XProc: An XML Pipeline Language

W3C Working Draft 6 July 2007

This Version:: http://www.w3.org/TR/2007/WD-xproc-20070706/
Latest Version:: http://www.w3.org/TR/xproc/
Previous versions:: http://www.w3.org/TR/2007/WD-xproc-20070405/ http://www.w3.org/TR/2006/WD-xproc-20061117/ http://www.w3.org/TR/2006/WD-xproc-20060928/
Editors:: Norman Walsh, Sun Microsystems, Inc. <Norman.Walsh@Sun.COM>; Alex Milowski, Invited expert <alex@milowski.org>

This document is also available in these non-normative formats: XML, Revision markup

Abstract

This specification describes the syntax and semantics of XProc: An XML Pipeline Language, a language for describing operations to be performed on XML documents.

An XML Pipeline specifies a sequence of operations to be performed on one or more XML documents. Pipelines generally accept one or more XML documents as input and producing one or more XML documents as output, though they are not required to do so. Some pipelines are entirely self-contained, starting with input derived inside the pipeline and producing no XML output.

Status of this Document

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/.

This document was produced by the XML Processing Model Working Group which is part of the XML Activity. Publication as a 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.

This is a public Working Draft. This draft addresses many, but not all, of the design questions that were incomplete in previous drafts. The library of standard steps, both required and optional, is still being reviewed and considered. The Working Group continues to encourage feedback from potential users. A revision marks draft, with respect to the 5 April 2007 specification, has been provided, though it is not obviously of great value due to editorial reorganization of the material.

The most significant changes in this draft are: a new mechanism for dealing with parameters, new defaulting rules for primary input and output ports, and revisions to the standard step library.

Please send comments about this document to public-xml-processing-model-comments@w3.org (public archives are available).

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

1 Introduction

2 Pipeline Concepts

2.1 Steps

2.2 Inputs and Outputs

2.3 Primary Inputs and Outputs

2.4 Options

2.5 Parameters

2.6 Connections

2.7 Environment

2.8 XProc Extension Functions for XProc

2.8.1 System Properties
2.8.2 Step Available
2.8.3 Iteration Count

3 Syntax Overview

3.1 XProc Namespaces
3.2 Scoping of Names
3.3 Global Attributes
3.4 Associating Documents with Ports
3.5 Documentation
3.6 Extension attributes
3.7 Extension elements
3.8 Ignored namespaces
3.9 Syntax Summaries

4 Steps

4.1 p:pipeline

4.2 p:for-each

4.3 p:viewport

4.4 p:choose

4.5 p:group

4.6 p:try/p:catch

4.7 Other Steps

4.7.1 Syntactic Shortcut for Option Values

5 Other pipeline elements

5.1 p:input Element

5.1.1 Document Inputs
5.1.2 Parameter Inputs

5.2 p:iteration-source Element

5.3 p:viewport-source Element

5.4 p:xpath-context Element

5.5 p:output Element

5.6 p:log Element

5.7 Options and Parameters

5.7.1 p:option Element
5.7.2 p:parameter Element
5.7.3 Option and Parameter Namespace Bindings

5.8 p:declare-step Element

5.9 p:pipeline-library Element

5.10 p:import Element

5.11 p:pipe Element

5.12 p:inline Element

5.13 p:document Element

5.14 p:empty Element

5.15 p:documentation Element

6 Errors

6.1 Static Errors
6.2 Dynamic Errors

Appendices

A Standard Step Library

A.1 Required Steps

A.1.1 Count
A.1.2 Delete
A.1.3 Equal
A.1.4 Error
A.1.5 Escape Markup
A.1.6 Identity
A.1.7 Insert
A.1.8 Label Elements
A.1.9 Load
A.1.10 Namespace Rename
A.1.11 Parameters
A.1.12 Rename
A.1.13 Replace
A.1.14 Set Attributes
A.1.15 Sink
A.1.16 Split Sequence
A.1.17 String Replace
A.1.18 Store
A.1.19 Unescape Markup
A.1.20 Unwrap
A.1.21 Wrap
A.1.22 Wrap Sequence
A.1.23 XInclude
A.1.24 XSLT

A.2 Optional Steps

A.2.1 HTTP Request

A.2.1.1 c:http-request
A.2.1.2 c:header
A.2.1.3 c:multipart
A.2.1.4 c:body
A.2.1.5 c:http-response
A.2.1.6 HTTP Request Examples

A.2.2 Relax NG Validate

A.2.3 XML Schema Validate

A.2.4 XSLT 2.0

A.2.5 XSL Formatter

A.2.6 XQuery 1.0

B Conformance

C References

D Glossary

E Pipeline Language Summary

F The Error Vocabulary

F.1 err:errors
F.2 err:error
F.3 Error Example

1 Introduction

An XML Pipeline specifies a sequence of operations to be performed on a collection of XML input documents. Pipelines take zero or more XML documents as their input and produce zero or more XML documents as their output.

A pipeline consists of steps. Like pipelines, steps take zero or more XML documents as their input and produce zero or more XML documents as their output. The inputs to 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 two kinds of steps: atomic steps and compound steps. Atomic steps carry out single operations and have no substructure as far as the pipeline is concerned, whereas compound steps include a subpipeline of steps within themselves.

This specification defines a standard library, Appendix A, Standard Step Library, of steps. Pipeline implementations may support additional types of steps as well.

Figure 1, “A simple, linear XInclude/Validate pipeline” is a graphical representation of a simple pipeline that performs XInclude processing and validation on a document.

Figure 1. A simple, linear XInclude/Validate pipeline

This is a pipeline that consists of two atomic steps, XInclude and Validate. The pipeline itself has two inputs, “Document” and “Schema”. How these inputs are connected to XML documents outside the pipeline is implementation-defined. The XInclude step reads the pipeline input “Document” and produces a result document. The Validate step reads the pipeline input “Schema” and the output from the XInclude step and produces a result document. The result of the validation, “Result Document”, is the result of the pipeline. How pipeline outputs are connected to XML documents outside the pipeline is implementation-defined.

The pipeline document for this pipeline is shown in Example 1, “A simple, linear XInclude/Validate pipeline”.

Example 1. A simple, linear XInclude/Validate pipeline

<p:pipeline name="fig1" xmlns:p="http://www.w3.org/2007/03/xproc">
  <p:input port="source" primary="yes"/>
  <p:input port="schemaDoc" sequence="yes" primary="no"/>
  <p:output port="result"/>

  <p:xinclude/>

  <p:validate-xml-schema>
    <p:input port="schema">
      <p:pipe step="fig1" port="schemaDoc"/>
    </p:input>
  </p:validate-xml-schema>
</p:pipeline>

Figure 2, “A validate and transform pipeline” is a more complex example: it performs schema validation with an appropriate schema and then styles the validated document.

Figure 2. A validate and transform pipeline

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.

Example 2. A validate and transform pipeline

<p:pipeline xmlns:p="http://www.w3.org/2007/03/xproc">

  <p:documentation xmlns="http://www.w3.org/1999/xhtml">
    <div>
      <p>This is documentation</p>
    </div>
  </p:documentation>

  <p:choose>
    <p:when test="/*[@version &lt; 2.0]">
      <p:validate-xml-schema name="val1">
        <p:input port="schema">
          <p:document href="v1schema.xsd"/>
        </p:input>
      </p:validate-xml-schema>
    </p:when>

    <p:otherwise>
      <p:validate-xml-schema name="val2">
        <p:input port="schema">
          <p:document href="v2schema.xsd"/>
        </p:input>
      </p:validate-xml-schema>
    </p:otherwise>
  </p:choose>

  <p:xslt name="xform">
    <p:input port="stylesheet">
      <p:document href="stylesheet.xsl"/>
    </p:input>
  </p:xslt>
</p:pipeline>

2 Pipeline Concepts

[Definition: A pipeline is a set of connected steps, outputs flowing into inputs, without any loops (no step can read its own output, directly or indirectly).] A pipeline is itself a step and must satisfy the constraints on steps.

The result of evaluating a pipeline is the result of evaluating the steps that it contains, in the order determined by the connections between them. A pipeline must behave as if it evaluated each step each time it occurs. Unless otherwise indicated, implementations must not assume that steps are functional (that is, that their outputs depend only on their explicit inputs, options, and parameters) or side-effect free.

2.1 Steps

[Definition: A step is the basic computational unit of a pipeline. Steps are either atomic or compound.] [Definition: An atomic step is a step that performs a unit of XML processing, such as XInclude or transformation, and has no internal subpipeline.] Atomic steps carry out fundamental XML operations and can perform arbitrary amounts of computation, but they are indivisible. An XSLT step, for example, performs XSLT processing; an XML Schema Validation 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 Appendix A, Standard Step Library, but implementations may provide others as well. 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 additional steps. 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.]

Every compound step contains one or more steps. [Definition: The steps that occur directly inside a compound step are called contained steps.] [Definition: A compound step which immediately contains another step is called its container.]

[Definition: The steps (and the connections between them) within a compound step form a subpipeline.] [Definition: The last step in a subpipeline is the last step in document order within its container. ]

A compound step can contain one or more subpipelines and it determines how and which of its subpipelines are evaluated.

Steps have “ports” into which inputs and outputs are connected or “bound”. Each step has a number of input ports and a number of output ports, all with unique names. 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.)

Steps have any number of options, all with unique names. A step can have zero options.

Steps may have access to any number of parameters, all with unique names. A step can have zero parameters.

2.2 Inputs and Outputs

Although some steps can read and write non-XML resources, what flows between steps through input ports and output ports are exclusively XML documents or sequences of XML documents. Each XML document (or document in a sequence) must conceptually be an [Infoset] with a Document Information Item at its root. The inputs and outputs can be implemented as sequences of characters, events, or object models, or any other representation the implementation chooses.

It is a dynamic error (err:XD0001) if a non-XML resource is produced on a step output or arrives on a step input.

An implementation may make it possible for a step to produce non-XML output (through channels other than a named output port)—for example, writing a PDF document to a URI—but that output cannot flow through the pipeline. Similarly, one can imagine a step that takes no pipeline inputs, reads a non-XML file from a URI, and produces an XML output. But the non-XML file cannot arrive on an input port to a step.

The common case is that each step has 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.

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 defines the input ports, output ports, and options of all steps of that type. For example, every p:xslt step has two inputs, named “source” and “stylesheet”, and one output named “result” and the same set of options.

The situation is slightly more complicated for compound steps because they don't have separate declarations; each instance of a compound step serves as its own declaration. Compound steps don't have declared inputs, but they do have declared outputs, and unlike atomic steps, on 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 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.

A compound step with two inputs and one output

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, connections are made to all of its inputs and outputs.

When a step is used, all of the declared inputs of the step must be connected. Each input may 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 the top-level p:pipeline step.

When an input accepts a sequence of documents, it may have one or more bindings to any of those locations.

All of the declared outputs of a step must be connected. Outputs may be connected to:

The input port of some other step.
One of the outputs declared on the top-level p:pipeline step.

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 must be connected as described above. From the perspective of the compound step itself, they are inputs into which something must be connected.

Within a compound step, the declared outputs of the step can be connected to:

The output port of some contained step.
A fixed, inline document or sequence of documents.
A document read from a URI.

Each input and output 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.

[Definition: The signature of a step is the set of inputs, outputs, and options that it is declared to accept.] Each atomic step (e.g. XSLT or XInclude) has a fixed signature, declared globally or built-in, which all its instances share, whereas each compound step has its own implicit signature.

[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 appear on a special “error output” port defined (only) in the catch clause of a try/catch. Outside of a try/catch, the disposition of error messages is implementation-dependent.

2.3 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 exactly one input port, or if one of its input ports is explicitly designated as the primary, 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 designated as not being the primary input port, or if a step has more than one input port and none is explicitly designated the primary, then the primary input port of that step is undefined.

[Definition: If a step has exactly one output port, or if one of its output ports is explicitly designated as the primary, 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 designated as not being the primary, or if a step has more than one output port and none is explicitly designated the primary, then the primary output port of that step is undefined.

The special significance of primary input and output ports is that they are connected automatically by the processor if no explicit binding 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 inputs and the first step in its subpipeline has an unbound primary input, then an implicit (and unnamed) primary input port will be added to the compound step. If a compound step has no declared outputs and the last step in its subpipeline has an unbound primary output, then an implicit (and also unnamed) primary output port will be added to the compound step.

The practical consequence of these rules is that straightforward, linear pipelines are much simpler to read, write, and understand. The following pipeline has a single input which is transformed by the XSLT step; the result of that XSLT step is the result of the pipeline:

<p:pipeline xmlns:p="http://www.w3.org/2007/03/xproc">
<p:xslt>
  <p:input port="stylesheet">
    <p:document href="docbook.xsl"/>
  </p:input>
</p:xslt>
</p:pipeline>

It is semantically equivalent to this pipeline:

<p:pipeline name="main" xmlns:p="http://www.w3.org/2007/03/xproc">
<p:input port="source"/>
<p:input port="result">
  <p:pipe step="transform" port="result"/>
</p:input>

<p:xslt name="transform">
  <p:input port="source">
    <p:pipe step="main" port="source"/>
  </p:input>
  <p:input port="stylesheet">
    <p:document href="docbook.xsl"/>
  </p:input>
</p:xslt>

</p:pipeline>

2.4 Options

Some steps accept options. Options are name/value pairs.

[Definition: An option is a name/value pair where the name is an expanded name and the value must be a string.] If a document, node, or other value is given, its [XPath 1.0] string value is computed and that string is used.

[Definition: The options declared on a step are its declared options.] All of the options specified on an atomic step must have been declared. 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:option element specifies a value or because the declaration included a default value, are its specified options.]

2.5 Parameters

Some steps accept parameters. Parameters are name/value pairs.

[Definition: A parameter is a name/value pair where the name is an expanded name and the value must be a string.] If a document, node, or other value is given, its [XPath 1.0] string value is computed and that string is used.

Unlike options, which have names known in advance to the pipeline, parameters are not declared and their names may be unknown to the pipeline author. Pipelines can dynamically construct sets of parameters. Steps can read dynamically constructed sets with parameter inputs.

2.6 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.

2.7 Environment

[Definition: The environment of a step is the static information available to each instance of a step in a pipeline.]

The environment consists of:

A set of readable ports. [Definition: The readable ports are the step name/output port name pairs that are visible to the step.] Inputs and outputs can only be connected to readable ports.
A set of in-scope options. [Definition: The in-scope options are the set of options that are visible to a step.] All of the in-scope options are available to the processor for computing option and parameter values. The actual options passed to a step are those that are declared for a step of its type and that have values either provided explicitly with p:option elements on the step or as defaults in the declaration of the step.
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.]

[Definition: The empty environment contains no readable ports, no in-scope options, and an undefined default readable port. ]

Unless otherwise specified, the environment of a contained step is its inherited environment. [Definition: The inherited environment of a contained step is an environment that is the same as the environment of its container with the standard modifications. ]

The standard modifications made to an inherited environment are:

All of the specified options of the container are added to the in-scope options. The value of any option in the environment with the same name as one of the options specified on the container is shadowed by the new value.

In other words, steps can access the most recently specified value of all of the options specified on any ancestor step.
The union of all the declared outputs of all of the step's sibling contained steps are added to the readable ports.

In other words, sibling steps can see each other's outputs in addition to the outputs visible to their container.
If there is a preceding sibling step element:
- If that preceding sibling has a primary output port, then that output port becomes the default readable port.
- Otherwise, the default readable port is undefined.
If there is not a preceding sibling step element, the default readable port is unchanged.

A step with no parent inherits the empty environment.

2.8 XProc Extension Functions for XProc

The XProc processor must support a few additional functions 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. Four aspects of the processor are exposed through the p:system-property function in the pipeline namespace:

Function: String p:system-property(String property)

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. 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.

The unique identifier must consist of ASCII alphanumeric characters and must start with an alphabetic character. Thus, the string is syntactically an XML name.

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 of XProc implemented by the processor; for processors implementing the version of XProc specified by this document, the number is “1.0”.

The value will always be a string in the lexical space of the decimal data type defined in [W3C XML Schema: Part 2]. This allows the value to be converted to a number for the purpose of magnitude comparisons.

2.8.2 Step Available

The p:step-available function reports whether or not a particular type of step is understood by the processor.

Function: Boolean p:step-available(String step-type)

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 Iteration Count

In the context of a p:for-each or a p:viewport, the p:iteration-count function reports the number of iterations that have occurred. In the context of other standard XProc compound steps, it returns 1.

Function: Integer p:iteration-count()

In the context of an extension compound step, the value is implementation-defined.

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.

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 and parameters 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
Parameters

3.1 XProc Namespaces

The XML syntax for XProc uses three namespaces:

http://www.w3.org/2007/03/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/2007/03/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-request and p:store, 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/2007/03/xproc-error: The namespace used for error reporting. When a step fails inside a p:try, it may produce error messages that can be inspected in the p:catch. The error namespace is used for those messages. The conventional prefix “err:” is used for this namespace.

3.2 Scoping of Names

The scope of the names of step types is the pipeline. Each pipeline processor has some number of built in step types and may declare (directly, or by reference to an external library) additional step types.

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 its ancestor's siblings are all in the same scope. All in-scope steps 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 names is essentially the same as the scope of step names, with the following caveat: whereas step names must be unique, option names may be repeated. An option specified on a step shadows any specification that may already be in-scope.

Parameter names are not scoped; they are distinct on each step.

3.3 Global Attributes

The following attributes may appear on any element in a pipeline:

The attribute xml:id with the semantics outlined in [xml:id].
The attribute xml:base with the semantics outlined in [XML Base].

3.4 Associating Documents with Ports

[Definition: A binding associates an input or output port with some data source.] A document or a sequence of documents can be bound to a port in four ways: by source, by URI, by providing it inline, or by making it explicitly empty. Each of these mechanisms is allowed on the p:input, p:output, p:xpath-context, p:iteration-source, and p:viewport-source elements.

Specified by URI

[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:identity step named “otherstep” comes from “http://example.com/input.xml”.

<p:identity name="otherstep">
  <p:input port="source">
    <p:document href="http://example.com/input.xml"/>
  </p:input>
</p:identity>

It is a dynamic error (err:XD0002) if the processor attempts to retrieve the URI specified on a p:document and fails. (For example, if the resource does not exist or is not accessible with the user's authentication credentials.)

Specified by source

[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 “document” input to the p:xinclude step named “expand” comes from the “result” port of the step named “otherstep”.

<p:xinclude name="expand">
  <p:input port="source">
    <p:pipe step="otherstep" port="result"/>
  </p:input>
</p:xinclude>

When a p:pipe is used, the specified port must be in the readable ports of the current environment. It is a static error (err:XS0003) if the port specified by a p:pipe is not in the readable ports of the environment.

Specified inline

[Definition: An inline document is specified directly in the body of the element that binds it.] 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.

<p:xslt name="xform">
  <p:input port="stylesheet">
    <p:inline>
      <xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                      version="1.0">
        ...
      </xsl:stylesheet>
    </p:inline>
  </p:input>
</p:xslt>

Inline documents are considered “quoted”, they are not interpolated or available to the pipeline processor in any way except as documents flowing through the pipeline.

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:

<p:xslt2 name="generate">
  <p:input port="source">
    <p:empty/>
  </p:input>
  <p:input port="stylesheet">
    <p:inline>
      <xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                      version="2.0">
        ...
      </xsl:stylesheet>
    </p:inline>
  </p:input>
  <p:option name="template-name" value="someName"/>
</p:xslt2>

If you omit the binding on a primary input port, a binding to the default readable port will be assumed. Making the binding explicitly empty guarantees that the binding 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 binding is provided, then the specified sequence of documents is made available on that port in the same order as the bindings.

3.5 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.)

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 extension elements.

3.6 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 any other conformant XProc processor would produce. They must not cause the processor to fail to signal an error that a conformant processor is required to signal. This means that an extension attribute must not change the effect of any XProc element except to the extent that the effect is implementation-defined or implementation-dependent.

A processor which encounters an extension attribute that it does not recognize must behave as if the attribute was not present.

3.7 Extension elements

The presence of an extension element must not cause the connections between steps to differ from the connections that any other conformant XProc processor would produce. They must not cause the processor to fail to signal an error that a conformant processor is required to signal. This means that an extension element must not change the effect of any XProc element except to the extent that the effect is implementation-defined or implementation-dependent.

There are three contexts in which an extension element might occur:

In an inline document. All elements in an inline document are considered quoted; no extension element can occur.
In a subpipeline. In a subpipeline, any element in a namespace that is in the set of ignored namespaces is an extension element. Every other element identifies a step.
In any other context, any element that is not in the pipeline namespace is an error.

3.8 Ignored namespaces

The element children of a p:pipeline can come from many different namespaces. Some of the children identify steps in the subpipeline, others may be extension elements. In order to determine which elements are extension elements and which are expected to identify steps, the pipeline may specify a set of “ignored namespaces”

The ignored namespaces are a set of namespaces which do not identify steps. They are ignored by the processor unless the processor happens to recognize one or more of them as extension elements.

Syntactically, a pipeline author can specify the set of ignored namespaces with the ignore-prefixes attribute. This attribute can appear on the p:pipeline and p:pipeline-library elements. It is a static error (err:XS0004) if the ignore-prefixes attribute appears on any other element in the pipeline namespace.

The value of the ignore-prefixes attribute is a sequence of tokens, each of which must be the prefix of an in-scope namespace. It is a static error (err:XS0005) if any token specified in the ignore-prefixes attribute is not the prefix of an in-scope namespace.

Elements in an ignored namespace are only ignored when they appear as the direct children of the p:pipeline or p:pipeline-library which specifies the ignored namespaces.

Any ignored namespaces that are specified in a pipeline library are not inherited by pipelines either within that library or that import that library, they only apply to the elements that appear as children of the p:pipeline-library element on which they are specified.

3.9 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>

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 p:documentation element is not shown, but it is allowed anywhere.
Attributes that are syntactic shortcuts for option values are not shown.

4 Steps

This section describes the core steps of XProc.

Every compound step in a pipeline has several parts: a set of inputs, a set of outputs, a set of options, a set of contained steps, and an environment.

Editorial Note

In previous drafts, inputs, outputs, and options occurred in a fixed order. In this draft, they may appear in any order (but before the contained steps). Is that problematic?

Except where otherwise noted, a compound step can have an arbitrary number of outputs, options, and contained steps.

It is a static error (err:XS0027) if a compound step has no contained steps.

4.1 p:pipeline

A pipeline is specified by the p:pipeline element. It encapsulates the behavior of a subpipeline. Its children declare the inputs, outputs, and options that the pipeline exposes and identify the steps in its subpipeline.

A pipeline can declare additional steps (e.g., ones 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 pipeline that imported it.

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 environment inherited by the contained steps of a p:pipeline is the empty environment with these modifications:

All of the declared inputs of the pipeline are added to the readable ports in the environment.
If the pipeline has a primary input port, that input is the default readable port, otherwise the default readable port is undefined.
All of the declared options of the pipeline are added to the in-scope options in the environment.

If the p:pipeline has a primary output port and that port has no binding, then it is bound to the primary output port of the last step in the subpipeline. It is a static error (err:XS0006) if the primary output port has no binding and the last step in the subpipeline does not have a primary output port.

There are two additional constraints on pipelines:

A p:pipeline must not itself be a contained step.
If a p:pipeline is part of a p:pipeline-library or if it is imported directly with p:import, then it must have a name or a type or both.

If the pipeline initially invoked by the processor has inputs or outputs, those ports are bound to documents outside of the pipeline in an implementation-defined manner.

If a pipeline has a type then that type may be used as the name of a step to invoke the pipeline. This most often occurs when the it has been imported into another pipeline, but pipelines may also invoke themselves recursively. If it does not have a type, then its name is used to invoke it as a step.

For pipelines that are part of a p:pipeline-library, see Section 5.9, “p:pipeline-library Element” for more details on how p:pipeline names are used to compute step names.

4.1.1 Examples

A pipeline might accept a document and a stylesheet as input; perform XInclude, validation, and transformation; and produce the formatted document as its output.

Example 3. A Sample Pipeline Document

<p:pipeline name="pipeline" xmlns:p="http://www.w3.org/2007/03/xproc">
<p:input port="document" primary="yes"/>
<p:input port="stylesheet"/>
<p:output port="result" primary="yes"/>

<p:xinclude/>

<p:validate-xml-schema>
  <p:input port="schema">
    <p:document href="http://example.com/path/to/schema.xsd"/>
  </p:input>
</p:validate-xml-schema>

<p:xslt>
  <p:input port="stylesheet">
    <p:pipe step="pipeline" port="stylesheet"/>
  </p:input>
</p:xslt>

</p:pipeline>

4.2 p:for-each

A for-each is specified by the p:for-each element. It processes a sequence of documents, applying its subpipeline to each document in turn.

<p:for-each name? = NCName> (p:iteration-source?, (p:output | p:option | p:log)*, subpipeline) </p:for-each>

When a pipeline needs to process a sequence of documents using a step that only accepts a single document, the p:for-each construct can be used as a wrapper around the step that accepts only a single document. The p:for-each will apply that step 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 subpipeline is connected to one or more output ports on the p:for-each, 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.

The p:iteration-source is an anonymous input: its binding provides a sequence of documents to the p:for-each step. If no iteration sequence is explicitly provided, then the iteration source is read from the default readable port.

A portion of each input document can be selected using the select attribute. If no selection is specified, the document node of each document is selected.

Each subtree selected by the p:for-each from each of the inputs that appear on the iteration source is wrapped in a document node and provided to the subpipeline.

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.

The environment inherited by the contained steps of a p:for-each is the inherited environment with these modifications:

The port named “current” on the p:for-each is added to the readable ports.
The port named “current” on the p:for-each is made the default readable port.

If the p:for-each has a primary output port and that port has no binding, then it is bound to the primary output port of the last step in the subpipeline. It is a static error (err:XS0006) if the primary output port has no binding and the last step in the subpipeline does not have a primary output port.

4.2.1 Examples

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.

Example 4. A Sample For-Each

<p:for-each name="chapters">
  <p:iteration-source select="//chapter"/>
  <p:output port="html-results">
    <p:pipe step="make-html" port="result"/>
  </p:output>
  <p:output port="fo-results">
    <p:pipe step="make-fo" port="result"/>
  </p:output>

  <p:xslt name="make-html">
    <p:input port="stylesheet">
      <p:document href="http://example.com/xsl/html.xsl"/>
    </p:input>
  </p:xslt>

  <p:xslt name="make-fo">
    <p:input port="source">
      <p:pipe step="chapters" port="current"/>
    </p:input>
    <p:input port="stylesheet">
      <p:document href="http://example.com/xsl/fo.xsl"/>
    </p:input>
  </p:xslt>
</p:for-each>

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 processes a single document, applying its subpipeline to one or more subsections of the document.

<p:viewport name? = NCName match = XPath expression> ((p:viewport-source?, p:output?, p:log?, p:option*), subpipeline) </p:viewport>

The result of the p:viewport is a copy of the original document with the selected subsections replaced by the results of applying the subpipeline to them.

The p:viewport-source is an anonymous input: its binding provides a single document to the p:viewport step. If no document is explicitly provided, then the viewport source is read from the default readable port.

The match attribute specifies an [XPath 1.0] expression that is a Pattern in [XSLT 1.0]. Each matching node in the source document is wrapped in a document node and provided to the viewport's subpipeline.

The processor provides each document, one at a time, to the subpipeline represented by the children of the p:viewport on a port named current.

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.

It is a dynamic error (err:XD0003) if the viewport source does not provide exactly one document.

The environment inherited by the contained steps of a p:viewport is the inherited environment with these modifications:

The port named “current” on the p:viewport is added to the readable ports.
The port named “current” on the p:viewport is made the default readable port.

If the p:viewport has a primary output port and that port has no binding, then it is bound to the primary output port of the last step in the subpipeline. It is a static error (err:XS0006) if the primary output port has no binding and the last step in the subpipeline does not have a primary output port.

4.3.1 Examples

A p:viewport might accept an XHTML document as its input, add an hr element before 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.

Example 5. A Sample Viewport

<p:viewport match="h:div[@class='chapter']"
            xmlns:h="http://www.w3.org/1999/xhtml">
  <p:insert at-start="true">
    <p:input port="insertion">
      <p:inline>
        <hr xmlns="http://www.w3.org/1999/xhtml"/>
      </p:inline>
    </p:input>
  </p:insert>
</p:viewport>

</p:pipeline>

The nodes which match h:div[@class='chapter'] (according to the rules of [XSLT 1.0]) 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 selects exactly one of a list of alternative subpipelines based on the evaluation of [XPath 1.0] expressions.

<p:choose name? = NCName> (p:xpath-context?, p:when*, p:otherwise?) </p:choose>

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 result of the p:choose is the result of the selected subpipeline.

In order to ensure that the result of the p:choose is consistent irrespective of the subpipeline chosen, each subpipeline must declare the same number 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.

It is a dynamic error (err:XD0004) if no subpipeline is selected by the p:choose and no default is provided.

The p:choose can specify the context node against which the [XPath 1.0] expressions that occur on each branch are evaluated. The context node is specified as a binding for the xpath-context. If no binding is provided, the default xpath-context is the document on the default readable port. It is a static error (err:XS0032) if no binding is provided and the default readable port is undefined.

It is a dynamic error (err:XD0005) if the xpath-context is bound to a sequence of documents.

Each conditional subpipeline is represented by a p:when element.

<p:when test = XPath expression> (p:xpath-context?, (p:output | p:option | p:log)*, subpipeline) </p:when>

Each p:when branch of the p:choose has a test attribute which must contain an [XPath 1.0] expression. That XPath expression's effective boolean value is the guard expression 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 binding for the xpath-context. If no context is specified on the p:when, the context of the p:choose is used. It is a static error (err:XS0008) if no context is specified in either the p:choose or the p:when and the default readable port is undefined.

The default branch is represented by a p:otherwise element.

<p:otherwise> ((p:output | p:option | p:log)*, subpipeline) </p:otherwise>

4.4.1 Examples

A p:choose might test the version attribute of the document element and validate with an appropriate schema.

Example 6. A Sample Choose

<p:choose name="version">
  <p:when test="/*[@version = 2]">
    <p:validate-xml-schema>
      <p:input port="schema">
        <p:document href="v2schema.xsd"/>
      </p:input>
    </p:validate-xml-schema>
  </p:when>

  <p:when test="/*[@version = 1]">
    <p:validate-xml-schema>
      <p:input port="schema">
        <p:document href="v1schema.xsd"/>
      </p:input>
    </p:validate-xml-schema>
  </p:when>

  <p:when test="/*[@version]">
    <p:identity/>
  </p:when>

  <p:otherwise>
    <p:error code="NOVERSION"
             description="Required version attribute missing."/>
  </p:otherwise>
</p:choose>

4.5 p:group

A group is specified by the p:group element. It encapsulates the behavior of its subpipeline.

<p:group name? = NCName> ((p:output | p:option | p:log)*, subpipeline) </p:group>

A p:group is a convenience wrapper for a collection of steps. The result of a p:group is the result of its subpipeline.

4.5.1 Examples

Example 7. An Example Group

<p:group>
  <p:option name="db-key" value="some-long-string-of-nearly-random-characters"/>

  <p:choose>
    <p:when test="/config/output = 'fo'">
      <p:xslt>
        <p:parameter name="key" select="$db-key"/>
        <p:input port="stylesheet">
          <p:document href="fo.xsl"/>
        </p:input>
      </p:xslt>
    </p:when>
    <p:when test="/config/output = 'svg'">
      <p:xslt>
        <p:parameter name="key" select="$db-key"/>
        <p:input port="stylesheet">
          <p:document href="svg.xsl"/>
        </p:input>
      </p:xslt>
    </p:when>
    <p:otherwise>
      <p:xslt>
        <p:parameter name="key" select="$db-key"/>
        <p:input port="stylesheet">
          <p:document href="html.xsl"/>
        </p:input>
      </p:xslt>
    </p:otherwise>
  </p:choose>
</p:group>

4.6 p:try/p:catch

A try/catch is specified by the p:try element. It isolates a subpipeline, preventing any errors that arise within it from being exposed to the rest of the pipeline.

<p:try name? = NCName> (p:group, p:catch) </p:try>

The p:group represents the initial subpipeline and the recovery (or “catch”) pipeline is identified with a p:catch element.

The p:try step evaluates the initial subpipeline and, if no errors occur, the results of that pipeline are the results of the step. However, if any errors occur, it abandons the first subpipeline, discarding any output that it might have generated, and evaluates the recovery subpipeline.

If the recovery subpipeline is evaluated, the results of the recovery subpipeline are the results of the p:try step. If the recovery subpipeline is evaluated and a step within that subpipeline fails, the p:try fails.

In order to ensure that the result 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.

The recovery subpipeline of a p:try is identified with a p:catch:

<p:catch name? = NCName> ((p:output | p:option | p:log)*, subpipeline) </p:catch>

The environment inherited by the contained steps of the p:catch is the inherited environment with this modification:

The port named “error” on the p:catch is added to the readable ports.

Editorial Note

Should the error port be made the default readable port?

4.6.1 Examples

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.

Example 8. An Example Try/Catch

<p:try>
  <p:group>
    <p:http-request>
      <p:input port="source">
        <p:inline>
          <c:http-request method="post" href="http://example.com/form-action">
            <c:entity-body content-type="application/x-www-form-urlencoded">
              <c:body>name=W3C&amp;spec=XProc</c:body>
            </c:entity-body>
          </c:http-request>
        </p:inline>
      </p:input>
    </p:http-request>
  </p:group>
  <p:catch>
    <p:identity>
      <p:input port="source">
        <p:inline>
          <c:error>HTTP Request Failed</c:error>
        </p:inline>
      </p:input>
    </p:identity>
  </p:catch>
</p:try>

4.7 Other Steps

Other steps are specified by elements that occur as contained steps and are not in any of the the ignored namespaces.

Other steps can be atomic:

<pfx:other-atomic-step name? = NCName> (p:input | p:option | p:parameter | p:log)* </pfx:other-atomic-step>

Or compound:

<pfx:other-compound-step name? = NCName> ((p:input | p:output | p:option | p:log)*, subpipeline) </pfx:other-compound-step>

Each atomic step must be the name of a p:pipeline type or must have been declared with a p:declare-step that appears in the pipeline, or an imported library, before it is used. 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.

If the step element name is the same as the type of a step declared with p:declare-step, then that step invokes the declared step.

If the step element name is the same as the type or name of a p:pipeline, then that step runs the pipeline identified by that type or name.

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.

Editorial Note

A pipeline author can make the set of parameters passed to a step explicit with a parameter input. If the step does not make an explicit binding for a parameter input, the default could be either to pass no parameters to the step or to behave as if the parameter input was bound to the pipeline parameters.

The working group is divided on this issue and this draft does not provide an answer to that question. Reader feedback is encouraged.

4.7.1 Syntactic Shortcut for Option Values

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:option syntax:

<ex:stepType>
  <p:option name="option-name" value="5"/>
</ex:stepType>

The second step uses the syntactic shortcut:

<ex:stepType option-name="5"/>

Note that there are significant 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”.
It can only be used to specify a constant value. Options that are computed with a select expression must be written using the longer form.

It is a static error (err:XS0027) if an option is specified with both the shortcut form and the long form. It is a static error (err:XS0031) to use an option on an atomic step that is not declared on steps of that type.

5 Other pipeline elements

5.1 p:input Element

A p:input identifies an input port for a step, declaring it if necessary. There are two kinds of inputs that may be declared, ordinary “document” inputs and “parameter” inputs. It is a static error (err:XS0033) to specify any kind of input other than “document” or “parameter”.

5.1.1 Document Inputs

<p:input port = NCName sequence? = yes|no primary? = yes|no kind? = "document" />

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. It is a static error (err:XS0012) if the port given does not match the name of an input port specified in the step's declaration.

On compound steps and p:declare-step, an input declaration can indicate if a sequence of documents is allowed to appear on the port and if the port is a primary input port.

If sequence is specified with the value “yes”, then a sequence is allowed. If sequence is not specified, or has the value “no”, then it is a dynamic error (err:XD0006) for a sequence of more than one document to appear on the declared port.

An input port is a primary input port if primary is specified with the value “yes” 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.

On p:declare-step, the p:input simply declares the input port. In all other contexts, the declaration may be accompanied by a binding for the input:

If no binding is provided, the input will be bound to the default readable port. It is a static error (err:XS0032) if no binding is provided and the default readable port is undefined. A select expression may also be provided. The select expression, if specified, applies the specified [XPath 1.0] select expression to the document(s) that are read. Each node that matches is wrapped in a document and provided to the input port. In other words,

<p:input port="source">
  <p:document href="http://example.org/input.html"/>
</p:input>

provides a single document, but

<p:input port="source" select="//html:div">
  <p:document href="http://example.org/input.html"/>
</p:input>

provides a sequence of zero or more documents, one for each matching html:div in http://example.org/input.html.

A select expression can equally be applied to input read from another step. This input:

<p:input port="source" select="//html:div">
  <p:pipe step="origin" port="result"/>
</p:input>

provides a sequence of zero or more documents, one for each matching html:div in the document (or each of the documents) that is read from the result port of the step named origin.

It is a dynamic error (err:XD0016) if the select expression on a p:input returns anything other than a possibly empty set of nodes.

5.1.2 Parameter Inputs

<p:input port = NCName sequence? = "yes" kind = "parameter" />

A parameter input port is a distinguished kind of input port. It exists only to receive computed parameters; if a step does not have a parameter input port then it cannot receive computed parameters. A parameter input port must satisfy all the constraints of a normal, document input.

When used on a step, parameter input ports always accept a sequence of documents. If no binding is provided, the parameter input will be bound to @@TBD.

All of the documents that appear on a parameter input must either be c:parameter documents or c:parameter-list documents.

A step which accepts a parameter input reads all of the documents presented on that port, using each c:parameter (either at the root or inside the c:parameter-list) to establish the value of the named parameter. If the same name appears more than once, the last value specified is used. If the step also has literal p:parameter elements, they are are also considered in document order. In other words, p:parameter elements that appear before the parameter input may be overridden by the computed parameters; p:parameter elements that appear after may override the computed values.

5.1.2.1 The c:parameter element

A c:parameter represents a parameter on a parameter input.

<c:parameter name = string namespace? = anyURI value = string />

The name attribute of the c:parameter must have the lexical form of a QName. If it contains a colon, then its expanded name is constructed using the namespace declarations in-scope on the c:parameter element. If it does not contain a colon and the namespace attribute is specified, then it is an expanded name in the specified namespace. If the namespace attribute is not specified, its expanded name has no namespace. It is a dynamic error (err:XD0013) if the name attribute of a c:parameter element contains a colon and a namespace attribute is specified.

Any extension attributes that appear on the c:parameter element are ignored.

5.1.2.2 The c:parameter-list element

A c:parameter-list represents a list of parameters on a parameter input.

<c:parameter-list> c:parameter* </c:parameter-list>

The c:parameter-list contains zero or more c:parameter elements. It is a dynamic error (err:XD0018) if the parameter list contains any elements other than c:parameter.

Any extension attributes that appear on the c:parameter-list element are ignored.

5.2 p:iteration-source Element

A p:iteration-source identifies input to a p:for-each.

<p:iteration-source select? = XPath expression> (p:empty | (p:pipe | p:document | p:inline)+)? </p:iteration-source>

The select attribute and binding of a p:iteration-source work the same way that they do in a p:input.

5.3 p:viewport-source Element

A p:viewport-source identifies input to a p:viewport.

<p:viewport-source> (p:pipe | p:document | p:inline)? </p:viewport-source>

Only one binding is allowed and it works the same way that bindings work on a p:input. It is a dynamic error (err:XD0006) for a sequence of more than one document to appear on the p:viewport-source. No select expression is allowed.

5.4 p:xpath-context Element

A p:xpath-context identifies a context against which an [XPath 1.0] expression will be evaluated for a p:when.

<p:xpath-context> (p:empty | p:pipe | p:document | p:inline)? </p:xpath-context>

It is a dynamic error (err:XD0019) if the context is bound to p:empty and the test expression refers to the context node.

5.5 p:output Element

A p:output identifies an output port, optionally declaring it, if necessary.

<p:output port = NCName sequence? = yes|no primary? = yes|no />

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. It is a static error (err:XS0013) if the port given does not match the name of an output port specified in the step's declaration.

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 “yes”, then a sequence is allowed. If sequence is not specified on p:output, or has the value “no”, then it is a dynamic error (err:XD0007) if the step produces a sequence of more than one document on the declared port.

An output declaration can indicate if it is to be considered the primary output for the step. If primary is specified with the value “yes”, then the named port will be treated as the primary output port. It is a static error (err:XS0014) to identify more than one output ports as primary.

On compound steps, the declaration may be accompanied by a binding for the output.

It is a static error (err:XS0029) to specify a binding for a p:output inside a p:declare-step.

If a binding is provided for a p:output, documents are read from that binding 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.

5.6 p:log Element

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 URI whatever document or documents appear on the specified port. How a sequence of documents is represented is implementation-defined.

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.

5.7 Options and Parameters

5.7.1 p:option Element

The p:option element is used both to declare options and to establish values for them. It can occur in three contexts:

On p:declare-step, it declares that the step accepts the named option. It may also provide a default value for the option.
On a compound step, it provides a value for the option, simultaneously declaring it.
On an atomic step, it provides a value for the option, overriding any default specified in the declaration.

5.7.1.1 Declaring Options

Options are declared on p:declare-step with p:option:

<p:option name = QName required? = yes|no />

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 in the XProc namespace.

An option may be declared as required or it may be given a default value. It is a static error (err:XS0017) to specify that the option is both required and has a default value.

If an option is required, it is a static error (err:XS0018) to invoke the step without specifying a value for that option.

5.7.1.2 Using Options

Options are used on a step with p:option. 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 in the XProc namespace.

The option must be given a value when it is used. It is a static error (err:XS0031) to use an option on an atomic step that is not declared on steps of that type.

5.7.1.3 Assigning Values to Options

When an option is declared, it may be given a default value. When it is used, it must be given a value.

The value can be specified in two ways: with a select or value attribute.

If a select expression is given, it is evaluated against the document specified in the binding and the [XPath 1.0] string value of the expression becomes the value of the option. It is a dynamic error (err:XD0008) if a document sequence is specified in the binding for a p:option.

<p:option name = QName select = XPath expression required? = yes|no> (p:empty | p:pipe | p:document | p:inline)? </p:option>

The select expression may refer to the values of other in-scope options by variable reference. It is a static error (err:XS0019) if the variable reference uses a QName that is not the name of an in-scope option.

If a select expression is used but no binding is provided, the implicit binding is to the default readable port. It is a static error (err:XS0032) if no binding is provided and the default readable port is undefined.

If a value attribute is specified, its content becomes the value of the option.

<p:option name = QName value = string required? = yes|no />

In the case where the value of an option is a constant, its value may also be specified on the parent step as specified in Section 4.7.1, “Syntactic Shortcut for Option Values”.

It is a static error (err:XS0016) if the value is not specified with either select or value, or if both are specified.

5.7.2 p:parameter Element

The p:parameter element is used to establish the value of a parameter. The parameter must be given a value when it is used.

The value can be specified in two ways: with a select or value attribute.

If a select expression is given, it is evaluated against the document specified in the binding and the [XPath 1.0] string value of the expression becomes the value of the parameter. It is a dynamic error (err:XD0008) if a document sequence is specified in the binding for a p:parameter.

<p:parameter name = QName select = XPath expression> (p:empty | p:pipe | p:document | p:inline)? </p:parameter>

The select expression may refer to the values of in-scope options by variable reference. It is a static error (err:XS0019) if the variable reference uses a QName that is not the name of an in-scope option.

If a value attribute is specified, its content becomes the value of the parameter.

<p:parameter name = QName value = string />

It is a static error (err:XS0016) if the value is not specified with either select or value, or if both are specified.

5.7.3 Option and Parameter Namespace Bindings

Option and parameter values carry with them not only their literal or computed string value but also the set of namespaces that were in-scope on the element which defined them.

This is necessary because QName values in options or parameters may subsequently need to be expanded by the steps which accept them.

When values are computed from several sources, the union of the sets of namespaces must be carried with the new value. The results of computing the union in the presence of conflicting declarations for a particular prefix are implementation-dependent.

5.8 p:declare-step Element

A p:declare-step provides the type and signature of an atomic step. It declares the inputs, outputs, and options for all steps of that type.

<p:declare-step type = QName> (p:input | p:output | p:option)* </p:declare-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.

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 type attribute is in no namespace. If the namespace URI of the type is the XProc namespace, then the declaration must be exactly as defined in this specification. Neither users nor implementors may define additional steps in the XProc namespace.

5.9 p:pipeline-library Element

A p:pipeline-library is a collection of step declarations and/or pipeline definitions.

<p:pipeline-library namespace? = anyURI ignore-prefixes? = prefix list> (p:import | p:declare-step | p:pipeline)* </p:pipeline-library>

Libraries can import pipelines and/or other libraries. It is a static error (err:XS0021) if the import references in a pipeline or pipeline library are circular.

If the p:pipeline-library specifies a namespace with the namespace attribute, then all of the untyped pipelines that occur in the library are in that namespace.

For example, given the following pipeline library:

<p:pipeline-library xmlns:p="http://www.w3.org/2007/03/xproc"
                    namespace="http://example.com/ns/pipelines">

<p:import href="ancillary-library.xml"/>
<p:import href="other-pipeline.xml"/>

<p:pipeline name="validate">
  <!-- definition of validate pipeline -->
</p:pipeline>

<p:pipeline name="format" type="my:format"
            xmlns:my="http://example.com/vanity/mine">
  <!-- definition of format pipeline -->
</p:pipeline>

</p:pipeline-library>

The pipeline named “validate” is in the namespace http://example.com/ns/pipelines. That means that it must be invoked from the importing pipeline with a qualified name of the form:

<ex:validate>
  …
</ex:validate>

(Assuming that the “ex” prefix is bound to http://example.com/ns/pipelines.)

The pipeline named “format” has an explicit type so it must be invoked with a qualified name of the form:

<my:format>
  …
</my:format>

(Assuming that the “my” prefix is bound to http://example.com/vanity/mine.)

The pipeline library namespace applies only to pipelines that are defined directly in the library; it does not apply to pipeline libraries that are imported or pipelines that are directly imported.

5.10 p:import Element

An p:import loads a pipeline or pipeline library, making it available in the pipeline or library which contains the p:import.

<p:import href = anyURI />

An import statement loads the specified URI and makes any pipelines declared within it available to the current pipeline. An imported pipeline has an implicit signature that consists of the inputs, outputs, and options declared on it.

It is a dynamic error (err:XD0009) if the URI of a p:import cannot be retrieved or if, once retrieved, it does not point to a p:pipeline-library or p:pipeline. It is a dynamic error (err:XD0010) to import a single pipeline if that pipeline does not have a name or a type. It is a dynamic error (err:XD0012) to import more than one pipeline with the same name or type (either directly or within a library).

5.11 p:pipe Element

A p:pipe reads from the output port of another step.

<p:pipe step = NCName port = NCName />

The p:pipe element connects to the output port of another step. It identifies the output port too 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.

In all cases except the p:output of a compound step, it is a static error (err:XS0022) if the port identified by a p:pipe is not in the readable ports of the environment of the step that contains the p:pipe.

It is a static error (err:XS0023) if the port identified by a p:pipe in the p:output of a compound step is not in the readable ports of the environment inherited by the contained steps of the compound step.

In other words, the output of a compound step must be bound to the output of one of its contained steps. All other bindings must be to ports that are already readable in the current environment.

5.12 p:inline Element

A p:inline provides a document inline.

<p:inline> anyElement </p:inline>

The content of the p:inline element is wrapped in a document node and passed as input. The base URI of the document is the base URI of the p:inline element.

Note

The nodes inside a p:inline element naturally inherit the namespaces that are in-scope at the point where they occur in the pipeline document. Implementations must assure that those namespaces remain in-scope in the resulting document.

It is a static error (err:XS0024) if the content of the p:inline element is not a well-formed XML document.

5.13 p:document Element

A p:document reads an XML document from a URI.

<p:document href = anyURI />

The document identified by the URI in the href attribute is loaded and returned.

It is a dynamic error (err:XD0011) if the document referenced by a p:document element does not exist, cannot be accessed, or is not a well-formed XML document.

The parser which the p:document element employs must be conformant to Namespaces in XML. It must not perform validation. It must not perform any other processing, such as expanding XIncludes.

Use the p:load step if you need to perform DTD-based validation or wish to perform other processing on the document before it is used by a step.

5.14 p:empty Element

A p:empty binds to an empty sequence of documents.

<p:empty />

5.15 p:documentation Element

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.

6 Errors

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, incorrect specification of inputs and outputs, and reference to unknown steps.

Static errors are fatal and must be detected before any steps are evaluated.

Static Errors

err:XS0001

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.

See: Connections

err:XS0002

All in-scope steps must have unique names: it is a static error if two steps with the same name appear in the same scope.

See: Scoping of Names

err:XS0003

It is a static error if the port specified by a p:pipe is not in the readable ports of the environment.

See: Associating Documents with Ports

err:XS0004

It is a static error if the ignore-prefixes attribute appears on any other element in the pipeline namespace.

See: Ignored namespaces

err:XS0005

It is a static error if any token specified in the ignore-prefixes attribute is not the prefix of an in-scope namespace.

See: Ignored namespaces

err:XS0027

It is a static error if a compound step has no contained steps.

See: Steps, Syntactic Shortcut for Option Values

err:XS0006

It is a static error if the primary output port has no binding and the last step in the subpipeline does not have a primary output port.

See: p:pipeline, p:for-each, p:viewport

err:XS0007

It is a static error if two subpipelines in a p:choose declare different outputs.

See: p:choose

err:XS0032

It is a static error if no binding is provided and the default readable port is undefined.

See: p:choose, Document Inputs, Assigning Values to Options, p:parameter Element

err:XS0008

It is a static error if no context is specified in either the p:choose or the p:when and the default readable port is undefined.

See: p:choose

err:XS0009

It is a static error if the p:group and p:catch subpipelines declare different outputs.

See: p:try/p:catch

err:XS0010

It is a static error if a pipeline contains a step whose specified inputs, outputs, and options do not match the signature for steps of that type.

See: Other Steps

err:XS0031

It is a static error to use an option on an atomic step that is not declared on steps of that type.

See: Syntactic Shortcut for Option Values, Using Options

err:XS0033

It is a static error to specify any kind of input other than “document” or “parameter”.

See: p:input Element

err:XS0011

It is a static error to identify two ports with the same name on the same step.

See: Document Inputs, Parameter Inputs, p:output Element

err:XS0012

It is a static error if the port given does not match the name of an input port specified in the step's declaration.

See: Document Inputs, Parameter Inputs

err:XS0030

It is a static error to specify that more than one input port is the primary.

See: Document Inputs

err:XS0013

It is a static error if the port given does not match the name of an output port specified in the step's declaration.

See: p:output Element

err:XS0014

It is a static error to identify more than one output ports as primary.

See: p:output Element

err:XS0029

It is a static error to specify a binding for a p:output inside a p:declare-step.

See: p:output Element

err:XS0026

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.

See: p:log Element

err:XS0028

It is a static error to declare an option in the XProc namespace.

See: Declaring Options, Using Options

err:XS0017

It is a static error to specify that the option is both required and has a default value.

See: Declaring Options

err:XS0018

If an option is required, it is a static error to invoke the step without specifying a value for that option.

See: Declaring Options

err:XS0019

It is a static error if the variable reference uses a QName that is not the name of an in-scope option.

See: Assigning Values to Options, p:parameter Element

err:XS0016

It is a static error if the value is not specified with either select or value, or if both are specified.

See: Assigning Values to Options, p:parameter Element

err:XS0025

It is a static error if the type attribute is in no namespace.

See: p:declare-step Element

err:XS0021

It is a static error if the import references in a pipeline or pipeline library are circular.

See: p:pipeline-library Element

err:XS0022

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 environment of the step that contains the p:pipe.

See: p:pipe Element

err:XS0023

It is a static error if the port identified by a p:pipe in the p:output of a compound step is not in the readable ports of the environment inherited by the contained steps of the compound step.

See: p:pipe Element

err:XS0024

It is a static error if the content of the p:inline element is not a well-formed XML document.

See: p:inline Element

6.2 Dynamic Errors

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.

Dynamic Errors

err:XD0001

It is a dynamic error if a non-XML resource is produced on a step output or arrives on a step input.

See: Inputs and Outputs

err:XD0002

It is a dynamic error if the processor attempts to retrieve the URI specified on a p:document and fails.

See: Associating Documents with Ports

err:XD0003

It is a dynamic error if the viewport source does not provide exactly one document.

See: p:viewport

err:XD0004

It is a dynamic error if no subpipeline is selected by the p:choose and no default is provided.

See: p:choose

err:XD0005

It is a dynamic error if the xpath-context is bound to a sequence of documents.

See: p:choose

err:XD0017

It is a dynamic error if the running pipeline attempts to invoke a step which the processor does not know how to perform.

See: Other Steps

err:XD0006

If sequence is not specified, or has the value “no”, then it is a dynamic error for a sequence of more than one document to appear on the declared port.

See: Document Inputs, p:viewport-source Element, p:xpath-context Element

err:XD0016

It is a dynamic error if the select expression on a p:input returns anything other than a possibly empty set of nodes.

See: Document Inputs

err:XD0013

It is a dynamic error if the name attribute of a c:parameter element contains a colon and a namespace attribute is specified.

See: The c:parameter element, Label Elements, XInclude

err:XD0018

It is a dynamic error if the parameter list contains any elements other than c:parameter.

See: The c:parameter-list element

err:XD0019

It is a dynamic error if the context is bound to p:empty and the test expression refers to the context node.

See: p:xpath-context Element

err:XD0007

If sequence is not specified on p:output, or has the value “no”, then it is a dynamic error if the step produces a sequence of more than one document on the declared port.

See: p:output Element

err:XD0008

It is a dynamic error if a document sequence is specified in the binding for a p:option.

See: Assigning Values to Options, p:parameter Element

err:XD0009

It is a dynamic error if the URI of a p:import cannot be retrieved or if, once retrieved, it does not point to a p:pipeline-library or p:pipeline.

See: p:import Element

err:XD0010

It is a dynamic error to import a single pipeline if that pipeline does not have a name or a type.

See: p:import Element

err:XD0012

It is a dynamic error to import more than one pipeline with the same name or type (either directly or within a library).

See: p:import Element

err:XD0011

It is a dynamic error if the document referenced by a p:document element does not exist, cannot be accessed, or is not a well-formed XML document.

See: p:document Element

A Standard Step Library

This appendix describes the standard XProc steps. A machine-readable description of these steps may be found in pipeline-library.xml.

Some steps in this appendix consume or produce an XML vocabulary defined in this section. In all cases, the namespace for that vocabulary is http://www.w3.org/2007/03/xproc-step and is represented by the prefix 'c:' in this appendix.

Also, in this section, several steps use this empty element for result information:

<c:result value? = string href? = anyURI />

Note

The steps described in this draft are intended mainly as a starting point for discussion and to present a flavor for the sorts of steps envisioned. The WG has not yet discussed them in detail.

A.1 Required Steps

This section describes standard steps that must be supported by any conforming processor.

A.1.1 Count

The Count step counts the number of documents in the source input sequence and returns a single document on result containing that number. The generated document contains a single c:result element whose value attribute is the string representation of the number of documents in the sequence.

<p:declare-step type="p:count">
<p:input port="source" sequence="yes"/>
<p:output port="result"/>
</p:declare-step>

A.1.2 Delete

The Delete step deletes the matching items from the source input document and produces the resulting document with the deletions on the result port. The matching items are specified by the match pattern in the match option. The match pattern may match multiple items to be deleted but nested matches are not considered as their ancestors would already be deleted.

<p:declare-step type="p:delete">
     <p:input port="source"/>
     <p:output port="result"/>
     <p:option name="match" required="yes"/>
</p:declare-step>

A.1.3 Equal

The Equal step accepts two, single documents and returns “1” if they are fn:deep-equal (as defined in [XPath 2.0 Functions and Operators]) to each other, “0” otherwise. The return value is expressed using a c:result document.

<p:declare-step type="p:equal">
     <p:input port="source" primary="yes"/>
     <p:input port="alternate"/>
     <p:output port="result"/>
     <p:option name="fail-if-not-equal" value="no"/>
</p:declare-step>

A.1.4 Error

The Error step generates an error using the options specified on the step. The error generated can be caught by a try/catch language construct like any other dynamic error.

<p:declare-step type="p:error">
<p:option name="code" required="yes"/>
<p:option name="description" required="yes"/>
</p:declare-step>

Note that this step has no inputs and no outputs. The containing language construct (e.g. a choose construct) controls whether the error is generated. Since the step generates an error upon invocation, there is no direct output. Instead, the error generates an instance of the err:errors element on the error port just like any other dynamic error.

For example, give the following invocation:


<p:error name="bad-document">
   <p:option name="code" value="12">
   <p:option name="description" value="The document element is unknown."/>
</p:error>

The error vocabulary element (and document) generated on the error output port is:


<err:errors name="bad-document" type="p:error">
<err:error code="12">
The document element is unknown
</err:errors>
</err:errors>

A.1.5 Escape Markup

The Escape Markup step applies XML serialization to the children of the document element and replaces those children with their serialization. The outcome is a single element with text content that represents the "escaped" syntax of the children if they were serialized.

<p:declare-step type="p:escape-markup">
<p:input port="source"/>
<p:output port="result"/>
</p:declare-step>

For example, the input:

<description>
<div xmlns="http://www.w3.org/1999/xhtml">
<p>This is a chunk of XHTML.</p>
</div>
</description>

and produces:

<description>
&lt;div xmlns="http://www.w3.org/1999/xhtml">
&lt;p>This is a chunk of XHTML.&lt;/p>
&lt;/div>
</description>

A.1.6 Identity

The identity step makes a verbatim copy of its input available on its output.

<p:declare-step type="p:identity">
<p:input port="source" sequence="yes"/>
<p:output port="result" sequence="yes"/>
</p:declare-step>

A.1.7 Insert

The Insert step inserts the insertion port's document as a child of matching elements in the source port's document. The insertion copies the document element of the insertion document into the position specified by the options on the step. In some cases, multiple insertions may be performed by this step.

<p:declare-step type="p:insert">
     <p:input port="source" primary="yes"/>
     <p:input port="insertion"/>
     <p:output port="result"/>
     <p:option name="match"/>
     <p:option name="at-start"/>
</p:declare-step>

The matching nodes in the source document are specified as a match pattern in the match option that must target an element. If no match pattern is supplied, the document element is the only match.

If the at-start option has a value 'yes', the insertion document will be inserted as the first child(ren) of the element, otherwise it will be inserted as the last child. If the at-start option is not specified, a value of 'yes' is assumed.

As the inserted elements are part of the output of the step they are not considered in determining matching elements.

A.1.8 Label Elements

The Label Elements step labels each element with a unique xml:id value. If the element already has an xml:id value, that value is preserved. A user may specify the prefix and/or suffix options for prefixing or suffixing the generated value of the xml:id attribute. These prefixes or suffixes do not affect existing xml:id values.

If a select option is specified, only elements which match that XPath 1.0 select expression are modified.

If an existing xml:id value conflicts with a previously generated value, the step fails.

It is a dynamic error (err:XD0013) if an existing xml:id value conflicts with a previously generated value and the step fails.

<p:declare-step type="p:label-elements">
     <p:input port="source"/>
     <p:output port="result"/>
     <p:option name="select"/>
     <p:option name="prefix"/>
     <p:option name="suffix"/>
</p:declare-step>

A.1.9 Load

The Load step has no inputs but takes a option that specifies a URI of an XML resource that should be loaded and provided as the result.

<p:declare-step type="p:load">
     <p:output port="result"/>
     <p:option name="href" required="yes"/>
     <p:option name="validate"/>
</p:declare-step>

Load attempts to read an XML document from the specified URI. If the document does not exist, or is not well-formed, the step fails. Otherwise, the document read is produced on the result port.

If the value of the validate option is 'yes', the XML processor is invoked as a validating XML processor and DTD validation is performed. If the document is not valid or the step doesn't support validating processors, the step fails.

A.1.10 Namespace Rename

The Namespace Rename step renames any namespace declaration or use of a namespace in a document to a new URI value. The source namespaces is identified by the from option and the target namespace is identified by the to option.

If the from option is the empty string, or is not specified, then elements and attributes in no namespace are renamed. If the to option is the empty string, or is not specified, then elements and attributes in the specified from namespace are renamed into no namespace.

If the XML namespace (http://www.w3.org/XML/1998/namespace) or the XMLNS namespace (http://www.w3.org/2000/xmlns/) is used in either the from or to options, the step fails.

<p:declare-step type="p:namespace-rename">
     <p:input port="source"/>
     <p:output port="result"/>
     <p:option name="from"/>
     <p:option name="to"/>
</p:declare-step>

It is not an error to specify the same namespace in the from and to options, but it will have no observable effect.

A.1.11 Parameters

The Parameters step exposes a set of parameters as a sequence of c:parameter documents.

<p:declare-step type="p:parameters">
<p:input port="parameters" kind="parameter" primary="no"/>
<p:output port="result" sequence="yes" primary="no"/>
</p:declare-step>

Each parameter passed to the step is converted into a c:parameter element and written to the result port as a document. The step resolves duplicate parameters in the normal way; the order in which the parameters are written is implementation dependent.

For consistency and user convenience, if any of the parameters have names that are in a namespace, the namespace attribute on the c:parameter element must be used. Each name will be an NCName.

A.1.12 Rename

The rename step renames elements, attributes, or processing-instruction targets in a document based on option values.

<p:declare-step type="p:rename">
     <p:input port="source"/>
     <p:output port="result"/>
     <p:option name="match" required="yes"/>
     <p:option name="name" required="yes"/>
</p:declare-step>

Each element, attribute, or processing-instruction matched by the match pattern specified in the match is renamed to the name specified by the name option.

The step fails if the specified name is not a valid name or if the renaming would introduce a syntactic error into the document (i.e., if it would create two attributes with the same name on the same element).

A.1.13 Replace

The Replace step replaces a matching node in the source input port's document with the document element of the replacement port's document. The result is a single document with all the matching nodes so replaced.

The elements to be replaced are specified by the match pattern in the match option. If there are multiple matches, only non-nested matches are replaced. That is, once an element is replaced, its descendants cannot be matched.

<p:declare-step type="p:replace">
     <p:input port="source" primary="yes"/>
     <p:input port="replacement"/>
     <p:output port="result"/>
     <p:option name="match" required="yes"/>
</p:declare-step>

A.1.14 Set Attributes

The Set Attributes step sets attribute values on the matching elements using the attribute values provided on the attributes port's document element. That is, it copies the attributes on the document element on the attributes port to the matching elements found in the source port's document. If the same attribute exists on the matching element, the value specified in the attribute port's document is used. The result port of this step produces a copy of the source port's document with the matching elements' attributes modified.

The matching elements are specified by the match pattern in the match option that must target an element. If there are multiple matches, all elements are processed.

<p:declare-step type="p:set-attributes">
     <p:input port="source" primary="yes"/>
     <p:input port="attributes"/>
     <p:output port="result"/>
     <p:option name="match"/>
</p:declare-step>

A.1.15 Sink

The Sink step accepts a sequence of documents and discards them. It has no output.

<p:declare-step type="p:sink">
<p:input port="source" sequence="yes"/>
</p:declare-step>

A.1.16 Split Sequence

The Split Sequence step accepts a sequence of documents and produces two subsequences of that input by applying a test expression to each document. For each document on the source input port, if the XPath expression provided by the test option evaluates to true, the document is reproduced on the “matched” output port. If the expression evaluates to false, the document is reproduced on the “notmatched” output port.

<p:declare-step type="p:split-sequence">
     <p:input port="source" sequence="yes"/>
     <p:output port="matched" sequence="yes" primary="yes"/>
     <p:output port="notmatched" sequence="yes"/>
     <p:option name="test" required="yes"/>
</p:declare-step>

When the test XPath expression is evaluated, the XPath context position is always bound to the position of the source document in the input sequence. The XPath context size is bound to the total number of documents in the sequence.

Note

In principle, this component cannot stream because it must buffer all of the input sequence in order to find the context size. In practice, if the test expression does not use the last() function, the implementation can stream and ignore the context size.

A.1.17 String Replace

The String Replace step matches nodes in the document provided on the source input port and replaces them with the string result of evaluating an XPath expression. The matched nodes are specified by the match pattern in the match option. For each matching node, the XPath expression provided by the replace option is applied and the string value that results is used in constructing the node's replacement in the output.

The output of this step is much like an identity transformation with exceptions for the set of matching nodes as follows:

For any matching node, the string value S is the result from evaluating the replace XPath expression with the matching node as the context node.
For a matching node that is not an attribute, the node is replaced by the string value S in the output document.
For a matching node that is an attribute, the string value of the attribute is replaced by the string value S in the output document.

<p:declare-step type="p:string-replace">
     <p:input port="source"/>
     <p:output port="result"/>
     <p:option name="match" required="yes"/>
     <p:option name="replace" required="yes"/>
</p:declare-step>

A.1.18 Store

The store step stores a serialized version of its input to a URI. The URI is either specified explicitly by the 'href' option or implicitly by the base URI of the document. This step outputs a reference to the location of the stored document.

<p:declare-step type="p:store">
     <p:input port="source"/>
     <p:output port="result" primary="no"/>
     <p:option name="href"/>
     <p:option name="method" value="xml"/>
     <p:option name="encoding" value="UTF-8"/>
</p:declare-step>

The step attempts to store the XML document to the specified URI. If that URI scheme is not supported or such storage is not allowed, the step fails.

The output of this step is a document containing a single c:result element whose href attribute contains the same value as the href option.

The 'method' option controls the serialization method used by this component with standard values of 'html', 'xml', 'xhtml', and 'text'.

Note

A more direct “serialize-to-octet-stream” step may also be required. One, for example, that supports the XSLT 2.0/XQuery 1.0 Serialization specification.

Note

We need a reference to the serialization step and methods.

A.1.19 Unescape Markup

The Unescape Markup step takes the text value of the document element and parses the content as if it was a unicode character stream containing XML. The outcome is a single element with children from the parsing of the XML content. This is the reverse of the p:escape-markup step.

When the text value is parsed, a document element wrapper must be assumed so that element siblings can be parsed back into XML. Further, if the 'namespace' option is specified, the default namespace is declared on that wrapper element.

If the 'content-type' option is specified, an implementation can use a different parser to produce XML content. Such a behavior is implementation defined. For example, for the mime type 'text/html', an implementation might provide an HTML to XHTML parser (e.g. Tidy).

<p:declare-step type="p:unescape-markup">
     <p:input port="source"/>
     <p:output port="result"/>
     <p:option name="namespace"/>
     <p:option name="content-type"/>
</p:declare-step>

For example, with the 'namespace' option set to the XHTML namespace, the following input:

<description>
&lt;p>This is a chunk.&lt;/p>
&lt;p>This is a another chunk.&lt;/p>
</description>

would produce:

<description>
<p xmlns="http://www.w3.org/1999/xhtml">This is a chunk.</p>
<p xmlns="http://www.w3.org/1999/xhtml">This is a another chunk.</p>
</description>

A.1.20 Unwrap

The Unwrap step matches a certain number of elements and replaces them with their children. The source port input document is processed by applying the match pattern specified by the match. If the match is an element, the element is replaced with its children in the output document produced on the result port. A single document is produced and if unwrapping causes a non-well-formed document (e.g. more than one document element), the step fails.

<p:declare-step type="p:unwrap">
     <p:input port="source"/>
     <p:output port="result"/>
     <p:option name="match" required="yes"/>
</p:declare-step>

A.1.21 Wrap

The Wrap step wraps matches items in the source port's input document with a new element. The document is processed by applying the match pattern specified by the match. For each match, the match is wrapped with a new element in the output document. The wrapper is used to specify the name of the element and takes a QName value whose prefixes are resolved as specified in Section 5.7.3, “Option and Parameter Namespace Bindings”. A single document is produced on the result output port.

If the group-by is specified, adjacent matches are grouped by evaluating the XPath specified by this option with the content node set to the matched node. If the XPath evaluates to the same string value, the matches are grouped into the same wrapper element. While processing the document for grouping, two matches are considered adjacent if they are either siblings or all intervening siblings are whitespace text, comments, or processing instruction nodes.

<p:declare-step type="p:wrap">
     <p:input port="source"/>
     <p:output port="result"/>
     <p:option name="wrapper" required="yes"/>
     <p:option name="match" required="yes"/>
     <p:option name="group-by"/>
</p:declare-step>

A.1.22 Wrap Sequence

The Wrap Sequence step converts the sequence of documents on the source port into a single document on the result port. The document produced has a document element whose name is specified via the wrapper option and whose children are the contents of the documents received on source port, in the order they were received. All of the top-level nodes (white space, comments, processing-instructions, and one element node) of each document are added to the children of the new wrapper document. The wrapper option takes a QName value whose prefixes are resolved as specified in Section 5.7.3, “Option and Parameter Namespace Bindings”

If the group-by is specified, adjacent documents in the sequence are grouped by evaluating the XPath specified by this option with the context node set to the document node. If the XPath evaluates to the same string value, the documents are grouped into the same wrapper element.

<p:declare-step type="p:wrap-sequence">
     <p:input port="source" sequence="yes"/>
     <p:output port="result"/>
     <p:option name="wrapper" required="yes"/>
</p:declare-step>

A.1.23 XInclude

The XInclude step applies xinclude processing semantics to the document. The referenced documents are calculated against the base URI and are not provided as input to the step.

<p:declare-step type="p:xinclude">
<p:input port="source"/>
<p:output port="result"/>
</p:declare-step>

It is a dynamic error (err:XD0013) if an XInclude error occurs during processing and the step fails.

A.1.24 XSLT

The xslt step applies an XSLT 1.0 transformation to a document. The transformation is supplied by a single document on the input port named 'transform'. That transformation is applied to the primary source document supplied on the input port named 'source'. The result of the transformation is a sequence of documents on its 'result' port.

<p:declare-step type="p:xslt">
     <p:input port="source" primary="yes"/>
     <p:input port="stylesheet"/>
     <p:input port="parameters" kind="parameter"/>
     <p:output port="result"/>
</p:declare-step>

All of the specified parameters are made available to the XSLT processor. If the XSLT processor signals a fatal error, the step fails. Otherwise, the result of the transformation is produced on the result port.

A.2 Optional Steps

A.2.1 HTTP Request

<p:declare-step type="p:http-request">
<p:input port="source"/>
<p:output port="result"/>
</p:declare-step>

The HTTP Request step provides interactions with resources identified by URIs over HTTP or HTTPS. The input document provided on the source port specifies the request by a single c:http-request element. This element specifies the method, resource, and other request properties as well as possibly including an entity body (content) for the request.

When the request is formulated, the step and/or protocol implementation may add headers as necessary to either complete the request or as appropriate for the content specified (e.g. transfer encodings). A user of this step is guaranteed that their requested headers and content will be sent with the exception of any conflicts with protocol-related headers. If the user of the step requests a header value (e.g. content-type) that conflicts with a value the step and/or protocol implementation must set, the step will fail.

The response received after making the request is handled as follows:

A single c:http-response element is produced on the output port result with the status attribute containing the status of the response received.
Each response header whose name does not start with "Content-" is translated into a c:header element.
Unless the status-only attribute has a value 'yes', the entity body of the response is converted into a c:body or c:multipart element via the rules given in this section.

A request or response may be multipart per RFC 1521. In those situations, the entity is represented by a c:multipart element that contains multiple c:body elements inside. In the case of a request, the media type of the c:multipart must be a multipart media type (i.e. have a main type of 'multipart').

If the media type of the response is a text type with a charset parameter that is a unicode character encoding, the children of the constructed c:body element is the translation of the text into a unicode character sequence

If the media type of the response is an XML media type, the content of the constructed c:body element is the result of parsing the body with an XML parser. If the content is not well-formed, the step fails.

For all other media types, the response is encoded as base64 and produced as text children of the c:body element.

In the case of a multipart response, the same rules apply when constructing a c:body element for each body part encountered.

The following sections describe the structure of the request and the response.

A.2.1.1 c:http-request

A HTTP request is represented by a c:http-request element.

<c:http-request method? = NCName href? = anyURI status-only? = yes|no override-content-type? = string> (c:header*, (c:multipart | c:body)?) </c:http-request>

The method attribute specifies the method to be used against the URI specified by the href attribute. If the href attribute is a relative URI, it will resolve against the base URI of the element.

Both the status-only and override-content-type attributes are not used in formulating the response. The override-content-type attribute controls interpretation of the response's content-type. If this attribute is specified, the response will be treated as if it returned that content-type. If the override-content-type value cannot be used (e.g. text/plain to override image/png), the step fails. The original media type value will still be provided in the response XML.

Also, if status-only attribute has the value yes, the entity of the response will not be processed to produce a c:body or c:multipart element.

A.2.1.2 c:header

A c:header is a name-value pair passed in the headers of the request or its response.

<c:header name = string value? = string />

A.2.1.3 c:multipart

The c:multipart element holds a set of body parts for a message request or response.

<c:multipart content-type? = string> c:body+ </c:multipart>

If the content-type attribute is not specified, a value of "multipart/mixed" will be assumed.

A.2.1.4 c:body

The c:body element holds the body or body part of the message. Each of the attributes holds controls some aspect of the encoding of the body or body part when the request is formulated. These are specified as follows:

The content-type attribute specifies the media type of the content. If the media type is not an XML type nor is it text, the content must be already be base64 encoded.
The encoding attribute specifies the request's Content-Transfer-Encoding header. If the value of encoding is 'base64' but the content type does not require such an encoding, the c:body element is assumed to contain base64 encoded content of that media type.
The id attribute specified the value of the Content-ID header for body parts.
The description attribute specified the value of the Content-Description header for body parts.

Any XML media type is assumed to be children of the c:body element in unescaped form. That is, the following is the request that sends a small XML document as the entity's body:


<c:http-request method="POST" href="http://example.com/someservice">
<c:body xmlns:c="http://www.w3.org/2007/03/xproc-step" content-type="application/xml">
<doc>
<title>My document</title>
</doc>
</c:body>
</c:http-request>

<c:body content-type = string encoding? = string id? = string description? = string> anyElement* </c:body>

A.2.1.5 c:http-response

The c:http-response element represents an HTTP response. The response's status code is encoded in the status attribute and the headers and entity body are processing into the c:header and c:multipart or c:body, respectively.

<c:http-response status? = integer> (c:header*, (c:multipart | c:body)?) </c:http-response>

A.2.1.6 HTTP Request Examples

For example, a form post would be formulated as:

<c:http-request method="POST" href="http://www.example.com/form-action" xmlns:c="http://www.w3.org/2007/03/xproc-step">
<c:body content-type="application/x-www-form-urlencoded">
name=W3C&spec=XProc
</c:body>
</c:http-request>

and if the response was an XHTML document, the response would be:

<c:http-response status="200" xmlns:c="http://www.w3.org/2007/03/xproc-step">
<c:header name="Date" value=" Wed, 09 May 2007 23:12:24 GMT"/>
<c:header name="Server" value="Apache/1.3.37 (Unix) PHP/4.4.5"/>
<c:header name="Vary" value="negotiate,accept"/>
<c:header name="TCN" value="choice"/>
<c:header name="P3P" value="policyref=&quot;http://www.w3.org/2001/05/P3P/p3p.xml&quot;"/>
<c:header name="Cache-Control" value="max-age=600"/>
<c:header name="Expries" value="Wed, 09 May 2007 23:22:24 GMT"/>
<c:header name="Last-Modified" value="Tue, 08 May 2007 16:10:49 GMT"/>
<c:header name="ETag" value="&quot;4640a109;42380ddc&quot;"/>
<c:header name="Accept-Ranges" value="bytes"/>
<c:header name="Keep-Alive" value="timeout=2, max=100"/>
<c:header name="Connection" value="Keep-Alive"/>
<c:body content-type="application/xhtml+xml">
<html xmlns="http://www.w3.org/1999/xhtml">
<head><title>OK</title></head>
<body><p>OK!</p></body>
</html>
</c:body>
</c:http-response>

A.2.2 Relax NG Validate

The Relax NG Validate step applies Relax validation to an XML document input.

<p:declare-step type="p:validate-relax-ng">
     <p:input port="source" primary="yes"/>
     <p:input port="schema"/>
     <p:output port="result"/>
     <p:option name="dtd-compatibility" value="no"/>
</p:declare-step>

If the dtd-annotations option is “yes”, then the conventions of the RELAX NG DTD Compatibility (@@ cite) are also applied.

A.2.3 XML Schema Validate

The XML Schema Validate step applies XML Schema's validity assessment to an XML document input. The set of known available schemata are specified via a sequence of documents provided on the schema input port.

If assert-valid option has a value of yes, the step will fail if the XML Schema validation assessment reports any errors. When XML Schema validation assessment is performed, the processor is invoked in the mode specified by the mode option. The result of the assessment produces an infoset with the PSVI annotations if the pipeline implementation supports such annotations. Otherwise, the input document is reproduced with any defaulting of attributes and elements performed as specified by the XML Schema recommendation.

<p:declare-step type="p:validate-xml-schema">
     <p:input port="source" primary="yes"/>
     <p:input port="schema" sequence="yes"/>
     <p:output port="result"/>
     <p:option name="assert-valid" value="yes"/>
     <p:option name="mode" value="strict"/>
</p:declare-step>

A.2.4 XSLT 2.0

The XSLT 2.0 step applies an XSLT 2.0 transformation to a document. The transformation is supplied on the port named 'transform' and the primary input document is supplied on the port named 'document'. The application of the transformation produces the primary result document on the port named 'result'.

<p:declare-step type="p:xslt2">
     <p:input port="source" sequence="yes" primary="yes"/>
     <p:input port="stylesheet"/>
     <p:input port="parameters" kind="parameter"/>
     <p:output port="result" primary="yes"/>
     <p:output port="secondary" sequence="yes"/>
     <p:option name="initial-mode"/>
     <p:option name="template-name"/>
     <p:option name="allow-version-mismatch" value="yes"/>
     <p:option name="output-base-uri"/>
     <p:option name="allow-collections" value="yes"/>
</p:declare-step>

If a sequence of documents is provided on the input port named 'source', the first document is assumed to be the primary input document. By default, this sequence is also the default collection unless the 'allow-collections' option is set to 'no'.

The invocation of the transformation is controlled by the 'initial-mode' and 'template-name' option that set the initial mode and/or named template in the XSLT transformation that should initiate processing. If these values do not match the transformation specified, a dynamic error must be thrown.

The 'allow-version-mismatch' option indicates whether an XSLT 1.0 transformation should be allowed to be run through the XSLT 2.0 processor. A value of 'yes' means that it should be allow.

The 'output-base-uri' option sets the context's output base URI per the XSLT 2.0 specification.

If more than one document is produced, the secondary result documents are produced on the output port named 'secondary'. Otherwise, the 'secondary' port produces an empty sequence.

A.2.5 XSL Formatter

The XSL Formatter step receives an XSL FO document and renders the content. The result of rendering is stored to the uri provided via the 'uri' option. A reference to that result is produced on the output port.

The output content type is controlled by the 'output' option which contains the mime type of the output format. A formatter may take any number of optional rendering parameters via the step's parameters. Such parameters are defined by the XSL implementation used and are implementation defined.

<p:declare-step type="p:xsl-formatter">
     <p:input port="source"/>
     <p:output port="result" primary="no"/>
     <p:option name="uri" required="yes"/>
     <p:option name="output"/>
</p:declare-step>

The output of this step is a document containing a single c:result whose href attribute points to the output of the XSL formatter.

A.2.6 XQuery 1.0

The XQuery 1.0 step applies an XQuery to a sequence of documents treated as the default collection. The 'source' input port allows a sequence of documents and specifies each document that should be in the default collection. The result of the xquery is a sequence of documents constructed from a XPath 2.0 sequence of elements. Each element in the sequence is assumed to be the document element of a separate document. It is an error if the sequence contains items other than elements.

<p:declare-step type="p:xquery">
     <p:input port="source" sequence="yes" primary="yes"/>
     <p:input port="query"/>
     <p:input port="parameters" kind="parameter"/>
     <p:output port="result" sequence="yes"/>
</p:declare-step>

The 'query' port must receive a single document whose element is 'query' in the step vocabulary namespace. As XQuery is not necessarily well-formed XML, the text descendants of this element are considered the query.

<c:query> string </c:query>

For example:

<c:query>
declare namespace atom="http://www.w3.org/2005/Atom";
/atom:feed/atom:entry
</c:query>

B Conformance

Conformant processors must implement all of the features described in this specification except those that are explicitly identified as optional.

Some aspects of processor behavior are not completely specified; those features are either implementation-dependent or implementation-defined.

[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.]

C References

[XML Core Req] XML Processing Model Requirements. Dmitry Lenkov, Norman Walsh, editors. W3C Working Group Note 05 April 2004

[Infoset] XML Information Set (Second Edition). John Cowan, Richard Tobin, editors. W3C Working Group Note 04 February 2004.

[XML 1.0] Extensible Markup Language (XML) 1.0 (Fourth Edition). Tim Bray, Jean Paoli, C. M. Sperberg-McQueen, et. al. editors. W3C Recommendation 16 August 2006.

[Namespaces 1.0] Namespaces in XML 1.0 (Second Edition). Tim Bray, Dave Hollander, Andrew Layman, et. al., editors. W3C Recommendation 16 August 2006.

[XML 1.1] Extensible Markup Language (XML) 1.1 (Second Edition). Tim Bray, Jean Paoli, C. M. Sperberg-McQueen, et. al. editors. W3C Recommendation 16 August 2006.

[Namespaces 1.1] Namespaces in XML 1.1 (Second Edition). Tim Bray, Dave Hollander, Andrew Layman, et. al., editors. W3C Recommendation 16 August 2006.

[XPath 1.0] XML Path Language (XPath) Version 1.0. James Clark and Steve DeRose, editors. W3C Recommendation. 16 November 1999.

[XSLT 1.0] XSL Transformations (XSLT) Version 1.0. James Clark, editor. W3C Recommendation. 16 November 1999.

[XPath 2.0] XML Path Language (XPath) 2.0. Anders Berglund, Scott Boag, Don Chamberlin, et. al., editors. W3C Recommendation. 23 January 2007.

[XPath 2.0 Functions and Operators] XQuery 1.0 and XPath 2.0 Functions and Operators. Ashok Malhotra, Jim Melton, and Norman Walsh, editors. W3C Recommendation. 23 January 2007.

[XSLT 2.0] XSL Transformations (XSLT) Version 2.0. Michael Kay, editor. W3C Recommendation. 23 January 2007.

[XSL 1.1] Extensible Stylesheet Language (XSL) Version 1.1. Anders Berglund, editor. W3C Recommendation. 5 December 2006.

[XQuery 1.0] XQuery 1.0: An XML Query Language. Scott Boag, Don Chamberlin, Mary Fernández, et. al., editors. W3C Recommendation. 23 January 2007.

[RELAX NG] ISO/IEC JTC 1/SC 34. ISO/IEC FDIS 19757-2:2002(E) Document Schema Definition Languages (DSDL) — Part 2: Grammar-based validation — RELAX NG 2002.

[Schematron] ISO/IEC JTC 1/SC 34. ISO/IEC FDIS 19757-2:2002(E) Document Schema Definition Languages (DSDL) — Part 3: Rule-based validation — Schematron 2004.

[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.

[W3C XML Schema: Part 2] XML Schema Part 2: Structures Second Edition. Paul V. Biron and Ashok Malhotra, 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.

[XInclude] XML Inclusions (XInclude) Version 1.0 (Second Edition). Jonathan Marsh, David Orchard, and Daniel Veillard, editors. W3C Recommendation. 15 November 2005.

[XML Base] XML Base. Jonathan Marsh, editor. W3C Recommendation. 27 June 2001.

[RFC 1521] RFC 1521: MIME (Multipurpose Internet Mail Extensions) Part One: Mechanisms for Specifying and Describing the Format of Internet Message Bodies. N. Borenstein, N. Freed, editors. Internet Engineering Task Force. September, 2003.

[RFC 2616] RFC 2616: Hypertext Transfer Protocol — HTTP/1.1. R. Fielding, J. Gettys, J. Mogul, et. al., editors. Internet Engineering Task Force. June, 1999.

[RFC 3548] RFC 3548: The Base16, Base32, and Base64 Data Encodings. S. Josefsson, Editor. Internet Engineering Task Force. July, 2003.

D Glossary

atomic step: An atomic step is a step that performs a unit of XML processing, such as XInclude or transformation, and has no internal subpipeline.
binding: A binding associates an input or output port with some data source.
by URI: A document is specified by URI if it is referenced with a URI.
by source: A document is specified by source if it references a specific port on another step.
compound step: A compound step is a step that contains additional steps. 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.
contained steps: The steps that occur directly inside a compound step are called contained steps.
container: A compound step which immediately contains another step is called its container.
declared inputs: The input ports declared on a step are its declared inputs.
declared options: The options declared on a step are its declared options.
declared outputs: The output ports declared on a step are its declared outputs.
default readable port: The default readable port, which may be undefined, is a specific step name/port name pair from the set of readable ports.
dynamic error: A dynamic error is one which occurs while a pipeline is being evaluated.
empty environment: The empty environment contains no readable ports, no in-scope options, and an undefined default readable port.
empty sequence: An empty sequence of documents is specified with the p:empty element.
environment: The environment of a step is the static information available to each instance of a step in a pipeline.
extension attribute: 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.
implementation-defined: 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.
implementation-dependent: 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.
in-scope options: The in-scope options are the set of options that are visible to a step.
inherited environment: The inherited environment of a contained step is an environment that is the same as the environment of its container with the standard modifications.
inline document: An inline document is specified directly in the body of the element that binds it.
last step: The last step in a subpipeline is the last step in document order within its container.
matches: 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.
option: An option is a name/value pair where the name is an expanded name and the value must be a string.
parameter: A parameter is a name/value pair where the name is an expanded name and the value must be a string.
pipeline: A pipeline is a set of connected steps, outputs flowing into inputs, without any loops (no step can read its own output, directly or indirectly).
primary input port: If a step has exactly one input port, or if one of its input ports is explicitly designated as the primary, then that input port is the primary input port of the step.
primary output port: If a step has exactly one output port, or if one of its output ports is explicitly designated as the primary, then that output port is the primary output port of the step.
readable ports: The readable ports are the step name/output port name pairs that are visible to the step.
signature: The signature of a step is the set of inputs, outputs, and options that it is declared to accept.
specified options: The options on a step which have specified values, either because a p:option element specifies a value or because the declaration included a default value, are its specified options.
static error: A static error is one which can be detected before pipeline evaluation is even attempted.
step: A step is the basic computational unit of a pipeline. Steps are either atomic or compound.
subpipeline: The steps (and the connections between them) within a compound step form a subpipeline.

E 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.

<p:for-each name? = NCName> (p:iteration-source?, (p:output | p:option | p:log)*, subpipeline) </p:for-each>

<p:viewport name? = NCName match = XPath expression> ((p:viewport-source?, p:output?, p:log?, p:option*), subpipeline) </p:viewport>

<p:choose name? = NCName> (p:xpath-context?, p:when*, p:otherwise?) </p:choose>

<p:when test = XPath expression> (p:xpath-context?, (p:output | p:option | p:log)*, subpipeline) </p:when>

<p:otherwise> ((p:output | p:option | p:log)*, subpipeline) </p:otherwise>

<p:group name? = NCName> ((p:output | p:option | p:log)*, subpipeline) </p:group>

<p:try name? = NCName> (p:group, p:catch) </p:try>

<p:catch name? = NCName> ((p:output | p:option | p:log)*, subpipeline) </p:catch>

<pfx:other-atomic-step name? = NCName> (p:input | p:option | p:parameter | p:log)* </pfx:other-atomic-step>

<pfx:other-compound-step name? = NCName> ((p:input | p:output | p:option | p:log)*, subpipeline) </pfx:other-compound-step>

<p:input port = NCName sequence? = yes|no primary? = yes|no kind? = "document" />

<p:input port = NCName sequence? = "yes" kind = "parameter" />

<p:iteration-source select? = XPath expression> (p:empty | (p:pipe | p:document | p:inline)+)? </p:iteration-source>

<p:viewport-source> (p:pipe | p:document | p:inline)? </p:viewport-source>

<p:xpath-context> (p:empty | p:pipe | p:document | p:inline)? </p:xpath-context>

<p:output port = NCName sequence? = yes|no primary? = yes|no />

<p:log port = NCName href = anyURI />

<p:option name = QName required? = yes|no />

<p:option name = QName select = XPath expression required? = yes|no> (p:empty | p:pipe | p:document | p:inline)? </p:option>

<p:option name = QName value = string required? = yes|no />

<p:parameter name = QName select = XPath expression> (p:empty | p:pipe | p:document | p:inline)? </p:parameter>

<p:parameter name = QName value = string />

<p:declare-step type = QName> (p:input | p:output | p:option)* </p:declare-step>

<p:pipeline-library namespace? = anyURI ignore-prefixes? = prefix list> (p:import | p:declare-step | p:pipeline)* </p:pipeline-library>

<p:import href = anyURI />

<p:pipe step = NCName port = NCName />

<p:inline> anyElement </p:inline>

<p:document href = anyURI />

<p:empty />

<p:documentation> any-well-formed-content* </p:documentation>

The core steps are also summarized here.