to satisfy the constraints of Namespaces in XML. This process is referred to as namespace fixup.]
Implementors are encouraged to perform namespace fixup before passing documents between steps, but they are not required to do so. Conversely, an implementation which does serialize between steps and therefore must perform such fixups, or reject documents that cannot be serialized, is also conformant.
Except where the semantics of a step explicitly require changes, processors are required to preserve the information in the documents and fragments they manipulate. In particular, the information corresponding to the [Infoset] properties [attributes], [base URI], [children], [local name], [namespace name], [normalized value], [owner], and [parent]must be preserved.
The information corresponding to [prefix], [in-scope namespaces], [namespace attributes], and [attribute type]should be preserved, with changes to the first three only as required for namespace fixup. In particular, processors are encouraged to take account of prefix information in creating new namespace bindings, to minimize negative impact on prefixed names in content.
Except for cases which are specifically called out in [XProc 2.0: Standard Step Library], the extent to which namespace fixup, and other checks for outputs which cannot be serialized, are performed on intermediate outputs is implementation-defined.
Whenever an implementation serializes pipeline contents, for example for pipeline outputs, logging, or as part of steps such as p:storeXPS or p:http-requestXPS, it is a dynamic error if that serialization could not be done so as to produce a document which is both well-formed and namespace-well-formed, as specified in XML and Namespaces in XML, regardless of what serialization method, if any, is called for.
2.6 Environment
[Definition: The environment is a context-dependent collection of information available within subpipelines.] Most of the information in the environment is static and can be computed for each subpipeline before evaluation of the pipeline as a whole begins. The in-scope bindings have to be calculated as the pipeline is being evaluated.
The environment consists of:
A set of readable ports. [Definition: The readable ports are a set of step name/port name pairs.] Inputs and outputs can only be connected to readable ports.
A default readable port. [Definition: The default readable port, which may be undefined, is a specific step name/port name pair from the set of readable ports.]
A set of in-scope bindings. [Definition: The in-scope bindings are a set of name-value pairs, based on option and variable bindings.]
[Definition: The empty environment contains no readable ports, an undefined default readable port and no in-scope bindings.]
Otherwise, the default readable port is unchanged.
The names and values from each p:variable present at the beginning of the container are added, in document order, to the in-scope bindings. A new binding replaces an old binding with the same name. See Section 5.5.1, “p:variable” for the specification of variable evaluation.
When a pipeline is invoked by a processor, an initial environment is constructed. [Definition: An initial environment is a connection for each of the readable ports and a set of option bindings used to construct the in-scope bindings.] This environment is used in place of the empty environment that might have otherwise been provided.
An invoked pipeline's initial environment is different from the environment constructed for the sub-pipeline of a declared step. The initial environment is constructed for the initial invocation of the pipeline by the processor by the outside application. Steps that are subsequently invoked construct an environment as specified in Section 5.6.2, “Declaring pipelines”.
When constructing an initial environment, an implementation is free to provide any set of mechanisms to construct connections for the input ports of the invoked step. These mechanisms are not limited to the variety of mechnisms described within this specification. Any extensions are implementation defined.
The set of in-scope bindings are constructed from a set of option name/value pairs. Each option value can be a simple string value, a specific data type instance (e.g. xs:dateTime), or a more complex value like a map item. How these values are specified is implementation defined.
2.7 XPaths in XProc
XProc uses XPath as an expression language. XPath expressions are evaluated by the XProc processor in several places: on compound steps, to compute the default values of options and the values of variables; on atomic steps, to compute the actual values of options.
XPath expressions are also passed to some steps. These expressions are evaluated by the implementations of the individual steps.
This distinction can be seen in the following example:
The select expression on the variable “home” is evaluated by the XProc processor. The value of the variable is “http://example.com/docs”.
The href option of the p:loadXPS step is evaluated by the XProc processor. The actual href option received by the step is simply the string literal “http://example.com/docs/document.xml”. (The select expression on the source input of the p:split-sequenceXPS step is also evaluated by the XProc processor.)
The XPath expression “@role='chapter'” is passed literally to the test option on the p:split-sequenceXPS step. That's because the nature of the p:split-sequenceXPS is that it evaluates the expression. Only some options on some steps expect XPath expressions.
The XProc processor evaluates all of the XPath expressions in select attributes on variables, options, and inputs, in match attributes on p:viewport, and in test attributes on p:when steps.
2.7.1 Processor XPath Context
When the XProc processor evaluates an XPath expression using XPath, unless otherwise indicated by a particular step, it does so with the following static context:
XPath 1.0 compatibility mode
False
Statically known namespaces
The namespace declarations in-scope for the containing element.
Default element/type namespace
The null namespace.
Default function namespace
The [XPath 2.0] function namespace. Function names that do not contain a colon always refer to the default function namespace, any in-scope binding for the default namespace does not apply. This specification does not provide a mechanism to override the default function namespace.
In-scope schema definitions
A basic XPath 2.0 XProc processor includes the following named type definitions in its in-scope schema definitions:
All the primitive atomic types defined in [W3C XML Schema: Part 2], with the exception of xs:NOTATION. That is: xs:string, xs:boolean, xs:decimal, xs:double, xs:float, xs:date, xs:time, xs:dateTime, xs:duration, xs:QName, xs:anyURI, xs:gDay, xs:gMonthDay, xs:gMonth, xs:gYearMonth, xs:gYear, xs:base64Binary, and xs:hexBinary.
The types xs:anyType, xs:anySimpleType, xs:yearMonthDuration, xs:dayTimeDuration, xs:anyAtomicType, xs:untyped, and xs:untypedAtomic defined in [XQuery 1.0 and XPath 2.0 Data Model (XDM)].
In-scope variables
The union of the in-scope specified options and variables are available as variable bindings to the XPath processor.
Note
An option that has neither a specified value nor a default value will not appear as an in-scope variable. Consequently, an attempt to refer to that variable will raise an error.
Implementation-defined but must include the Unicode code point collation. The version of Unicode supported is implementation-defined, but it is recommended that the most recent version of Unicode be used.
Default collation
Unicode code point collation.
Base URI
The base URI of the element on which the expression occurs.
Statically known documents
None.
Statically known collections
None.
And the following dynamic context:
context item
A document. The document is either specified with a connection or is taken from the default readable port. It is a dynamic error (err:XD0008) if a document sequence appears where a document to be used as the context node is expected.
The result of evaluating an expression when the context node has a non-XML content type is implementation-defined.
If there is no explicit connection and there is no default readable port then the context node is undefined.
context position and context size
The context position and context size are both “1”.
Variable values
The union of the in-scope options and variables are available as variable bindings to the XPath processor.
When a step evaluates an XPath expression using XPath 2.0, unless otherwise indicated by a particular step, it does so with the following static context:
XPath 1.0 compatibility mode
False
Statically known namespaces
The namespace declarations in-scope for the containing element or made available through p:namespaces.
Default element/type namespace
The null namespace.
Default function namespace
The [XPath 2.0] function namespace. Function names that do not contain a colon always refer to the default function namespace, any in-scope binding for the default namespace does not apply. This specification does not provide a mechanism to override the default function namespace.
The set of available documents (those that may be retrieved with a URI) is implementation-dependent.
Available collections
None.
Default collection
None.
Note
Some steps may also provide for implementation-defined or implementation-dependent amendments to the contexts. Those amendments are in addition to any specified by XProc.
2.8 XPath Extension Functions
The XProc processor must support the additional functions described in this section in XPath expressions evaluated by the processor.
2.8.1 System Properties
XPath expressions within a pipeline document can interrogate the processor for information about the current state of the pipeline. Various aspects of the processor are exposed through the p:system-property function in the pipeline namespace:
p:system-property($property as xs:string) as xs:string
The $property string must have the form of a QName; the QName is expanded into a name using the namespace declarations in scope for the expression. It is a dynamic error (err:XD0015) if the specified QName cannot be resolved with the in-scope namespace declarations. The p:system-property function returns the string representing the value of the system property identified by the QName. If there is no such property, the empty string must be returned.
Implementations must provide the following system properties, which are all in the XProc namespace:
p:episode
Returns a string which should be unique for each invocation of the pipeline processor. In other words, if a processor is run several times in succession, or if several processors are running simultaneously, each invocation of each processor should get a distinct value from p:episode.
Returns a string which identifies the current language, for example, for message localization purposes. The exact format of the language string is implementation-defined but should be consistent with the xml:lang attribute.
p:product-name
Returns a string containing the name of the implementation, as defined by the implementer. This should normally remain constant from one release of the product to the next. It should also be constant across platforms in cases where the same source code is used to produce compatible products for multiple execution platforms.
p:product-version
Returns a string identifying the version of the implementation, as defined by the implementer. This should normally vary from one release of the product to the next, and at the discretion of the implementer it may also vary across different execution platforms.
p:vendor
Returns a string which identifies the vendor of the processor.
p:vendor-uri
Returns a URI which identifies the vendor of the processor. Often, this is the URI of the vendor's web site.
p:version
Returns the version(s) of XProc implemented by the processor as a space-separated list. For example, a processor that supports XProc 1.0 would return “1.0”; a processor that supports XProc 1.0 and 2.0 would return “1.0 2.0”; a processor that supports only XProc 2.0 would return “2.0”.
p:xpath-version
Returns the version(s) of XPath implemented by the processor for evaluating XPath expressions on XProc elements. The result is a space-separated list of versions supported. For example, a processor that only supports XPath 2.0 would return “2.0”; a processor that supports XPath 2.0 and XPath 3.0 could return “2.0 3.0”; a processor that supports only XPath 2.0 would return “2.0”.
p:psvi-supported
Returns true if the implementation supports passing PSVI annotations between steps, false otherwise.
Implementations may support additional system properties but such properties must be in a namespace and must not be in the XProc namespace.
2.8.2 Step Available
The p:step-available function reports whether or not a particular type of step is understood by the processor.
p:step-available($step-name as xs:string) as xs:boolean
The $step-type string must have the form of a QName; the QName is expanded into a name using the namespace declarations in-scope for the expression. The p:step-available function returns true if and only if the processor knows how to evaluate steps of the specified type.
2.8.3 Value Available
The p:value-available function reports whether or not a particular in-scope option has a value.
p:value-available($option-name as xs:string) as xs:boolean
p:value-available($option-name as xs:string, $fail-if-unknown as xs:boolean) as xs:boolean
The $option-name string must have the form of a QName; the QName is expanded into a name using the namespace declarations in-scope for the expression. The p:value-available function returns true if and only if the name specified is the name of an in-scope binding and the binding has a value. It is a dynamic error (err:XD0033) if the name specified is not the name of an in-scope option or variable.
In the two-argument form, it is not an error to specify a name that is not the name of an in-scope option or variable if $fail-if-unknown is false; the function simply returns false. The semantics of the two-argument form when $fail-if-unknown is true are precisely the same as the single argument form.
If the path option is specified in the call to ex:dir-list, then the first p:when clause will be evaluated and the specified value will be used. If the option is not specified, then the p:otherwise clause will be evaluated and "." will be used instead.
2.8.4 Iteration Position
Both p:for-each and p:viewport process a sequence of documents. The iteration position is the position of the current document in that sequence: the first document has position 1, the second 2, etc. The p:iteration-position function returns the iteration position of the nearest ancestor p:for-each or p:viewport.
Both p:for-each and p:viewport process a sequence of documents. The iteration size is the total number of documents in that sequence. The p:iteration-size function returns the iteration size of the nearest ancestor p:for-each or p:viewport.
p:iteration-size() as xs:integer
If there is no p:for-each or p:viewport among the ancestors of the element on which the expression involving p:iteration-size occurs, it returns 1.
2.8.6 Version Available
Returns true if and only if the processor supports the version specified.
p:version-available($version as xs:decimal) as xs:boolean
A version 1.0 processor will return true() when p:version-available(1.0) is evaluated.
2.8.7 XPath Version Available
Returns true if and only if the processor supports the XPath version specified.
p:xpath-version-available($version as xs:decimal) as xs:boolean
A processor that supports XPath 2.0 will return true() when p:xpath-version-available(2.0) is evaluated.
2.8.8 Make Map
XProc uses maps to pass parameters to steps. Sometimes it is convenient to represent these maps as XML documents. This function reads such an XML document and produces a map.
p:make-map($param-set as item()) as map(xs:QName,item())
The map returned contains (exclusively) the parameters that are represented by the $param-set item.
Only c:paramXPS children of the c:param-setXPS element are considered, all other nodes are ignored. The parameters represented by those c:paramXPS children are added to the map that is returned. It is a dynamic error (err:XD1002) if any of the c:paramXPS elements are invalid.
Editorial Note
Must tie down what “valid” means wrt the c:param element.
p:document-properties($doc as document-node()) as map(xs:string,xs:string)
The map returned contains (exclusively) the document properties associated with the $doc specified.
Editorial Note
This function is only defined on XML documents but clearly the intent is that it should work on any kind of document. How can we do that?
2.8.10 Other XPath Extension Functions
It is implementation-defined if the processor supports any other XPath extension functions. Additional extension functions, if any, must not use any of the XProc namespaces.
2.9 PSVIs in XProc
XML documents flow between steps in an XProc pipeline. Section 3, “Infoset Conformance” identifies the properties of those documents that must be available. Implementations may also have the ability to pass PSVI annotations between steps.
Whether or not the pipeline processor supports passing PSVI annotations between steps is implementation-defined.The exact PSVI properties that are preserved when documents are passed between steps is implementation-defined.
A pipeline can use the p:psvi-supported system property to determine whether or not PSVI properties can be passed between steps.
A pipeline can assert that PSVI support is required with the psvi-required attribute:
On a p:pipeline or p:declare-step, psvi-required indicates whether or not the declared step requires PSVI support. It is a dynamic error (err:XD0022) if a processor that does not support PSVI annotations attempts to invoke a step which asserts that they are required.
On a p:library, the psvi-required attribute provides a default value for all of its p:pipeline and p:declare-stepchildren that do not specify a value themselves.
Many of the steps that an XProc pipeline can use are transformative in nature. The p:deleteXPS step, for example, can remove elements and attributes; the p:label-elementsXPS step can add attributes; etc. If PSVI annotations were always preserved, the use of such steps could result in documents that were inconsistent with their schema annotations.
In order to avoid these inconsistencies, most steps must not produce PSVI annotated results even when PSVI passing is supported.
If PSVI passing is supported, the following constraints apply:
Implementations must faithfully transmit any PSVI properties produced on step outputs to the steps to which they are connected.
When only a subset of the input is processed by a step (because a select expression appears on an input port or a match expression is used to process only part of the input), any PSVI annotations that appear on the selected input must be preserved in the resulting documents passed to the step.
Note that ID/IDREF constraints, and any other whole-document constraints, may not be satisfied within the selected portion, irrespective of what its PSVI properties claim.
If an output of a compound step is connected to an output which includes PSVI properties, those properties must be preserved on the output of the compound step, except for the output of p:viewport which must not contain any PSVI properties.
If an implementation supports XPath 2.0, the data model constructed with which to evaluate XPath expressions and match patterns should take advantage of as much PSVI information as possible.
Except as specified above, or in the descriptions of individual steps, implementations must not include PSVI properties in the outputs of steps defined by this specification. It is implementation-defined what PSVI properties, if any, are produced by extension steps.
A processor that supports passing PSVI properties between steps is always free to do so. Even if psvi-required="false" is explicitly specified, it is not an error for a step to produce a result that includes additional PSVI properties, provide it does not violate the constraints above.
2.10 Value Templates
An attribute or text node in a pipeline may, in particular circumstances, contain embedded expressions enclosed between curly brackets. Attributes and text nodes that use (or are permitted to use) this mechanism are referred to respectively as attribute value templates and text value templates..
[Definition: Collectively, attribute value templates and text value templates are referred to as value templates.]
A value template is a string that contains zero or more expressions delimited by curly brackets. Outside an expression, a doubled left or right curly brackets (“{{” or “}}”) represents a literal, single bracket and does not start or end an expression. Once an expression begins, it extends to the first unmatched right curly bracket that is not within a string literal or comment.
Value templates are not recursive. Curly brackets inside an expression are part of that expression and are not recognized as nested value templates.
It is a static error (err:XS1003) if an expression does not have a closing right curly bracket or if an unescaped right curly bracket occurs outside of an expression.
It is a static error if the string contained between matching curly brackets in a value template, when interpreted as an XPath expression, contains errors. The error is signaled using the appropriate XPath error code.
2.10.1 Attribute Value Templates
This section describes how (the values of) attribute nodes are processed when they are designated attribute value templates. Option shortcuts are attribute value templates. In an element syntax summary, the value of other such attributes are surrounded by curly brackets. No other attributes are attribute value templates.
[Definition: In an attribute that is designated as an attribute value template, an expression can be used by surrounding the expression with curly brackets ({}), following the general rules for value templates].
Curly brackets are not treated specially in an attribute value in an XProc pipeline unless the attribute is specifically designated as one that permits an attribute value template. Option shortcuts permit attribute value templates. In an element syntax summary, the value of other such attributes is surrounded by curly brackets.
An attribute value template can be seen as an alternating sequence of zero or more “fixed” (non-expression) parts and expression parts. Each expression is evaluated with an undefined context node.
The result of the attribute value template is the concatentation of the fixed parts and the string-value of the result of evaluating each expression part.
Note
This process can generate dynamic errors, for example if the sequence contains an element with a complex content type (which cannot be atomized).
The string value of the attribute is the result of the attribute value template as an xs:untypedAtomic.
2.10.2 Text Value Templates
This section describes how text nodes are processed when they are designated text value templates. All text nodes that are the descendants of a p:inline element are text value templates unless the p:inline has an expand-text attribute with the value “false”. Text nodes that are not the descendants of a p:inline are never text value templates.
[Definition: In a text node that is designated as a text value template, expressions can be used by surrounding each expression with curly brackets ({}), following the general rules for value templates.]
A text value template can be seen as an alternating sequence of zero or more “fixed” (non-expression) parts and expression parts. Each expression is evaluated with an undefined context node.
This produces a sequence of strings (the fixed parts) and items (the results of evaluating each expression). Any items that are non-string atomic values are converted to strings by taking their string value. Strings are converted into text nodes.
The result of the text value template is this sequence of nodes. Adjacent text nodes are combined into a single text node.
Note
Unlike XSLT, in XProc, text value templates are not atomized and converted to single text nodes. It is possible to insert nodes with text value templates in XProc, for example, if the XPath expressions refer to variables that have node content.
2.11 Variables
Variables are name/value pairs. Pipeline authors can create variables to hold computed values.
[Definition: A variable is a name/value pair. The name must be an expanded name. The value may be any XDM value.]
Variables and options share the same scope and may shadow each other.
2.12 Options
Some steps accept options. Options are name/value pairs, like variables. Unlike variables, the value of an option can be changed by the caller.
[Definition: An option is a name/value pair. The name must be an expanded name. The value may be any XDM value.]
[Definition: The options declared on a step are its declared options.] Option names are always expressed as literal values, pipelines cannot construct option names dynamically.
[Definition: The options on a step which have specified values, either because a p:with-option element specifies a value or because the declaration included a default value, are its specified options.]
How outside values are specified for pipeline options on the pipeline initially invoked by the processor is implementation-defined. In other words, the command line options, APIs, or other mechanisms available to specify such options values are outside the scope of this specification.
Some steps require a set of name/value pairs for the operations they perform. For example, an XSLT stylesheet might have required parameters or an XQuery query might have external variables. In the XProc Step Library, the standard way to pass such values to the step is to use an option named “parameters” whose value is a map item value [XSLT 3.0]. The map item contains the mapping of between the names and the values whose interpretation is specific to the step.
2.13 Security Considerations
An XProc pipeline may attempt to access arbitrary network resources: steps such as p:loadXPS and p:http-requestXPS can attempt to read from an arbitrary URI; steps such as p:storeXPS can attempt to write to an arbitrary location; p:execXPS can attempt to execute an arbitrary program. Note, also, that some steps, such as p:xsltXPS and p:xqueryXPS, include extension mechanisms which may attempt to execute arbitrary code.
In some environments, it may be inappropriate to provide the XProc pipeline with access to these resources. In a server environment, for example, it may be impractical to allow pipelines to store data. In environments where the pipeline cannot be trusted, allowing the pipeline to access arbitrary resources or execute arbitrary code may be a security risk.
It is a dynamic error (err:XD0021) for a pipeline to attempt to access a resource for which it has insufficient privileges or perform a step which is forbidden. Which steps are forbidden, what privileges are needed to access resources, and under what circumstances these security constraints apply is implementation-dependent.
Steps in a pipeline may call themselves recursively which could result in pipelines which will never terminate.
A conformant XProc processor may limit the resources available to any or all steps in a pipeline. A conformant implementation may raise dynamic errors, or take any other corrective action, for any security problems that it detects.
2.14 Versioning Considerations
A pipeline author may identify the version of XProc for which a particular pipeline was authored by setting the version attribute. The version attribute can be specified on p:declare-step, p:pipeline, or p:library. If specified, the value of the version attribute must be a xs:decimal. It is a static error (err:XS0063) if the value of the version attribute is not a xs:decimal.
The version of XProc defined by this specification is “2.0”.
A pipeline author must identify the version of XProc on the document element of a pipeline document. It is a static error (err:XS0062) if a required version attribute is not present.
The version identified applies to the element on which the version attribute appears and all of its descendants, unless or until another version is explicitly identified.
When a processor encounters an explicit version (other than a version which it implements), it proceeds in backwards- or forwards-compatible mode.
2.14.1 Backwards-compatible Mode
If the processor encounters a request for a previous version of XProc (e.g., if a "2.0" processor encounters an explicit request for the "1.0" language), it must process the pipeline as if it was a processor for the requested version: it must enforce the semantics of the requested version, it must report steps not known in that version as errors, etc. It is a static error (err:XS0060) if the processor encounters an explicit request for a previous version of the language and it is unable to process the pipeline using those semantics.
2.14.2 Forwards-compatible Mode
If the processor encounters an explicit version which it does not recognize, it processes the pipeline in forwards-compatible mode. Forwards-compatible mode relaxes several static errors, turning them into dynamic errors so that a pipeline author can write a pipeline which conditionally uses new language features.
In forwards-compatible mode:
On any element in the XProc namespace, unrecognized attributes (other than extension attributes) are ignored.
On any step in the XProc namespace, unknown options are ignored.
If a step in the XProc namespace includes an unknown input port with an explicit connection, the connection is treated normally for the purpose of computing the dependencies in the pipeline but it is otherwise ignored. Unknown input ports must not be treated as primary input ports; it will always be an error if they are used but not explicitly connected.
If a step in the pipeline includes an explicit connection to an unknown output port on a step in the XProc namespace, the connection is treated normally for the purpose of computing the dependencies in the pipeline. An empty sequence of documents must appear on that connection.
As a consequence of the rules above, future specifications must not change the semantics of existing step types without changing their names. Although they may add new input and output ports, such changes should be done with care; they should in some sense be limited to ancillary inputs and outputs and they must not be primary input ports.
2.14.2.1 Examples
In forwards-compatible mode, it is not a static error to encounter the following step:
The processor will simply ignore the “ancillary” port.
Suppose that XProc version 2.0 changes the definition of the p:xsltXPS step so that it has an additional output port, messages. Then consider the following pipeline:
When run by a "2.0" or later processor, it will count the documents that appear on the messages port. When run by a “1.0” processor in forwards-compatible mode, the binding to the “messages” port is not a static error. Dynamically, the "1.0" processor will always produce a count of zero, because an empty sequence of documents will always appear on the messages port.
3 Syntax Overview
This section describes the normative XML syntax of XProc. This syntax is sufficient to represent all the aspects of a pipeline, as set out in the preceding sections. [Definition: XProc is intended to work equally well with [XML 1.0] and [XML 1.1]. Unless otherwise noted, the term “XML” refers equally to both versions.][Definition: Unless otherwise noted, the term Namespaces in XML refers equally to [Namespaces 1.0] and [Namespaces 1.1].]Support for pipeline documents written in XML 1.1 and pipeline inputs and outputs that use XML 1.1 is implementation-defined.
Elements in a pipeline document represent the pipeline, the steps it contains, the connections between those steps, the steps and connections contained within them, and so on. Each step is represented by an element; a combination of elements and attributes specify how the inputs and outputs of each step are connected and how options are passed.
Conceptually, we can speak of steps as objects that have inputs and outputs, that are connected together and which may contain additional steps. Syntactically, we need a mechanism for specifying these relationships.
Containment is represented naturally using nesting of XML elements. If a particular element identifies a compound step then the step elements that are its immediate children form its subpipeline.
The connections between steps are expressed using names and references to those names.
Six kinds of things are named in XProc:
Step types,
Steps,
Input ports),
Output ports,
Options and variables
3.1 XProc Namespaces
There are three namespaces associated with XProc:
http://www.w3.org/ns/xproc
The namespace of the XProc XML vocabulary described by this specification; by convention, the namespace prefix “p:” is used for this namespace.
http://www.w3.org/ns/xproc-step
The namespace used for documents that are inputs to and outputs from several standard and optional steps described in this specification. Some steps, such as p:http-requestXPS and p:storeXPS, have defined input or output vocabularies. We use this namespace for all of those documents. The conventional prefix “c:” is used for this namespace.
http://www.w3.org/ns/xproc-error
The namespace used for errors. The conventional prefix “err:” is used for this namespace.
This specification also makes use of the prefix “xs:” to refer to the [W3C XML Schema: Part 1] namespace http://www.w3.org/2001/XMLSchema.
3.2 Scoping of Names
Names are used to identify step types, steps, ports, options and variables. Step types, options, and variables are named with QNames. Steps and ports are named with NCNames. The scope of a name is a measure of where it is available in a pipeline. [Definition: If two names are in the same scope, we say that they are visible to each other. ]
The scope of the names of the step types is the pipeline in which they are declared, including any declarations imported from libraries via p:import. Nested pipelines inherit the step types in scope for their parent.
Any types that are in the scope of any p:library that is imported.
All the step types in a pipeline or library must have unique names: it is a static error (err:XS0036) if any step type name is built-in and/or declared or defined more than once in the same scope.
The scope of the names of the steps themselves is determined by the environment of each step. In general, the name of a step, the names of its sibling steps, the names of any steps that it contains directly, the names of its ancestors, and the names of the siblings of its ancestors are all in a common scope. All steps in the same scope must have unique names: it is a static error (err:XS0002) if two steps with the same name appear in the same scope.
The scope of an input or output port name is the step on which it is defined. The names of all the ports on any step must be unique.
Taken together, these uniqueness constraints guarantee that the combination of a step name and a port name uniquely identifies exactly one port on exactly one in-scope step.
The scope of option and variable names is determined by where they are declared. When an option is declared with p:option (or a variable with p:variable), unless otherwise specified, its scope consists of the sibling elements that follow its declaration and the descendants of those siblings. It is a static error (err:XS0004) if an option or variable declaration duplicates the name of any other option or variable in the same environment. That is, no option or variable may lexically shadow another option or variable with the same name.
3.3 Base URIs and xml:base
When a relative URI appears in an option value, the base URI against which it must be made absolute is the base URI of the p:option element. If an option value is specified using a syntactic shortcut, the base URI of the step on which the shortcut attribute appears must be used. In general, whenever a relative URI appears, its base URI is the base URI of the nearest ancestor element.
The pipeline author can control the base URIs of elements within the pipeline document with the xml:base attribute. The xml:base attribute may appear on any element in a pipeline and has the semantics outlined in [XML Base].
3.4 Unique identifiers
A pipeline author can provide a globally unique identifier for any element in a pipeline with the xml:id attribute.
The xml:id attribute may appear on any element in a pipeline and has the semantics outlined in [xml:id].
[Definition: A document is specified by URI if it is referenced with a URI.] The href attribute on the p:document element is used to refer to documents by URI.
In this example, the input to the p:identityXPS step named “otherstep” comes from “http://example.com/input.xml”.
[Definition: A document is specified by source if it references a specific port on another step.] The step and port attributes on the p:pipe element are used for this purpose.
In this example, the “source” input to the p:xincludeXPS step named “expand” comes from the “result” port of the step named “otherstep”.
See the description of p:pipe for a complete description of the ports that can be connected.
Specified inline
[Definition: An inline document is specified directly in the body of the element to which it connects.] The content of the p:inline element is used for this purpose.
In this example, the “stylesheet” input to the XSLT step named “xform” comes from the content of the p:input element itself.
Inline documents are considered “quoted”. The pipeline processor passes them literally to the port, even if they contain elements from the XProc namespace or other namespaces that would have other semantics outside of the p:inline.
Specified explicitly empty
[Definition: An empty sequence of documents is specified with the p:empty element.]
In this example, the “source” input to the XSLT 2.0 step named “generate” is explicitly empty:
If you omit the connection on a primary input port, a connection to the default readable port will be assumed. Making the connection explicitly empty guarantees that the connection will be to an empty sequence of documents.
Note that a p:input or p:output element may contain more than one p:pipe, p:document, or p:inline element. If more than one connection is provided, then the specified sequence of documents is made available on that port in the same order as the connections.
3.6 Documentation
Pipeline authors may add documentation to their pipeline documents with the p:documentation element. Except when it appears as a descendant of p:inline, the p:documentation element is completely ignored by pipeline processors, it exists simply for documentation purposes. If a p:documentation is provided as a descendant of p:inline, it has no special semantics, it is treated literally as part of the document to be provided on that port. The p:documentation element has no special semantics when it appears in documents that flow through the pipeline.
Pipeline processors that inspect the contents of p:documentation elements and behave differently on the basis of what they find are not conformant. Processor extensions must be specified with p:pipeinfo.
3.7 Processor annotations
Pipeline authors may add annotations to their pipeline documents with the p:pipeinfo element. The semantics of p:pipeinfo elements are implementation-defined. Processors should specify a way for their annotations to be identified, perhaps with extension attributes.
Where p:documentation is intended for human consumption, p:pipeinfo elements are intended for processor consumption. A processor might, for example, use annotations to identify some particular aspect of an implementation, to request additional, perhaps non-standard features, to describe parallelism constraints, etc.
When a p:pipeinfo appears as a descendant of p:inline, it has no special semantics; in that context it must be treated literally as part of the document to be provided on that port. The p:pipeinfo element has no special semantics when it appears in documents that flow through the pipeline.
3.8 Extension attributes
[Definition: An element from the XProc namespace may have any attribute not from the XProc namespace, provided that the expanded-QName of the attribute has a non-null namespace URI. Such an attribute is called an extension attribute.]
The presence of an extension attribute must not cause the connections between steps to differ from the connections that would arise in the absence of the attribute. They must not cause the processor to fail to signal an error that would be signaled in the absence of the attribute.
A processor which encounters an extension attribute that it does not implement must behave as if the attribute was not present.
3.9 Conditional Element Exclusion
Any element in the XProc namespace may have a use-when attribute which must contain an XPath expression that can be evaluated statically. If the attribute is present and the effective boolean value of the expression is false, then the element and all of its descendants are effectively excluded from the pipeline document. If a node is effectively excluded, the processor must behave as if the element was not present in the document.
Elements that are not in the XProc namespace may also have a use-when attribute, but the attribute must be in the XProc namespace. The semantics of a p:use-when attribute on an element not in the XProc namespace are the same as the semantics of a use-when attribute on an element in the XProc namespace.
Conditional element exclusion occurs before any static analysis of the pipeline.
Note
The effective exclusion of use-when processing occurs after XML parsing and has no effect on well-formedness or validation errors which will be reported in the usual way. Note also that use-when is not performed when it occurs on the descendant of a p:inline element.
For the purposes of evaluating a use-when expression, the context node, position, and size are all undefined. No in-scope bindings are available. There are no readable ports. There are no available documents or available collections.
There are some additional restrictions on the XPath extension functions that are available in a use-when expression:
The p:episode system property should not be used. The value of the p:episode system property in a use-when expression is implementation-dependent.
The p:step-available function cannot be used to test for the availability of extension steps (because the libraries that declare them may not have been imported). The results of testing for steps not in the XProc namespace in a use-when expression are implementation-dependent.
The steps available and possibly other aspects of the expression may depend on the version specified for a pipeline, see Section 2.14, “Versioning Considerations”. For example, in a “1.0” pipeline, the processor should not report that “2.0” steps are available.
It is a static error (err:XS0061) if a use-when expression refers to the context or attempts to refer to any documents or collections.
3.10 Syntax Summaries
The description of each element in the pipeline namespace is accompanied by a syntactic summary that provides a quick overview of the element's syntax:
<p:some-element
some-attribute? = some-type>
(some |
elements |
allowed)*,
other-elements?
</p:some-element>
The content model fragments in these tableaux are presented in a simple, compact notation. In brief:
A name represent exactly one occurrence of an element with that name.
Parentheses are used for grouping.
Elements or groups separated by a comma (“,”) represent an ordered sequence: a followed by b followed by c: (a,b,c).
Elements or groups separated by a vertical bar (“|”) represent a choice: a or b or c: (a | b | c).
Elements or groups separated by an ampersand (“&”) represent an unordered sequence: a and b and c, in any order: (a & b & c).
An element or group followed by a question mark (“?”) is optional; it may or may not occur but if it occurs it can occur only once.
An element or group followed by an asterisk (“*”) is optional and may be repeated; it may or may not occur and if it occurs it can occur any number of times.
An element or group followed by a plus (“+”) is required and may be repeated; it must occur at least once, and it can occur any number of times.
For clarity of exposition, some attributes and elements are elided from the summaries:
An xml:id attribute is allowed on any element. It has the semantics of [xml:id].
An xml:base attribute is allowed on any element. It has the semantics of [XML Base].
The types given for attributes should be understood as follows:
ID, NCName, NMTOKEN, NMTOKENS, anyURI, boolean, integer, string: As per [W3C XML Schema: Part 2] including whitespace normalization as appropriate.
QName: With whitespace normalization as per [W3C XML Schema: Part 2] and according to the following definition: In the context of XProc, a QName is almost always a QName in the Namespaces in XML sense. Note, however, that p:option values can get their namespace declarations in a non-standard way (with p:namespaces) and QNames that have no prefix are always in no-namespace, irrespective of the default namespace.
QNameList: As a whitespace separated list of QNames, per the definition above.
PrefixList: As a list with [item type]NMTOKEN, per [W3C XML Schema: Part 2], including whitespace normalization.
ExcludeInlinePrefixes: As a PrefixList per the definition above, with the following extensions: the tokens #all and #default may appear.
XPathExpression, XSLTMatchPattern: As a string per [W3C XML Schema: Part 2], including whitespace normalization, and the further requirement to be a conformant Expression per [XPath 2.0] or Match pattern per [XSLT 2.0].
MediaTypes: As a whitespace separated list of media types as defined in [Media Types].
It is a dynamic error (err:XD0028) if any attribute value does not satisfy the type required for that attribute.
It is a static error (err:XS0044) if any element in the XProc namespace or any step has element children other than those specified for it by this specification. In particular, the presence of atomic steps for which there is no visible declaration may raise this error.
It is a static error (err:XS0037) if any step directly contains text nodes that do not consist entirely of whitespace.
It is a dynamic error (err:XD0019) if any option value does not satisfy the type required for that option.
It is a dynamic error (err:XD0012) if any attempt is made to dereference a URI where the scheme of the URI reference is not supported. Implementations are encouraged to support as many schemes as is practical and, in particular, they should support both the file: and http(s): schemes. The set of URI schemes actually supported is implementation-defined.
It is a dynamic error (err:XD0030) if a step is unable or incapable of performing its function. This is a general error code for “step failed” (e.g., if the input isn't of the expected type or if attempting to process the input causes the implementation to abort). Users and implementers who create extension steps are encouraged to use this code for general failures.
In most steps which use a select expression or match pattern, any kind of node can be identified by the expression or pattern. However, some expressions and patterns on some steps are only applicable to some kinds of nodes (e.g., it doesn't make sense to speak of adding attributes to a comment!).
It is a dynamic error (err:XC0023XPS) if a select expression or match pattern returns a node type that is not allowed by the step.
If an XProc processor can determine statically that a dynamic error will always occur, it may report that error statically provided that the error does not occur among the descendants of a p:try. Dynamic errors inside a p:trymust not be reported statically. They must be raised dynamically so that p:catch processing can be performed on them.
4 Steps
This section describes the core language steps of XProc; the full vocabulary of standard, atomic steps is described in [XProc 2.0: Standard Step Library].
The following dynamic errors are described in the atomic step vocabulary. They are repeated here so that the list of dynamic errors is wholly contained within this specification.
Editorial Note
This is not the right long term solution.
It is a dynamic error (err:XD0014) for any unqualified attribute names other than “name”, “namespace”, or “value” to appear on a c:param element.
It is a dynamic error (err:XD0025) if the namespace attribute is specified on c:param, the name contains a colon, and the specified namespace is not the same as the in-scope namespace binding for the specified prefix.
On steps which allow independent specification of a namespace and a name, it is a dynamic error (err:XD0034) to specify a new namespace or prefix if the lexical value of the specified name contains a colon.
4.1 p:pipeline
A p:pipeline declares a pipeline that can be evaluated by an XProc processor. It encapsulates the behavior of a subpipeline. Its children declare inputs, outputs, and options that the pipeline exposes and identify the steps in its subpipeline. (A p:pipeline is a simplified form of step declaration.)
All p:pipeline pipelines have an implicit primary input port named “source” and an implicit primary output port named “result”. Any input or output ports that the p:pipeline declares explicitly are in addition to those ports and may not be declared primary.
Viewed from the outside, a p:pipeline is a black box which performs some calculation on its inputs and produces its outputs. From the pipeline author's perspective, the computation performed by the pipeline is described in terms of contained steps which read the pipeline's inputs and produce the pipeline's outputs.
The version attribute identifies the version of XProc for which this pipeline was authored. If the p:pipeline has no ancestors in the XProc namespace, then it must have a version attribute. See Section 2.14, “Versioning Considerations”.
If a pipeline does not have a type then that pipeline cannot be invoked as a step.
The p:pipeline element is just a simplified form of step declaration. A document that reads:
A for-each is specified by the p:for-each element. It is a compound step that processes a sequence of documents, applying its subpipeline to each document in turn.
When a pipeline needs to process a sequence of documents using a subpipeline that only processes a single document, the p:for-each construct can be used as a wrapper around that subpipeline. The p:for-each will apply that subpipeline to each document in the sequence in turn.
The result of the p:for-each is a sequence of documents produced by processing each individual document in the input sequence. If the p:for-each has one or more output ports, what appears on each of those ports is the sequence of documents that is the concatenation of the sequence produced by each iteration of the loop on the port to which it is connected. If the iteration source for a p:for-each is an empty sequence, then the subpipeline is never run and an empty sequence is produced on all of the outputs.
The p:for-each has a single anonymous input: its connection is provided by the p:input. If no iteration sequence is explicitly provided, then the iteration source is read from the default readable port.
The processor provides each document, one at a time, to the subpipeline represented by the children of the p:for-each on a port named current.
For each declared output, the processor collects all the documents that are produced for that output from all the iterations, in order, into a sequence. The result of the p:for-each on that output is that sequence of documents.
Note that outputs declared for a p:for-each serve a dual role. Inside the p:for-each, they are used to read results from the subpipeline. Outside the p:for-each, they provide the aggregated results.
The sequence attribute on a p:output inside a p:for-each only applies inside the step. From the outside, all of the outputs produce sequences.
In the case where no XPath expression that must be evaluated by the processor makes any reference to p:iteration-size, its value does not actually have to be calculated (and the entire input sequence does not, therefore, need to be buffered so that its size can be calculated before processing begins).
4.2.2 Example
A p:for-each might accept a sequence of chapters as its input, process each chapter in turn with XSLT, a step that accepts only a single input document, and produce a sequence of formatted chapters as its output.
The //chapter elements of the document are selected. Each chapter is transformed into HTML and XSL Formatting Objects using an XSLT step. The resulting HTML and FO documents are aggregated together and appear on the html-results and fo-results ports, respectively, of the chapters step itself.
4.3 p:viewport
A viewport is specified by the p:viewport element. It is a compound step that processes a single XML document, applying its subpipeline to one or more subtrees of the document.
The result of the p:viewport is a copy of the original document where the selected subtrees have been replaced by the results of applying the subpipeline to them.
The p:viewport has a single anonymous input: its connection is provided by the p:input. If no document is explicitly provided, then the viewport source is read from the default readable port. If the p:viewport input is a sequence, each document in the sequence is processed in turn producing a sequence on the output.
The match attribute specifies an XSLT match pattern. Each matching node in the source document is wrapped in a document node, as necessary, and provided, one at a time, to the viewport's subpipeline on a port named current. The base URI of the resulting document that is passed to the subpipeline is the base URI of the matched element or document. It is a dynamic error (err:XD0010) if the match expression on p:viewport does not match an element or document.
After a match is found, the entire subtree rooted at that match is processed as a unit. No further attempts are made to match nodes among the descendants of any matched node.
What appears on the output from the p:viewport will be a copy of the input document where each matching node is replaced by the result of applying the subpipeline to the subtree rooted at that node. In other words, if the match pattern matches a particular element then that element is wrapped in a document node and provided on the current port, the subpipeline in the p:viewport is evaluated, and the result that appears on the output port replaces the matched element.
If no documents appear on the output port, the matched element will effectively be deleted. If exactly one document appears, the contents of that document will replace the matched element. If a sequence of documents appears, then the contents of each document in that sequence (in the order it appears in the sequence) will replace the matched element.
The output of the p:viewport itself is a single document that appears on a port named “result”. Note that the semantics of p:viewport are special. The output port in the p:viewport is used only to access the results of the subpipeline. The output of the step itself appears on a port with the fixed name “result” that is never explicitly declared.
In the case where no XPath expression that must be evaluated by the processor makes any reference to p:iteration-size, its value does not actually have to be calculated (and the entire input sequence does not, therefore, need to be buffered so that its size can be calculated before processing begins).
4.3.2 Example
A p:viewport might accept an XHTML document as its input, add an hr element at the beginning of all div elements that have the class value “chapter”, and return an XHTML document that is the same as the original except for that change.
The nodes which match h:div[@class='chapter'] in the input document are selected. An hr is inserted as the first child of each h:div and the resulting version replaces the original h:div. The result of the whole step is a copy of the input document with a horizontal rule as the first child of each selected h:div.
4.4 p:choose
A choose is specified by the p:choose element. It is a multi-container step that selects exactly one of a list of alternative subpipelines based on the evaluation of XPath expressions.
A p:choose has no inputs. It contains an arbitrary number of alternative subpipelines, exactly one of which will be evaluated.
The list of alternative subpipelines consists of zero or more subpipelines guarded by an XPath expression, followed optionally by a single default subpipeline.
The p:choose considers each subpipeline in turn and selects the first (and only the first) subpipeline for which the guard expression evaluates to true in its context. If there are no subpipelines for which the expression evaluates to true, the default subpipeline, if it was specified, is selected.
After a subpipeline is selected, it is evaluated as if only it had been present.
The outputs of the p:choose are taken from the outputs of the selected subpipeline. The p:choose has the same number of outputs as the selected subpipeline with the same names. If the selected subpipeline has a primary output port, the port with the same name on the p:choose is also a primary output port.
In order to ensure that the output of the p:choose is consistent irrespective of the subpipeline chosen, each subpipeline must declare the same number of outputs with the same names. If any of the subpipelines specifies a primary output port, each subpipeline must specify exactly the same output as primary. It is a static error (err:XS0007) if two subpipelines in a p:choose declare different outputs.
As a convenience to authors, it is not an error if some subpipelines declare outputs that can produce sequences and some do not. Each output of the p:choose is declared to produce a sequence if that output is declared to produce a sequence in any of its subpipelines.
The p:choose can specify the context node against which the XPath expressions that occur on each branch are evaluated. The context node is specified as a connection in the p:input. If no explicit connection is provided, the default readable port is used. If the context node is connected to p:empty, or is unconnected and the default readable port is undefined, the context item is undefined. It is a dynamic error (err:XD0005) if more than one document appears on the connection for this input port.
Each conditional subpipeline is represented by a p:when element. The default branch is represented by a p:otherwise element.
4.4.1 p:when
A when specifies one subpipeline guarded by a test expression.
Each p:when branch of the p:choose has a test attribute which must contain an XPath expression. That XPath expression's effective boolean value is the guard for the subpipeline contained within that p:when.
The p:when can specify a context node against which its test expression is to be evaluated. That context node is specified as a connection for the p:input. If no context is specified on the p:when, the context of the p:choose is used. It is a dynamic error (err:XD0005) if more than one document appears on the connection for this input port.
4.4.2 p:otherwise
An otherwise specifies the default branch; the subpipeline selected if no test expression on any preceding p:when evaluates to true.
A group is specified by the p:group element. In a p:try, it is a non-step wrapper, everywhere else, it is a compound step. A group encapsulates the behavior of its subpipeline.
A try/catch is specified by the p:try element. It is a multi-container step that isolates a subpipeline, preventing any dynamic errors that arise within it from being exposed to the rest of the pipeline.
The p:group represents the initial subpipeline and the recovery (or “catch”) pipelines are identified with p:catch elements. The p:finally pipeline always runs after the p:try.
The p:try step evaluates the initial subpipeline and, if no errors occur, the outputs of that pipeline are the outputs of the p:try step. However, if any errors occur, the p:try abandons the first subpipeline, discarding any output that it might have generated, and considers the recovery subpipelines.
Each p:catch pipeline is considered in document order. All except the last must have a code attribute. If any of the specified error codes matches the error that was raised in the p:group, then that p:catch is selected as the recovery pipeline. The last p:catchmust not have a code attribute; it is selected if no preceding p:catch has a matching error code. It is a static error (err:XS1001) if the code attribute is missing from any but the last p:catch, if the last p:catch has a code, or if any error code is repeated..
If the recovery subpipeline is evaluated, the outputs of the recovery subpipeline are the outputs of the p:try step. If the recovery subpipeline is evaluated and a step within that subpipeline fails, the p:try fails.
The outputs of the p:try are taken from the outputs of the initial subpipeline or the recovery subpipeline if an error occurred in the initial subpipeline. The p:try has the same number of outputs as the selected subpipeline with the same names. If the selected subpipeline has a primary output port, the port with the same name on the p:try is also a primary output port.
In order to ensure that the output of the p:try is consistent irrespective of whether the initial subpipeline provides its output or the recovery subpipeline does, both subpipelines must declare the same number of outputs with the same names. If either of the subpipelines specifies a primary output port, both subpipelines must specify exactly the same output as primary. It is a static error (err:XS0009) if the p:group and p:catch subpipelines declare different outputs.
As a convenience to authors, it is not an error if an output port can produce a sequence in the initial subpipeline but not in the recovery subpipeline, or vice versa. Each output of the p:try is declared to produce a sequence if that output is declared to produce a sequence in either of its subpipelines.
A pipeline author can cause an error to occur with the p:error step.
The recovery subpipeline of a p:try is identified with a p:catch:
What appears on the error output port is an error document. The error document may contain messages generated by steps that were part of the initial subpipeline. Not all messages that appear are indicative of errors; for example, it is common for all xsl:message output from the XSLT component to appear on the error output port. It is possible that the component which fails may not produce any messages at all. It is also possible that the failure of one component may cause others to fail so that there may be multiple failure messages in the document.
Irrespective of which pipeline is evaluated, the last thing that the p:try step does is evaluate the p:finally pipeline. This happens even if the p:try fails.
<p:finally
name? = NCName>
subpipeline
</p:finally>
The p:finally has no inputs and no outputs. It exists only to handle recovery and resource cleanup tasks. If cleanup tasks require access to readable ports, put them in the p:catch block of an enclosing p:try.
Editorial Note
I'm not actually sure p:finally is worth doing, but I've sketched it in for completeness. Also, should p:catch be entirely optional, allowing just try/group/finally?
4.6.1 The Error Vocabulary
In general, it is very difficult to predict error behavior. Step failure may be catastrophic (programmer error), or it may be the result of user error, resource failures, etc. Steps may detect more than one error, and the failure of one step may cause other steps to fail as well.
The p:try/p:catch mechanism gives pipeline authors the opportunity to process the errors that caused the p:try to fail. In order to facilitate some modicum of interoperability among processors, errors that are reported on the error output port of a p:catchshould conform to the format described here.
4.6.1.1 c:errors
The error vocabulary consists of a root element, c:errors which contains zero or more c:error elements.
Each specific error is represented by an c:error element:
<c:error
name? = NCName
type? = QName
code? = QName
href? = anyURI
line? = integer
column? = integer
offset? = integer>
(string |
anyElement)*
</c:error>
The name and type attributes identify the name and type, respectively, of the step which failed.
The code is a QName which identifies the error. For steps which have defined error codes, this is an opportunity for the step to identify the error in a machine-processable fashion. Many steps omit this because they do not include the concept of errors identified by QNames.
If the error was caused by a specific document, or by the location of some erroneous construction in a specific document, the href, line, column, and offset attributes identify this location. Generally, the error location is identified either with line and column numbers or with an offset from the beginning of the document, but not usually both.
The content of the c:error element is any well-formed XML. Specific steps, or specific implementations, may provide more detail about the format of the content of an error message.
It is not an error for steps to generate non-standard error output as long as it is well-formed.
4.6.2 Example
A pipeline might attempt to process a document by dispatching it to some web service. If the web service succeeds, then those results are passed to the rest of the pipeline. However, if the web service cannot be contacted or reports an error, the p:catch step can provide some sort of default for the rest of the pipeline.
In addition to the six step types described in the preceding sections, XProc provides a standard library of atomic step types. The full vocabulary of standards steps is described in [XProc 2.0: Standard Step Library].
All of the standard, atomic steps are invoked in the same way:
Where “p:atomic-step” must be in the XProc namespace and must be declared in either the standard library for the XProc version supported by the processor or explicitly imported by the surrounding pipeline (see Section 2.14, “Versioning Considerations”).
4.8 Extension Steps
Pipeline authors may also have access to additional steps not defined or described by this specification. Atomic extension steps are invoked just like standard steps:
Extension steps must not be in the XProc namespace and there must be a visible step declaration at the point of use (see Section 3.2, “Scoping of Names”).
If the relevant step declaration has no subpipeline, then that step invokes the declared atomic step, which the processor must know how to perform. These steps are implementation-defined extensions.
If the relevant step declaration has a subpipeline, then that step runs the declared subpipeline. These steps are user- or implementation-defined extensions. Pipelines can refer to themselves (recursion is allowed), to pipelines defined in imported libraries, and to other pipelines in the same library if they are in a library.
It is a static error (err:XS0010) if a pipeline contains a step whose specified inputs, outputs, and options do not match the signature for steps of that type.
It is a dynamic error (err:XD0017) if the running pipeline attempts to invoke a step which the processor does not know how to perform.
Namespace qualified attributes on a step are extension attributes. Attributes, other than name, that are not namespace qualified are treated as a syntactic shortcut for specifying the value of an option. In other words, the following two steps are equivalent:
The first step uses the standard p:with-option syntax:
There are some limitations to this shortcut syntax:
It only applies to option names that are not in a namespace.
It only applies to option names that are not otherwise used on the step, such as “name”.
If the option value includes curly braces, it is treated as an attribute value template. The context node for attribute value templates in an option shortcut value comes from the default readable port for the step on which they occur. If there is no such port, the context node is undefined.
The syntactic shortcuts apply equally to standard atomic steps and extension atomic steps.
5 Other pipeline elements
5.1 p:input
A p:input identifies an input port for a step. In some contexts, p:input declares that a port with the specified name exists and identifies the properties of that port. In other contexts, it provides a connection for a port declared elsewhere. And in some contexts, it does both.
5.1.1 Input declarations
The declaration of an input identifies the name of the port, whether or not the port accepts a sequence, whether or not the port is a primary input port, what content types it accepts, and may provide a default connection for the port.
The port attribute defines the name of the port. It is a static error (err:XS0011) to identify two ports with the same name on the same step.
The sequence attribute determines whether or not a sequence of documents is allowed on the port. If sequence is not specified, or has the value false, then it is a dynamic error (err:XD0006) unless exactly one document appears on the declared port.
The primary attribute is used to identify the primary input port. An input port is a primary input port if primary is specified with the value true or if the step has only a single input port and primary is not specified. It is a static error (err:XS0030) to specify that more than one input port is the primary.
The content-types attribute lists one or more (space separated) content types that this input port will accept. A content type must be of the form “type/subtype+ext” where any of type, subtype, and ext can be specified as “*” meaning “any”. The “+ext” is optional. Here are some examples of content types for matching:
text/plain, plain text documents
text/*, any kind of text document.
*/*+xml, any XML content type.
*/*, any content type.
If a connection is provided in the declaration, then select may be used to select a portion of the input identified by the p:empty, p:document, or p:inline elements in the p:input. This select expression applies only if the default connection is used. If an explicit connection is provided by the caller, then the default select expression is ignored.
Note
The p:pipe element is explicitly excluded from a declaration because it would make the default value of an input dependent on the execution of some part of the pipeline. Default values are designed so that they can be computed statically.
On a p:declare-step for an atomic step, the p:input simply declares the input port. It is a static error (err:XS0042) to attempt to provide a connection for an input port on the declaration of an atomic step.
If the port is specified, then this is a binding for the specified port. If no port is specified, then:
In a p:viewport or p:for-each, it is a binding for the step's single, anonymous input port.
In a p:choose or p:when, it is a binding for the context item for the test expression(s).
Elsewhere, it is a binding for the primary input port of the step in which it occurs. It is a static error (err:XS1002) if there is no primary input port.
A select expression may also be provided with a connection. The select expression, if specified, applies the specified XPath select expression to the document(s) that are read. It is a dynamic error (err:XD1004) if the select is used and any input document is not an XML document.
Each selected node is wrapped in a document (unless it is a document) and provided to the input port. In other words,
provides a sequence of zero or more documents, one for each html:div in http://example.org/input.html. (Note that in the case of nested html:div elements, this may result in the same content being returned in several documents.)
A select expression can equally be applied to input read from another step. This input:
provides a sequence of zero or more documents, one for each html:div in the document (or each of the documents) that is read from the result port of the step named origin.
The base URI of the document that results from a select expression is the base URI of the matched element or document. It is a dynamic error (err:XD0016) if the select expression on a p:input returns atomic values or anything other than element or document nodes (or an empty sequence).
An input declaration may include a default connection. If no connection is provided for an input port which has a default connection, then the input is treated as if the default connection appeared.
A p:input element with no children (e.g., “<p:input/>”) is equivalent to an input connection that contains only “<p:pipe/>”, which is in turn equivalent to a binding to the default readable port.
An input declaration may contain foreign element, scoped outside of XProc vocabulary (http://www.w3.org/ns/xproc) namespace. Each element is treated as if wrapped with a p:inline element. For definition of this implicit behaviour see p:inline.
A default connection does not satisfy the requirement that a primary input port is automatically connected by the processor, nor is it used when no default readable port is defined. In other words, a p:declare-step or a p:pipeline can define defaults for all of its inputs, whether they are primary or not, but defining a default for a primary input usually has no effect. It's never used by an atomic step since the step, when it's called, will always connect the primary input port to the default readable port (or cause a static error). The only case where it has value is on a p:pipeline when that pipeline is invoked directly by the processor. In that case, the processor must use the default connection if no external connection is provided for the port.
5.2 p:output
A p:output identifies an output port, optionally connecting an input for it, if necessary.
<p:output
port? = NCName
sequence? = boolean
primary? = boolean />
The port attribute defines the name of the port. It is a static error (err:XS0011) to identify two ports with the same name on the same step.
An output declaration can indicate if a sequence of documents is allowed to appear on the declared port. If sequence is specified with the value true, then a sequence is allowed. If sequence is not specified on p:output, or has the value false, then it is a dynamic error (err:XD0007) if the step does not produce exactly one document on the declared port.
The primary attribute is used to identify the primary output port. An output port is a primary output port if primary is specified with the value true or if the step has only a single output port and primary is not specified. It is a static error (err:XS0014) to identify more than one output port as primary.
If a connection is provided for a p:output, documents are read from that connection and those documents form the output that is written to the output port. In other words, placing a p:document inside a p:output causes the processor to read that document and provide it on the output port. It does not cause the processor to write the output to that document.
A p:log element is a debugging aid. It associates a URI with a specific output port on a step:
<p:log
port? = NCName
href? = anyURI />
The semantics of p:log are that it writes to the specified IRI whatever document or documents appear on the specified port. If the href attribute is not specified, the location of the log file or files is implementation-defined.
How each document or sequence of documents is represented in a p:log is implementation-defined. Pipelines are not expected to be able to consume their own logging output. The ability of a step to read the p:log output of some former step is implementation-dependent.
It is a static error (err:XS0026) if the port specified on the p:log is not the name of an output port on the step in which it appears or if more than one p:log element is applied to the same port.
Implementations may, at user option, ignore all p:log elements.
Note
This element represents a potential security risk: running unexamined 3rd-party pipelines could result in vital system resources being overwritten.
5.4 p:serialization
The p:serialization element allows the user to request serialization properties on a p:pipeline output.
If the pipeline processor serializes the output on the specified port, it must use the serialization options specified. If the processor is not serializing (if, for example, the pipeline has been called from another pipeline), then the p:serializationmust be ignored. The processor may reject statically a pipeline that requests serialization options that it cannot provide.
The default value of any serialization options not specified on a particular p:serialization element is implementation-defined. The allowed options are defined by [Serialization]. It is a dynamic error (err:XD0020) if the combination of serialization options specified or defaulted is not allowed. Implementations must check that all of the specified serialization options are allowed if they serialize the specified output. If the specified output is not being serialized (because it is being returned as the result of a call from within another pipeline, for example) implementations may but are not required to check that the specified options are allowed.
It is a static error (err:XS0039) if the port specified on the p:serialization is not the name of an output port on the pipeline in which it appears or if more than one p:serialization element is applied to the same port.
5.5 Variables and Options
Variables and options provide a mechanism for pipeline authors to construct temporary results and hold onto them for reuse.
Variables are created in compound steps and, like XSLT variables, are single assignment, though they may be shadowed by subsequent declarations of other variables with the same name.
Options can be declared on atomic or compound steps. The value of an option can be specified by the caller invoking the step. Any value specified by the caller takes precedence over any default value specified in the declaration.
5.5.1 p:variable
A p:variable declares a variable and associates a value with it. Variable declarations may optionally specify the type of the variable using an XPath Sequence Type.
The name of the variable must be a QName. If it does not contain a prefix then it is in no namespace. It is a static error (err:XS0028) to declare an option or variable in the XProc namespace.
The variable's value is specified with a select attribute. The select attribute must be specified. The content of the select attribute is an XPath expression which will be evaluated to provide the value of the variable.
If a select expression is given, it is evaluated as an XPath expression using the appropriate context as described in Section 2.7, “XPaths in XProc”, for the enclosing container, with the addition of bindings for all preceding-sibling p:variable and p:option elements.
The type of the value may be specified in the as attribute using an XPath Sequence Type[citation needed]. If an atomic type, or sequence of atomic types, is specified, the value provided for the option will be atomized according to the standard XPath rules. It is a dynamic error (err:XD1001) if the computed value does not match the specified sequence type.
Since all in-scope bindings are present in the Processor XPath Context as variable bindings, select expressions may refer to the value of in-scope bindings by variable reference. If a variable reference uses a QName that is not the name of an in-scope binding, an XPath evaluation error will occur.
If a select expression is given, the readable ports available for document connections are the readable ports in the environment inherited by the first step in the surrounding container's contained steps. However, in order to avoid ordering paradoxes, it is a static error (err:XS0019) for a variable's document connection to refer to the output port of any step in the surrounding container's contained steps.
If a select expression is given but no document connection is provided, the implicit connection is to the default readable port in the environment inherited by the first step in the surrounding container's contained steps. If there is no default readable port, the connection is treated as if p:empty was specified.
It is a dynamic error (err:XD0008) if a sequence of more than one document appears on the connection for a p:variable. If p:empty is given or implied as the document connection, the context item is undefined. It is a dynamic error (err:XD0026) if the select expression makes reference to the context node, size, or position when the context item is undefined.
A p:option declares an option and may associate a default value with it. The p:option tag can only be used in a p:declare-step or a p:pipeline (which is a syntactic abbreviation for a step declaration).
The name of the option must be a QName. If it does not contain a prefix then it is in no namespace. It is a static error (err:XS0028) to declare an option or variable in the XProc namespace.
It is a static error (err:XS0004) to declare two or more options on the same step with the same name.
<p:option
name = QName
as? = XPathSequenceType
required? = boolean />
An option may declare its type. The type is specified in the as attribute using an XPath Sequence Type[citation needed]. If an atomic type, or sequence of atomic types, is specified, the value provided for the option will be atomized according to the standard XPath rules. It is a dynamic error (err:XD1001) if the computed value does not match the specified sequence type.
An option may be declared as required. If an option is required, it is a static error (err:XS0018) to invoke the step without specifying a value for that option.
If an option is not declared to be required, it may be given a default value. The value is specified with a select attribute.
<p:option
name = QName
as? = XPathSequenceType
required? = boolean
select = XPathExpression />
If a select attribute is specified, its content is an XPath expression which will be evaluated to provide the value of the option, which may differ from one instance of the step type to another.
The select expression is only evaluated when its actual value is needed by an instance of the step type being declared. In this case, it is evaluated as described in Section 5.5.3, “p:with-option” except that
The context item is undefined.
the variable bindings consist only of bindings for options whose declaration precedes the p:option itself in the surrounding step signature;
the in-scope namespaces are the in-scope namespaces of the p:option itself.
It is a static error (err:XS0017) to specify that an option is both requiredand has a default value.
It is a dynamic error (err:XD0026) if the select expression makes reference to the context node, size, or position.
Regardless of the implicit type of the expression, the value is an xs:untypedAtomic.
5.5.3 p:with-option
A p:with-option provides an actual value for an option when a step is invoked.
The name of the option must be a QName. If it does not contain a prefix then it is in no namespace. It is a static error (err:XS0031) to use an option name in p:with-option if the step type being invoked has not declared an option with that name. (This error does not apply for steps in the XProc namespace when the processor is operating in forwards-compatible mode.)
It is a static error (err:XS0004) to include more than one p:with-option with the same option name as part of the same step invocation.
The actual value is specified with a select attribute. The select attribute must be specified. The value of the select attribute is an XPath expression which will be evaluated to provide the value of the variable.
The type of the value may be specified in the as attribute using an XPath Sequence Type[citation needed]. If an atomic type, or sequence of atomic types, is specified, the value provided for the option will be atomized according to the standard XPath rules. It is a dynamic error (err:XD1001) if the computed value does not match the specified sequence type.
All in-scope bindings for the step instance itself are present in the Processor XPath Context as variable bindings, so select expressions may refer to any option or variable bound in those in-scope bindings by variable reference. If a variable reference uses a QName that is not the name of an in-scope binding or preceding sibling option, an XPath evaluation error will occur.
If a select expression is used but no document connection is provided, the implicit connection is to the default readable port. If there is no default readable port, the connection is treated as if p:empty was specified.
It is a dynamic error (err:XD0008) if a sequence of more than one document appears on the connection for a p:with-option. If p:empty is given or implied as the document connection, the context item is undefined. It is a dynamic error (err:XD0026) if the select expression makes reference to the context node, size, or position when the context item is undefined.
Variable and option values carry with them not only their literal or computed string value but also a set of namespaces. To see why this is necessary, consider the following step:
The p:deleteXPS step will delete elements that match the expression “html:div”, but that expression can only be correctly interpreted if there's a namespace binding for the prefix “html” so that binding has to travel with the option.
The default namespace bindings associated with a variable or option value are computed as follows:
If the select attribute was used to specify the value and it consisted of a single VariableReference (per [XPath 2.0]), then the namespace bindings from the referenced option or variable are used.
If the select attribute was used to specify the value and it evaluated to a node-set, then the in-scope namespaces from the first node in the selected node-set (or, if it's not an element, its parent) are used.
Otherwise, the in-scope namespaces from the element providing the value are used. (For options specified using syntactic shortcuts, the step element itself is providing the value.)
The default namespace is never included in the namespace bindings for a variable or option value. Unqualified names are always in no-namespace.
Unfortunately, in more complex situations, there may be no single variable or option that can reliably be expected to have the correct set of namespace bindings. Consider this pipeline:
In this case, the match option passed to the p:deleteXPS step needs both the namespace binding of “h” specified in the ex:delete-in-div pipeline definition and the namespace binding of “html” specified in the divchild option on the call of that pipeline. It's not sufficient to provide just one of the sets of bindings.
The namespace bindings specified by a p:namespaces element are determined as follows:
If the binding attribute is specified, it must contain the name of a single in-scope binding. The namespace bindings associated with that binding are used. It is a static error (err:XS0020) if the binding attribute on p:namespaces is specified and its value is not the name of an in-scope binding.
If the element attribute is specified, it must contain an XPath expression which identifies a single element node (the input connection for this expression is the same as the connection for the p:option which contains it). The in-scope namespaces of that node are used.
If neither binding nor element is specified, the in-scope namespaces on the p:namespaces element itself are used.
Irrespective of how the set of namespaces are determined, the except-prefixes attribute can be used to exclude one or more namespaces. The value of the except-prefixes attribute must be a sequence of tokens, each of which must be a prefix bound to a namespace in the in-scope namespaces of the p:namespaces element. All bindings of prefixes to each of the namespaces thus identified are excluded. It is a static error (err:XS0051) if the except-prefixes attribute on p:namespaces does not contain a list of tokens or if any of those tokens is not a prefix bound to a namespace in the in-scope namespaces of the p:namespaces element.
If a p:variable, p:with-option includes one or more p:namespaces elements, then the union of all the namespaces specified on those elements are used as the bindings for the variable or option value. In this case, the in-scope namespaces on the p:variable and p:with-option are ignored. It is a dynamic error (err:XD0013) if the specified namespace bindings are inconsistent; that is, if the same prefix is bound to two different namespace names.
For example, this would allow the preceding example to work:
The p:namespaces element provides namespace bindings for both of the prefixes necessary to correctly interpret the expression ultimately passed to the p:deleteXPS step (the binding for html: is explicitly provided and the binding for h: is in-scope).
Note
The use of p:namespaces here, when all of the bindings are provided with explicit namespace declarations, is unnecessary. The bindings could simply be placed on the parent p:with-option element. We use p:namespaces here only to make the example parallel to the one which follows.
The preceding solution has the weakness that it depends on knowing the bindings that will be used by the caller. A more flexible solution would use the binding attribute to copy the bindings from the caller's option value.
This example will succeed as long as the caller-specified option does not bind the “h” prefix to something other than the XHTML namespace.
5.6 p:declare-step
A p:declare-step provides the type and signature of an atomic step or pipeline. It declares the inputs, outputs, and options for all steps of that type.
The value of the type can be from any namespace provided that the expanded-QName of the value has a non-null namespace URI. It is a static error (err:XS0025) if the expanded-QName value of the type attribute is in no namespace or in the XProc namespace. Except as described in Section 2.14, “Versioning Considerations”, the XProc namespace must not be used in the type of steps. Neither users nor implementers may define additional steps in the XProc namespace.
Irrespective of the context in which the p:declare-step occurs, there are initially no option or variable names in-scope inside a p:declare-step. That is, p:option and p:variable elements can refer to values declared by their preceding siblings, but not by any of their ancestors.
When a declared step is evaluated directly by the XProc processor (as opposed to occurring as an atomic step in some container), how the input and output ports are connected to documents is implementation-defined.
A step declaration is not a step in its own right. Sibling steps cannot refer to the inputs or outputs of a p:declare-step using p:pipe; only instances of the type can be referenced.
The version attribute identifies the version of XProc for which this step declaration was authored. If the p:declare-step has no ancestors in the XProc namespace, then it must have a version attribute. See Section 2.14, “Versioning Considerations”.
When declaring an atomic step, the subpipeline in the declaration must be empty. And, conversely, if the subpipeline in a declaration is empty, the declaration must be for an atomic step.
Implementations may use extension attributes to provide implementation-dependent information about a declared step. For example, such an attribute might identify the code which implements steps of this type.
It is not an error for a pipeline to include declarations for steps that a particular processor does not know how to implement. It is, of course, an error to attempt to evaluate such steps.
If p:log or p:serialization elements appear in the declaration of an atomic step, they will only be used if the atomic step is directly evaluated by the processor. They have no effect if the step appears in a subpipeline; only the serialization options of the “top level” step or pipeline are used because that is the only step which the processor is required to serialize.
5.6.2 Declaring pipelines
When a p:declare-step declares a pipeline, that pipeline encapsulates the behavior of the specified subpipeline. Its children declare inputs, outputs, and options that the pipeline exposes and identify the steps in its subpipeline.
The subpipeline may include declarations of additional steps (e.g., other pipelines or other step types that are provided by a particular implementation or in some implementation-defined way) and import other pipelines. If a pipeline has been imported, it may be invoked as a step within the subpipeline that imported it.
The requested xpath-versionmust be used to evaluate XPath expressions subject to the constraints outlined in Section 2.7, “XPaths in XProc”.
The psvi-required attribute allows the author to declare that a step relies on the processor's ability to pass PSVI annotations between steps, see Section 2.9, “PSVIs in XProc”. If the attribute is not specified, the value “false” is assumed.
5.7 p:library
A p:library is a collection of step declarations and/or pipeline definitions.
The version attribute identifies the version of XProc for which this library was authored. If the p:library has no ancestors in the XProc namespace, then it must have a version attribute. See Section 2.14, “Versioning Considerations”.
The steps declared in a pipeline library are referred to by their type. It is not an error to put a p:pipeline or p:declare-step without a type in a p:library, but there is no standard mechanism for instantiating it or referring to it. It is effectively invisible.
Attempts to retrieve the library identified by the URI value may be redirected at the parser level (for example, in an entity resolver) or below (at the protocol level, for example, via an HTTP Location: header). In the absence of additional information outside the scope of this specification within the resource, the base URI of the library is always the URI of the actual resource returned. In other words, it is the URI of the resource retrieved after all redirection has occurred.
As imports are processed, a processor may encounter new p:import elements whose library URI is the same as one it has already processed in some other context. This may happen as a consequence of resolving the URI. If the actual base URI is the same as one that has already been processed, the implementation must recognize it as the same library and should not need to process the resource. Also, a duplicate, circular chain of imports, or a re-entrant import is not an error and implementations must take the necessary steps to avoid infinite loops and/or incorrect notification of duplicate step definitions. It is not an error for a library to import itself. An example of such steps is listed in Appendix G, Handling Circular and Re-entrant Library Imports (Non-Normative).
A library is considered the same library if the URI of the resource retrieved is the same. If a pipeline or library author uses two different URI values that resolve to the same resource, they must not be considered the same imported library.
5.9 p:pipe
A p:pipe connects an input to a port on another step.
<p:pipe
step? = NCName
port? = NCName />
The p:pipe element connects to a readable port of another step. It identifies the readable port to which it connects with the name of the step in the step attribute and the name of the port on that step in the port attribute.
If the step attribute is not specified, it defaults to the step which provides the default readable port. If the port attribute is not specified, it defaults to the primary output port of the step identified (explicitly or implicitly).
It is a static error (err:XS1004) if the step attribute is not specified, and there is no default readable port. It is a static error (err:XS1005) if the port attribute is not specified, and the step identified has no primary output port.
A p:pipe that is a connection for an p:output of a compound step may connect to one of the readable ports of the compound step or to an output port on one of the compound step's contained steps. In other words, the output of a compound step can simply be a copy of one of the available inputs or it can be the output of one of its children.
5.10 p:inline
A p:inline provides a document inline.
<p:inline
exclude-inline-prefixes? = ExcludeInlinePrefixes
expand-text? = boolean
content-type? = ContentType
encoding? = string>
anyElement
</p:inline>
How the content of a p:inline element is interpreted depends on the content-type and encoding attributes. If no content-type is provided, the value “application/xml” is assumed.
It is a static error (err:XS1008) if the content type value specifies a character set and the encoding attribute is absent.
It is a static error (err:XS1009) if an encoding is specified and the content of the p:inline contains any XML markup. In other words, the entire content must be a single text node. CDATA sections and character references do not count as markup for this purpose because they will already have been replaced by the XML parser that read the pipeline.
If the encoding attribute is present, the content must be decoded. The encoding value “base64” must be supported and identifies the content as being base64-encoded. An implementation may support encodings other than base64, but these encodings and their names are implementation-defined.It is a static error (err:XS1006) if the encoding specified is not supported by the implementation. It is a dynamic error (err:XD1005) if the body is not correctly encoded per the value of the encoding attribute.
If an encoding (1) is specified, the content of the inline is decoded (2) to produce an sequence of bytes (3).
The resulting byte sequence (4) is interpreted according to the charset parameter on the content type. If no charset parameter is provided, the bytes are interpreted using the UTF-8 charset.
If no encoding is specified, the content must be XML.
Several errors can occur:
The encoding value “base64” must be supported and identifies the content as being base64-encoded. An implementation may support encodings other than base64, but these encodings and their names are implementation-defined.It is a static error (err:XS1006) if the encoding specified is not supported by the implementation.
It is a static error (err:XS1009) if an encoding is specified and the content of the p:inline contains any XML markup. In other words, the entire content must be a single text node. CDATA sections and character references do not count as markup for this purpose because they will already have been replaced by the XML parser that read the pipeline.
It is a dynamic error (err:XD1005) if the body is not correctly encoded per the value of the encoding attribute.
It is a static error (err:XS1008) if the content type value specifies a character set and the encoding attribute is absent.
The interpretation of the (possibily decoded) content depends on the content-type attribute.
5.10.1 Inline non-XML content
If the content-type is not an XML media type, then the content is non-XML.
If the content-type is not specified or specifies an XML media type, then the content is XML. It is a static error (err:XS0024) if the content of the p:inline element does not consist of exactly one element, optionally preceded and/or followed by any number of processing instructions, comments or whitespace characters.
The in-scope namespaces of the inline document differ from the in-scope namespace of the content of the p:inline element in that bindings for all its excluded namespaces, as defined below, are removed:
The XProc namespace itself (http://www.w3.org/ns/xproc) is excluded.
A namespace URI designated by using an exclude-inline-prefixes attribute on the enclosing p:inline is excluded.
A namespace URI designated by using an exclude-inline-prefixes attribute on any ancestor p:declare-step, p:pipeline, or p:library is also excluded. (In other words, the effect of several exclude-inline-prefixes attributes among the ancestors of p:inline is cumulative.)
The value of each prefix in the exclude-inline-prefixes attribute is interpreted as follows:
The value of the attribute is either #all, or a whitespace-separated list of tokens, each of which is either a namespace prefix or #default. The namespace bound to each of the prefixes is designated as an excluded namespace. It is a static error (err:XS0057) if the exclude-inline-prefixes attribute does not contain a list of tokens or if any of those tokens (except #all or #default) is not a prefix bound to a namespace in the in-scope namespaces of the element on which it occurs.
The default namespace of the element on which exclude-inline-prefixes occurs may be designated as an excluded namespace by including #default in the list of namespace prefixes. It is a static error (err:XS0058) if the value #default is used within the exclude-inline-prefixes attribute and there is no default namespace in scope.
The value #all indicates that all namespaces that are in scope for the element on which exclude-inline-prefixes occurs are designated as excluded namespaces.
The XProc processor must include all in-scope prefixes that are not explicitly excluded. If the namespace associated with an excluded prefix is used in the expanded-QName of a descendant element or attribute, the processor may include that prefix anyway, or it may generate a new prefix.
The declaration for “c” must be present because it was not excluded. The “part” element uses the namespace bound to “b”, so some binding must be present. In this example, the original prefix has been preserved, but it would be equally correct if a different prefix had been used.
If the expand-text attribute is not false, then each text node within the p:inline is evaluated as a text value template. The context node for these expressions is undefined.
5.10.3 Implicit inlines
As an authoring convenience, if a single element node, optionally preceded and/or followed by whitespace, in any namespace other than the XProc namespace, occurs where a p:inline is allowed, it is treated as if it was enclosed within a p:inline element (with no attributes). Any preceding or following whitespace is discarded.
The following example demonstrates this implicit behaviour:
An explicit p:inline is required if the author wants to include top level comments, processing instructions, or whitespace, or if the document element is in the XProc namespace.
5.11 p:document
A p:document reads a document from a URI.
<p:document
href = anyURI
override-content-type? = ContentType />
The value of the href attribute must be an anyURI. It is interpreted as an IRI reference. If it is relative, it is made absolute against the base URI of the p:document element.
The semantics of p:document are the same as a the semantics of p:loadXPS where the href option comes from the href attribute, the override-content-type option comes from the override-content-type attribute, and the dtd-validate option is always false.
It is a dynamic error (err:XD0011) if the resource referenced by a p:document element does not exist, cannot be accessed, or has an XML content type and is not a well-formed XML document.
Use the p:loadXPS step if you need to perform DTD-based validation.
Note
A p:document always reads from the specified IRI. In the context of a p:input, this seems perfectly natural. In the context of a p:output, this may seem a little asymmetrical. Putting a p:document in a p:output causes the pipeline to read from the specified IRI and provide that document as an output on that port.
A p:documentation contains human-readable documentation.
<p:documentation>
any-well-formed-content*
</p:documentation>
There are no constraints on the content of the p:documentation element. Documentation is ignored by pipeline processors. See Section 3.6, “Documentation”.
5.14 p:pipeinfo
A p:pipeinfo contains ancillary information for steps in the pipeline.
Errors in a pipeline can be divided into two classes: static errors and dynamic errors.
6.1 Static Errors
[Definition: A static error is one which can be detected before pipeline evaluation is even attempted.] Examples of static errors include cycles and incorrect specification of inputs and outputs.
Static errors are fatal and must be detected before any steps are evaluated.
A [Definition: A dynamic error is one which occurs while a pipeline is being evaluated.] Examples of dynamic errors include references to URIs that cannot be resolved, steps which fail, and pipelines that exhaust the capacity of an implementation (such as memory or disk space).
If a step fails due to a dynamic error, failure propagates upwards until either a p:try is encountered or the entire pipeline fails. In other words, outside of a p:try, step failure causes the entire pipeline to fail.
[Definition: An implementation-dependent feature is one where the implementation has discretion in how it is performed. Implementations are not required to document or explain how implementation-dependent features are performed.]
[Definition: An implementation-defined feature is one where the implementation has discretion in how it is performed. Conformant implementations must document how implementation-defined features are performed.]
1 Implementation-defined features
The following features are implementation-defined:
It is implementation-defined what additional step types, if any, are provided. See Section 2.1, “Steps”.
The level of support for typed values in XDM instances in an XProc pipeline is implementation-defined. See Section 2.2.1.1, “XML Documents”.
How pipeline outputs are connected to documents outside the pipeline is implementation-defined. See Section 2.3, “Inputs and Outputs”.
In Version 2.0 of XProc, how (or if) implementers provide local resolution mechanisms and how (or if) they provide access to intermediate results by URI is implementation-defined. See Section 2.3.1, “External Documents”.
Except for cases which are specifically called out in , the extent to which namespace fixup, and other checks for outputs which cannot be serialized, are performed on intermediate outputs is implementation-defined. See Section 2.5.1, “Namespace Fixup on XML Outputs”.
The version of Unicode supported is implementation-defined, but it is recommended that the most recent version of Unicode be used. See Section 2.7.1, “Processor XPath Context”.
The result of evaluating an expression when the context node has a non-XML content type is implementation-defined. See Section 2.7.1, “Processor XPath Context”.
The exact format of the language string is implementation-defined but should be consistent with the xml:lang attribute. See Section 2.8.1, “System Properties”.
Whether or not the pipeline processor supports passing PSVI annotations between steps is implementation-defined. See Section 2.9, “PSVIs in XProc”.
The exact PSVI properties that are preserved when documents are passed between steps is implementation-defined. See Section 2.9, “PSVIs in XProc”.
It is implementation-defined what PSVI properties, if any, are produced by extension steps. See Section 2.9, “PSVIs in XProc”.
How outside values are specified for pipeline options on the pipeline initially invoked by the processor is implementation-defined. See Section 2.12, “Options”.
Support for pipeline documents written in XML 1.1 and pipeline inputs and outputs that use XML 1.1 is implementation-defined. See Section 3, “Syntax Overview”.
The presence of other compound steps is implementation-defined; XProc provides no standard mechanism for defining them or describing what they can contain. See Section 4.8, “Extension Steps”.
If the href attribute is not specified, the location of the log file or files is implementation-defined. See Section 5.3, “p:log”.
How each document or sequence of documents is represented in a p:log is implementation-defined. See Section 5.3, “p:log”.
The default value of any serialization options not specified on a particular p:serialization element is implementation-defined. See Section 5.4, “p:serialization”.
When a declared step is evaluated directly by the XProc processor (as opposed to occurring as an atomic step in some container), how the input and output ports are connected to documents is implementation-defined. See Section 5.6, “p:declare-step”.
The subpipeline may include declarations of additional steps (e.g., other pipelines or other step types that are provided by a particular implementation or in some implementation-defined way) and import other pipelines. See Section 5.6.2, “Declaring pipelines”.
An implementation may support encodings other than base64, but these encodings and their names are implementation-defined. See Section 5.10, “p:inline”.
It is implementation-defined whether additional information items and properties, particularly those made available in the PSVI, are preserved between steps. See Section 3, “Infoset Conformance”.
2 Implementation-dependent features
The following features are implementation-dependent:
The evaluation order of steps not connected to one another is implementation-dependent See Section 2, “Pipeline Concepts”.
Resolving a URI locally may involve resolvers of various sorts and possibly appeal to implementation-dependent mechanisms such as catalog files. See Section 2.3.1, “External Documents”.
Whether (and when and how) or not the intermediate results that pass between steps are ever written to a filesystem is implementation-dependent. See Section 2.3.1, “External Documents”.
The set of available documents (those that may be retrieved with a URI) is implementation-dependent. See Section 2.7.2, “Step XPath Context”.
Which steps are forbidden, what privileges are needed to access resources, and under what circumstances these security constraints apply is implementation-dependent. See Section 2.13, “Security Considerations”.
The ability of a step to read the p:log output of some former step is implementation-dependent. See Section 5.3, “p:log”.
Implementations may use extension attributes to provide implementation-dependent information about a declared step. See Section 5.6.1, “Declaring atomic steps”.
This specification conforms to the XML Information Set [Infoset]. The information corresponding to the following information items and properties must be available to the processor for the documents that flow through the pipeline.
The Document Information Item with [base URI] and [children] properties.
Element Information Items with [base URI], [children], [attributes], [in-scope namespaces], [prefix], [local name], [namespace name], [parent] properties.
Attribute Information Items with [namespace name], [prefix], [local name], [normalized value], [attribute type], and [owner element] properties.
Character Information Items with [character code], [parent], and, optionally, [element content whitespace] properties.
Processing Instruction Information Items with [base URI], [target], [content] and [parent] properties.
Comment Information Items with [content] and [parent] properties.
Namespace Information Items with [prefix] and [namespace name] properties.
It is implementation-defined whether additional information items and properties, particularly those made available in the PSVI, are preserved between steps.
B References
1 Normative References
[XProc V2.0 Requirements] XProc V2.0 Requirements. Alex Milowski, James Fuller, and Norman Walsh editors. W3C Working Draft 5 November 2013.
[XProc 2.0: Standard Step Library] XProc 2.0: Standard Step Library. Alex Milowski, Henry Thompson, and Norman Walsh editors. W3C Working Draft 15 December 2014.
[XPath 2.0] XML Path Language (XPath) 2.0. Anders Berglund, Scott Boag, Don Chamberlin, et. al., editors. W3C Recommendation. 23 January 2007.
[XQuery 1.0 and XPath 2.0 Data Model (XDM)] XQuery 1.0 and XPath 2.0 Data Model (XDM). Mary Fernández, Ashok Malhotra, Jonathan Marsh, et. al., editors. W3C Recommendation. 23 January 2007.
[W3C XML Schema: Part 1] XML Schema Part 1: Structures Second Edition. Henry S. Thompson, David Beech, Murray Maloney, et. al., editors. World Wide Web Consortium, 28 October 2004.
[xml:id] xml:id Version 1.0. Jonathan Marsh, Daniel Veillard, and Norman Walsh, editors. W3C Recommendation. 9 September 2005.
[XML Base] XML Base (Second Edition). Jonathan Marsh and Richard Tobin, editors. W3C Recommendation. 28 January 2009.
[Serialization] XSLT 2.0 and XQuery 1.0 Serialization. Scott Boag, Michael Kay, Joanne Tong, Norman Walsh, and Henry Zongaro, editors. W3C Recommendation. 23 January 2007.
In an attribute that is designated as an attribute value template, an expression can be used by surrounding the expression with curly brackets ({}), following the general rules for value templates
The bag-merger of two or more bags (where a bag is an unordered list or, equivalently, something like a set except that it may contain duplicates) is a bag constructed by starting with an empty bag and adding each member of each of the input bags in turn to it. It follows that the cardinality of the result is the sum of the cardinality of all the input bags.
The steps that occur directly within, or within non-step wrappers directly within, a step are called that step's contained steps. In other words, “container” and “contained steps” are inverse relationships.
An element from the XProc namespace may have any attribute not from the XProc namespace, provided that the expanded-QName of the attribute has a non-null namespace URI. Such an attribute is called an extension attribute.
An implementation-defined feature is one where the implementation has discretion in how it is performed. Conformant implementations must document how implementation-defined features are performed.
An implementation-dependent feature is one where the implementation has discretion in how it is performed. Implementations are not required to document or explain how implementation-dependent features are performed.
A step matches its signature if and only if it specifies an input for each declared input, it specifies no inputs that are not declared, it specifies an option for each option that is declared to be required, and it specifies no options that are not declared.
To produce a serializable XML document, the XProc processor must sometimes add additional namespace nodes, perhaps even renaming prefixes, to satisfy the constraints of Namespaces in XML. This process is referred to as namespace fixup.
If a step has a document input port which is explicitly marked “primary='true'”, or if it has exactly one document input port and that port is not explicitly marked “primary='false'”, then that input port is the primary input port of the step.
If a step has a document output port which is explicitly marked “primary='true'”, or if it has exactly one document output port and that port is not explicitly marked “primary='false'”, then that output port is the primary output port of the step.
The options on a step which have specified values, either because a p:with-option element specifies a value or because the declaration included a default value, are its specified options.
The step type exports of an XProc element, against the background of a set of URIs of resources already visited (call this set Visited), are defined by cases.
In a text node that is designated as a text value template, expressions can be used by surrounding each expression with curly brackets ({}), following the general rules for value templates.
If two names are in the same scope, we say that they are visible to each other.
D Pipeline Language Summary
This appendix summarizes the XProc pipeline language. Machine readable descriptions of this language are available in RELAX NG (and the RELAX NG compact syntax), W3C XML Schema, and DTD syntaxes.
It is a static error if there are any loops in the connections between steps: no step can be connected to itself nor can there be any sequence of connections through other steps that leads back to itself.
In all cases except the p:output of a compound step, it is a static error if the port identified by a p:pipe is not in the readable ports of the step that contains the p:pipe.
It is a static error if the content of the p:inline element does not consist of exactly one element, optionally preceded and/or followed by any number of processing instructions, comments or whitespace characters.
It is a static error if the port specified on the p:log is not the name of an output port on the step in which it appears or if more than one p:log element is applied to the same port.
All the step types in a pipeline or library must have unique names: it is a static error if any step type name is built-in and/or declared or defined more than once in the same scope.
It is a static error if the port specified on the p:serialization is not the name of an output port on the pipeline in which it appears or if more than one p:serialization element is applied to the same port.
It is a static error if any element in the XProc namespace or any step has element children other than those specified for it by this specification. In particular, the presence of atomic steps for which there is no visible declaration may raise this error.
It is a static error if the except-prefixes attribute on p:namespaces does not contain a list of tokens or if any of those tokens is not a prefix bound to a namespace in the in-scope namespaces of the p:namespaces element.
It is a static error if the URI of a p:import cannot be retrieved or if, once retrieved, it does not point to a p:library, p:declare-step, or p:pipeline.
It is a static error if the exclude-inline-prefixes attribute does not contain a list of tokens or if any of those tokens (except #all or #default) is not a prefix bound to a namespace in the in-scope namespaces of the element on which it occurs.
It is a static error if the processor encounters an explicit request for a previous version of the language and it is unable to process the pipeline using those semantics.
It is a static error if an expression does not have a closing right curly bracket or if an unescaped right curly bracket occurs outside of an expression.
It is a static error if the step attribute is not specified, and there is no default readable port. It is a static error if the port attribute is not specified, and the step identified has no primary output port.
If sequence is not specified on p:output, or has the value false, then it is a dynamic error if the step does not produce exactly one document on the declared port.
It is a dynamic error if the resource referenced by a p:document element does not exist, cannot be accessed, or has an XML content type and is not a well-formed XML document.
It is a dynamic error if the select expression on a p:input returns atomic values or anything other than element or document nodes (or an empty sequence).
It is a dynamic error if the namespace attribute is specified on c:param, the name contains a colon, and the specified namespace is not the same as the in-scope namespace binding for the specified prefix.
On steps which allow independent specification of a namespace and a name, it is a dynamic error to specify a new namespace or prefix if the lexical value of the specified name contains a colon.
If an input port provides a set of acceptable content types, it is a dynamic error if an input document that arrives on the port has a content type that does not match any content type in that set.
An XProc processor may find it necessary to add missing namespace declarations to ensure that a document can be serialized. While this process is implementation defined, the purpose of this appendix is to provide guidance as to what an implementation might do to either prevent such situations or fix them as before serialization.
When a namespace binding is generated, the prefix associated with the QName of the element or attribute in question should be used. From an Infoset perspective, this is accomplished by setting the [prefix] on the element or attribute. Then when an implementation needs to add a namespace binding, it can reuse that prefix if possible. If reusing the prefix is not possible, the implementation must generate a new prefix that is unique to the in-scope namespace of the element or owner element of the attribute.
An implementation can avoid namespace fixup by making sure that the standard step library does not output documents that require fixup. The following list contains suggestions as to how to accomplish this within the steps:
Any step that outputs an element in the step vocabulary namespace http://www.w3.org/ns/xproc-step must ensure that namespace is declared. An implementation should generate a namespace binding using the prefix “c”.
When attributes are added by p:add-attributeXPS or p:set-attributesXPS, the step must ensure the namespace of the attributes added are declared. If the prefix used by the QName is not in the in-scope namespaces of the element on which the attribute was added, the step must add a namespace declaration of the prefix to the in-scope namespaces. If the prefix is amongst the in-scope namespace and is not bound to the same namespace name, a new prefix and namespace binding must be added. When a new prefix is generated, the prefix associated with the attribute should be changed to reflect that generated prefix value.
When an element is renamed by p:renameXPS, the step must ensure the namespace of the element is declared. If the prefix used by the QName is not in the in-scope namespaces of the element being renamed, the step must add a namespace declaration of the prefix to the in-scope namespaces. If the prefix is amongst the in-scope namespace and is not bound to the same namespace name, a new prefix and namespace binding must be added. When a new prefix is generated, the prefix associated with the element should be changed to reflect that generated prefix value.
If the element does not have a namespace name and there is a default namespace, the default namespace must be undeclared. For each of the child elements, the original default namespace declaration must be preserved by adding a default namespace declaration unless the child element has a different default namespace.
When an attribute is renamed by p:renameXPS, the step must ensure the namespace of the renamed attribute is declared. If the prefix used by the QName is not in the in-scope namespaces of the element on which the attribute was added, the step must add a namespace declaration of the prefix to the in-scope namespaces. If the prefix is amongst the in-scope namespace and is not bound to the same namespace name, a new prefix and namespace binding must be added. When a new prefix is generated, the prefix associated with the attribute should be changed to reflect that generated prefix value.
When an element wraps content via p:wrapXPS, there may be in-scope namespaces coming from ancestor elements of the new wrapper element. The step must ensure the namespace of the element is declared properly. By default, the wrapper element will inherit the in-scope namespaces of the parent element if one exists. As such, there may be a existing namespace declaration or default namespace.
If the prefix used by the QName is not in the in-scope namespaces of the wrapper element, the step must add a namespace declaration of the prefix to the in-scope namespaces. If the prefix is amongst the in-scope namespace and is not bound to the same namespace name, a new prefix and namespace binding must be added. When a new prefix is generated, the prefix associated with the wrapper element should be changed to reflect that generated prefix value.
If the element does not have a namespace name and there is a default namespace, the default namespace must be undeclared. For each of the child elements, the original default namespace declaration must be preserved by adding a default namespace declaration unless the child element has a different default namespace.
When the wrapper element is added for p:wrap-sequenceXPS or p:packXPS, the prefix used by the QName must be added to the in-scope namespaces.
When a element is removed via p:unwrapXPS, an in-scope namespaces that are declared on the element must be copied to any child element except when the child element declares the same prefix or declares a new default namespace.
In the output from p:xsltXPS, if an element was generated from the xsl:element or an attribute from xsl:attribute, the step must guarantee that an namespace declaration exists for the namespace name used. Depending on the XSLT implementation, the namespace declaration for the namespace name of the element or attribute may not be declared. It may also be the case that the original prefix is available. If the original prefix is available, the step should attempt to re-use that prefix. Otherwise, it must generate a prefix for a namespace binding and change the prefix associated the element or attribute.
G Handling Circular and Re-entrant Library Imports (Non-Normative)
When handling imports, an implementation needs to be able to detect the following situations, and distinguish them from cases where multiple import chains produce genuinely conflicting step definitions:
Circular imports: A imports B, B imports A.
Re-entrant imports: A imports B and C, B imports D, C imports D.
One way to achieve this is as follows:
[Definition: The step type exports of an XProc element, against the background of a set of URIs of resources already visited (call this set Visited), are defined by cases.]
Let RU be the actual resolved URI of the resource identified by the href of the element. If RU is a member of Visited, then an empty bag, otherwise update Visited by adding RU to it, and return the step type exports of the document element of the retrieved representation
all other elements
An empty bag
The changes to Visited mandated by the p:import case above are persistent, not scoped. That is, not only the recursive processing of the imported resource but also subsequent processing of siblings and ancestors must be against the background of the updated value. In practice this means either using a side-effected global variable, or not only passing Visited as an argument to any recursive or iterative processing, but also returning its updated value for subsequent use, along with the bag of step types.
Given a pipeline library document with actual resolved URI DU, it is a static error (err:XS0036) if the step type exports of the document element of the retrieved representation, against the background of a singleton set containing DU as the initial Visited set, contains any duplicates.
Given a top-level pipeline document with actual resolved URI DU, it is a static error (err:XS0036) if the bag-merger of the step type exports of the document element of the retrieved representation with the step type exports of its children, against the background of a singleton set containing DU as the initial Visited set, contains any duplicates.
Given a non-top-level p:pipeline or p:declare-step element, it is a static error (err:XS0036) if the bag-merger of the step type exports of its parent with the step type exports of its children, against the background of a copy of the Visited set of its parent as the initial Visited set, contains any duplicates.
The phrase "a copy of the Visited set" in the preceding paragraph is meant to indicate that checking of non-top-level p:pipeline or p:declare-step elements does not have a persistent impact on the checking of its parent. The contrast is that whereas changes to Visited pass both up and down through p:import, they pass only down through p:pipeline and p:declare-step.
[Definition: The bag-merger of two or more bags (where a bag is an unordered list or, equivalently, something like a set except that it may contain duplicates) is a bag constructed by starting with an empty bag and adding each member of each of the input bags in turn to it. It follows that the cardinality of the result is the sum of the cardinality of all the input bags.]
H Sequential steps, parallelism, and side-effects
XProc imposes as few constraints on the order in which steps must be evaluated as possible and almost no constraints on parallel execution.
In the simple, and we believe overwhelmingly common case, inputs flow into the pipeline, through the pipeline from one step to the next, and results are produced at the end. The order of the steps is constrained by the input/output connections between them. Implementations are free to execute them in a purely sequential fashion or in parallel, as they see fit. The results are the same in either case.
This is not true for pipelines which rely on side effects, such as the state of the filesystem or the state of the web. Consider the following pipeline:
There's no guarantee that “style” step will execute after the “save-xslt” step. In this case, the solution is straightforward. Even if you need the saved stylesheet, you don't need to rely on it in your pipeline:
Now the result is independent of the implementation strategy.
Implementations are free to invent additional control structures using p:pipeinfo and extension attributes to provide greater control over parallelism in their implementations.
I The application/xproc+xml media type
This appendix registers a new MIME media type, “application/xproc+xml”.
1 Registration of MIME media type application/xproc+xml
MIME media type name:
application
MIME subtype name:
xproc+xml
Required parameters:
None.
Optional parameters:
charset
This parameter has identical semantics to the charset parameter of the application/xml media type as specified in [RFC 3023] or its successors.
Encoding considerations:
By virtue of XProc content being XML, it has the same considerations when sent as “application/xproc+xml” as does XML. See [RFC 3023], Section 3.2.
Security considerations:
Several XProc elements may refer to arbitrary URIs. In this case, the security issues of [RFC 2396], section 7, should be considered.
In addition, because of the extensibility features of XProc, it is possible that “application/xproc+xml” may describe content that has security implications beyond those described here. However, only in the case where the processor recognizes and processes the additional content, or where further processing of that content is dispatched to other processors, would security issues potentially arise. And in that case, they would fall outside the domain of this registration document.
Interoperability considerations:
This specification describes processing semantics that dictate behavior that must be followed when dealing with, among other things, unrecognized elements.
Because XProc is extensible, conformant "application/xproc+xml" processors can expect that content received is well-formed XML, but it cannot be guaranteed that the content is valid XProc or that the processor will recognize all of the elements and attributes in the document.
Published specification:
This media type registration is for XProc documents as described by this specification which is located at http://www.w3.org/TR/xproc/.
Applications which use this media type:
There is no experimental, vendor specific, or personal tree predecessor to “application/xproc+xml”, reflecting the fact that no applications currently recognize it. This new type is being registered in order to allow for the deployment of XProc on the World Wide Web, as a first class XML application.
Additional information:
Magic number(s):
There is no single initial octet sequence that is always present in XProc documents.
File extension(s):
XProc documents are most often identified with the extension “.xpl”.
Macintosh File Type Code(s):
TEXT
Person & email address to contact for further information:
The XProc specification is a work product of the World Wide Web Consortium's XML Processing Model Working Group. The W3C has change control over these specifications.
2 Fragment Identifiers
For documents labeled as “application/xproc+xml”, the fragment identifier notation is exactly that for “application/xml”, as specified in [RFC 3023] or its successors.
J Change Log
This appendix summarizes significant changes in this draft.
This First Public Working Draft contains a number of significant changes to the XProc pipeline language. Future drafts will attempt to address the remaining issues.
In this draft:
An attempt has been made to simplify how parameters are handled. There are no more parameter input ports. Instead, parameters are handled with a map and an otherwise ordinary option named “parameters”. (See issue #28)
Documents have accessible, arbitrary metadata (key/value pairs). (See issue #46)
Non-XML documents can flow between steps. (See issue #29)
