QName"> must"> XML"> ]> xproc 2008-05-01 XML Revision markup Norman Walsh Sun Microsystems, Inc. Norman.Walsh@Sun.COM Alex Milowski Invited expert alex@milowski.org Henry S. Thompson University of Edinburgh ht@inf.ed.ac.uk 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 zero or more XML documents. Pipelines generally accept zero or more XML documents as input and produce zero or more XML documents as output. Pipelines are made up of simple steps which perform atomic operations on XML documents and constructs similar to conditionals, iteration, and exception handlers which control which steps are executed. 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. Since the last public working draft, the Working Group has considered several hundred comments in nearly 150 threads. We've responded to many of these by changing the specification. Some of the significant changes in this draft are: Removed implicit pipeline inputs and outputs. See . Support either XPath 1.0 or XPath 2.0 as the pipeline expression language. Support both XSLT 1.0 and XSLT 2.0 on a single p:xslt step. Added p:hash, p:uuid, p:www-form-urldecode, and p:www-form-urlencode. Added . Added a p:language system property. Added fixup-xml-base and fixup-xml-lang options to p:xinclude. Renamed c:http-request and c:http-response to simply c:request and c:response. Added psvi-required attribute to pipelines. Fairly substantial syntax changes. A p:pipeline is now just syntactic sugar for a particular p:declare-step. Changed definition of p:error to better address localization issues. Significantly reworked the syntax and semantics of variables, options, and parameters. Added p:variable. Imposed a syntactic distinction between declaration (p:option) and use (p:with-option/p:with-param) of options and parameters. Clarified the scope of variables and options. Removed value attribute from p:variable, p:option, p:with-option, and p:with-param. Removed automatic declaration of parameter input ports. Added p:base-uri and p:resolve-uri functions to support (XPath 1.0) pipelines that need access to the base URI of documents. Removed ignored namespaces, added p:pipeinfo. Redefined the p:label-elements step to use a step-local variable in the XPath context. 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.

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 inputs and produce zero or more XML documents as their outputs. The inputs of a step come from the web, from the pipeline document, from the inputs to the pipeline itself, or from the outputs of other steps in the pipeline. The outputs from a step are consumed by other steps, are outputs of the pipeline as a whole, or are discarded. There are 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 control the execution of other steps, which they include in the form of one or more subpipelines. This specification defines a standard library, , of steps. Pipeline implementations may support additional types of steps as well. is a graphical representation of a simple pipeline that performs XInclude processing and validation on a document.

A simple, linear XInclude/Validate pipeline

This is a pipeline that consists of two atomic steps, XInclude and Validate with XML Schema. The pipeline itself has two inputs, “source” (a source document) and “schemas” (a sequence of W3C XML Schemas). The XInclude step reads the pipeline input “source” and produces a result document. The Validate with XML Schema step reads the pipeline input “schemas” and the result of the XInclude step and produces its own result document. The result of the validation, “result”, is the result of the pipeline. (For consistency across the step vocabulary, the standard input is usually named “source” and and the standard output is usually named “result”.) The pipeline document determines how the steps are connected together inside the pipeline. How inputs are connected to XML documents outside the pipeline is implementation-defined. How pipeline outputs are connected to XML documents outside the pipeline is implementation-defined. The pipeline document for this pipeline is shown in . <p:declare-step xmlns:p="http://www.w3.org/ns/xproc" name="xinclude-and-validate"> <p:input port="source" primary="true"/> <p:input port="schemas" sequence="true"/> <p:output port="result"> <p:pipe step="validated" port="result"/> </p:output> <p:xinclude name="included"> <p:input port="source"> <p:pipe step="xinclude-and-validate" port="source"/> </p:input> </p:xinclude> <p:validate-with-xml-schema name="validated"> <p:input port="source"> <p:pipe step="included" port="result"/> </p:input> <p:input port="schema"> <p:pipe step="xinclude-and-validate" port="schemas"/> </p:input> </p:validate-with-xml-schema> </p:declare-step> The example in is very verbose. It makes all of the connections seen in the figure explicit. In practice, pipelines do not have to be this verbose. XProc supports defaults for many common cases: If you use p:pipeline instead of p:declare-step, the “source” input port and “result” output port are implicitly declared for you. Where inputs and outputs are connected between sequential sibling steps, they do not have to be made explicit. The same pipeline, using XProc defaults, is shown in . <p:pipeline xmlns:p="http://www.w3.org/ns/xproc" name="xinclude-and-validate"> <p:input port="schemas" sequence="true"/> <p:xinclude/> <p:validate-with-xml-schema> <p:input port="schema"> <p:pipe step="xinclude-and-validate" port="schemas"/> </p:input> </p:validate-with-xml-schema> </p:pipeline> is a more complex example: it performs schema validation with an appropriate schema and then styles the validated document.

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. <p:pipeline xmlns:p="http://www.w3.org/ns/xproc"> <p:choose> <p:when test="/*[@version &lt; 2.0]"> <p:validate-with-xml-schema> <p:input port="schema"> <p:document href="v1schema.xsd"/> </p:input> </p:validate-with-xml-schema> </p:when> <p:otherwise> <p:validate-with-xml-schema> <p:input port="schema"> <p:document href="v2schema.xsd"/> </p:input> </p:validate-with-xml-schema> </p:otherwise> </p:choose> <p:xslt> <p:input port="stylesheet"> <p:document href="stylesheet.xsl"/> </p:input> </p:xslt> </p:pipeline> This example, like the preceding, relies on XProc defaults for simplicity. It is always valid to write the fully explicit form if you prefer. The media type for pipeline documents is application/xml. Often, pipeline documents are identified by the extension .xpl.

A pipeline is a set of connected steps, with outputs of one step flowing into inputs of another. A pipeline is itself a step and must satisfy the constraints on steps. Connections between steps occur where the input of one step is bound to the output of another. The result of evaluating a pipeline (or subpipeline) is the result of evaluating the steps that it contains, in an order consistent with the connections between them. A pipeline must behave as if it evaluated each step each time it occurs. Unless otherwise indicated, implementations must not assume that steps are functional (that is, that their outputs depend only on their inputs, options, and parameters) or side-effect free. The pattern of connections between steps will not always completely determine their order of evaluation. The evaluation order of steps not connected to one another is implementation-dependent.

A step is the basic computational unit of a pipeline. A typical step has zero or more inputs, from which it receives XML documents to process, zero or more outputs, to which it sends XML document results, and can have options and/or parameters. There are three kinds of steps: atomic, compound, and multi-container. 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; a Validate with XML Schema step validates one input with respect to some set of XML Schemas, etc. There are many types of atomic steps. The standard library of atomic steps is described in , but implementations may provide others as well. What additional step types, if any, are provided is implementation-defined. 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. A compound step is a step that contains a subpipeline. That is, a compound step differs from an atomic step in that its semantics are at least partially determined by the steps that it contains. Finally, there are two “multi-container steps”: p:choose and p:try. A multi-container step is a step that contains several alternate subpipelines. Each subpipeline is identified by a non-step wrapper element: p:when and p:otherwise in the case of p:choose, p:group and p:catch in the case of p:try. The runtime semantics of a multi-container step are that it behaves as if it evaluated exactly one of its subpiplines. In this sense, they function like compound steps. A compound step or multi-container step is a container for the steps directly within it or within non-step wrappers directly within it. The steps that occur directly within, or within non-step wrappers directly within, a step are called that step's contained steps. In other words, “container” and “contained steps” are inverse relationships. The ancestors of a step are its container, the container of its container, and all other containers above it. Sibling steps (and the connections between them) form a subpipeline. The last step in a subpipeline is its last step in document order. subpipeline = p:variable*, (p:for-each|p:viewport|p:choose|p:group|p:try|p:standard-step|pfx:user-pipeline)+ Note that user-defined pipelines, pfx:user-pipeline, are atomic; although a pipeline declaration, contains a subpipeline, a step which invokes a user-defined pipeline does not. 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; a step can have zero input ports and/or zero output ports. (All steps have an implicit output port for reporting errors that must not be declared.) The names of all ports on each step must be unique on that step (you can't have two input ports named “source”, nor can you have an input port named “schema” and an output port named “schema”). Steps may have any number of options, all with unique names. A step can have zero options. Steps may have parameter input ports, on which parameters can be passed. The parameters passed on a particular parameter input port must be uniquely named. If multiple parameters with the same name are used, only one of the values will actually be available to the step. A step can have zero, one, or many parameter input ports, and each parameter port can have zero or more parameters passed on it. All of the different instances of steps (atomic or compound) in a pipeline can be distinguished from one another by name. If the pipeline author does not provide a name for a step, a default name is manufactured automatically.

The name attribute on any step can be used to give it a name. The name must be unique within its scope, see . If the pipeline author does not provide an explicit name, the processor manufactures a default name. All default names are of the form “!1.m.n…” where “m” is the position of the step's highest ancestor within the pipeline document or library which contains it, “n” is the position of the next-highest ancestor, and so on, including both steps and non-step wrappers. For example, consider the pipeline in . The p:pipeline step has no name, so it gets the default name “!1”; the p:choose gets the name “!1.2”; the first p:when gets the name “!1.2.1”, etc. If the p:choose had had a name, it would not have received a default name, but it would still have been counted and its first p:when would still have been “!1.2.1”. Providing every step in the pipeline with an interoperable name has several benefits: It allows implementors to refer to all steps in an interoperable fashion, for example, in error messages. Pragmatically, we say that readable ports are identified by a step name/port name pair. By manufacturing names for otherwise anonymous steps, we include implicit bindings without changing our model. In a valid pipeline that runs successfully to completion, the manufactured names aren't visible (except perhaps in debugging or logging output). The format for defaulted names does not conform to the requirements of an NCName. This is an explicit design decision; it prevents pipelines from using the defaulted names on p:pipe elements. If an explicit connection is required, the pipeline author must provide an explicit name for the step.

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. For the purposes of this specification, an XML document is an . Implementations are free to transmit infosets as sequences of characters, sequences of events, object models, or any other representation that preserves the necessary infoset properties (see ). Most steps in this specification manipulate XML documents, or portions of XML documents. In these cases, we speak of changing elements, attributes, or nodes without prejudice to the actual representation used by an implementation. 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 data cannot arrive on an input port to a step. It is a dynamic error if a non-XML resource is produced on a step output or arrives on a step input. The common case is that each step has one or more inputs and one or more outputs. illustrates symbolically an atomic step with two inputs and one output.

An atomic step with two inputs and one output

All atomic steps are defined by a p:declare-step. The declaration of an atomic step type defines the input ports, output ports, and options of all steps of that type. For example, every p:validate-with-xml-schema step has two inputs, named “source” and “schema”, one output named “result”, and the same set of options. Like atomic steps, top level, user-defined pipelines also have declarations. The situation is slightly more complicated for the other compound steps because they don't have separate declarations; each instance of the compound step serves as its own declaration. On these compound steps, the number and names of the outputs can be different on each instance of the step. illustrates symbolically a compound step with one subpipeline and one output. As you can see from the diagram, the output from the compound step comes from one of the outputs of the subpipeline within the step.

A compound step with two inputs and one output

The input ports declared on a step are its declared inputs. The output ports declared on a step are its declared outputs. When a step is used in a pipeline, it is connected to other steps through its inputs and outputs. When a step is used, all of the declared inputs of the step must be connected. Each input can be connected to: The output port of some other step. A fixed, inline document or sequence of documents. A document read from a URI. One of the inputs declared on one of its ancestors. A special port provided by an ancestor compound step, for example, “current” in a p:for-each or p:viewport. When an input accepts a sequence of documents, the documents can come from any combination of these locations. The declared outputs of a step may be connected to: The input port of some other step. One of the outputs declared on its container. The primary output port of a step must be connected, but other outputs can remain unconnected. Any documents produced on an unconnected output port are discarded. Output ports on compound steps have a dual nature: from the perspective of the compound step's siblings, its outputs are just ordinary outputs and must be connected as described above. From the perspective of the subpipeline inside the compound step, 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. It is also not an error to connect a port that is declared to produce a single document to a port that is declared to accept a sequence. A single document is the same as a sequence of one document. An output port may be connected to more than one input port. At runtime this will result in distinct copies of the output. The signature of a step is the set of inputs, outputs, and options that it is declared to accept. The declaration for a step provides a fixed signature which all its instances share. A step matches its signature if and only if it specifies an input for each declared input, it specifies no inputs that are not declared, it specifies an option for each option that is declared to be required, and it specifies no options that are not declared. In other words, every input and required option must be specified and only inputs and options that are declared may be specified. Options that aren't required do not have to be specified. Steps may also produce error, warning, and informative messages. These messages are captured and provided on the error port inside of a p:catch. Outside of a try/catch, the disposition of error messages is implementation-dependent.

It's common for some of the documents used in processing a pipeline to be read from URIs. Sometimes this occurs directly, for example with a p:document element. Sometimes it occurs indirectly, for example if an implementation allows the URI of a pipeline input to be specified on the command line or if an p:xslt step encounters an xsl:import in the stylesheet that it is processing. It's also common for some of the documents produced in processing a pipeline to be written to locations which have, or at least could have, a URI. The process of dereferencing a URI to retrieve a document is often more interesting than it seems at first. On the web, it may involve caches, proxies, and various forms of indirection. Resolving a URI locally may involve resolvers of various sorts and possibly appeal to implementation-dependent mechanisms such as catalog files. In XProc, the situation is made even more interesting by the fact that many intermediate results produced by steps in the pipeline have base URIs. Whether or not (and when and how) the intermediate results that pass between steps are ever written to a filesystem is implementation-dependent. In Version 1.0 of XProc, how (or if) implementers provide local resolution mechanisms and how (or if) they provide access to intermediate results by URI is implementation-defined. Version 1.0 of XProc does not require implementations to guarantee that multiple attempts to dereference the same URI always produce consistent results. On the one hand, this is a somewhat unsatisfying state of affairs because it leaves room for interoperability problems. On the other, it is not expected to cause such problems very often in practice. If these problems arise in practice, implementers are encouraged to use the existing extension mechanisms to give users the control needed to circumvent them. Should such mechanisms become widespread, a standard mechanism could be added in some future version of the language.

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. If a step has a document input port which is explicitly marked “primary='true'”, or if it has exactly one document input port and that port is not explicitly marked “primary='false'”, then that input port is the primary input port of the step. If a step has a single input port and that port is explicitly marked “primary='false'”, or if a step has more than one input port and none is explicitly marked as the primary, then the primary input port of that step is undefined. A step can have at most one primary input port. If a step has a document output port which is explicitly marked “primary='true'”, or if it has exactly one document output port and that port is not explicitly marked “primary='false'”, then that output port is the primary output port of the step. If a step has a single output port and that port is explicitly marked “primary='false'”, or if a step has more than one output port and none is explicitly marked as the primary, then the primary output port of that step is undefined. A step can have at most one primary output port. The special significance of primary input and output ports is that they are connected automatically by the processor if no explicit 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 outputs and the last step in its subpipeline has an unbound primary output, then an implicit primary output port will be added to the compound step (and consequently the last step's primary output will be bound to it). This implicit output port has no name. It inherits the sequence property of the port bound to it.

Steps are connected together by their input ports and output ports. 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.

XProc processors are expected, and sometimes required, to perform namespace fixup. Unless the semantics of a step explicitly says otherwise: The in-scope namespaces associated with a node (even those that are inherited from namespace bindings that appear among its ancestors in the document in which it appears initially) are assumed to travel with it. Changes to one part of a tree (wrapping or unwrapping a node or renaming an element, for example) do not change the in-scope namespaces associated with the descendants of the node so changed. As a result, some steps can produce XML documents which have no direct serialization (because they include nodes with conflicting or missing namespace declarations, for example). To produce a serializable XML document, the XProc processor must sometimes add additional namespace nodes, perhaps even renaming prefixes, to satisfy the constraints of Namespaces in XML. This process is referred to as namespace fixup. Implementors are encouraged to perform namespace fixup before passing documents between steps, but they are not required to do so. Conversely, an implementation which does serialize between steps and therefore must perform such fixups, or reject documents that cannot be serialized, is also conformant. Except where the semantics of a step explicitly require changes, processors are required to preserve the information in the documents and fragments they manipulate. In particular, the information corresponding to the properties attributes, base URI, children, local name, namespace name, normalized value, owner, and parent must be preserved. The information corresponding to prefix, in-scope namespaces, namespace attributes, and attribute type should be preserved, with changes to the first three only as required for namespace fixup. In particular, processors are encouraged to take account of prefix information in creating new namespace bindings, to minimize negative impact on prefixed names in content. Except for cases which are specifically called out in , the extent to which namespace fixup, and other checks for outputs which cannot be serialized, are performed on intermediate outputs is implementation-defined. Whenever an implementation serializes pipeline contents, for example for pipeline outputs, logging, or as part of steps such as p:store or p:http-request, it is a dynamic error if that serialization could not be done so as to produce a document which is both well-formed and namespace-well-formed, as specified in XML and Namespaces in XML, regardless of what serialization method, if any, is called for.

The environment is a context-dependent collection of information available withing sub-pipelines. Most of the information in the environment is static and can be computed for each subpipeline before evaluation of the pipeline as a whole begins. The in-scope bindings have to be calculated as the pipeline is being evaluated. The environment consists of: A set of readable ports. The readable ports are a set of step name/port name pairs. Inputs and outputs can only be connected to readable ports. A default readable port. The default readable port, which may be undefined, is a specific step name/port name pair from the set of readable ports. A set of in-scope bindings. The in-scope bindings are a set of name-value pairs, based on option and variable bindings. The empty environment contains no readable ports, an undefined default readable port and no in-scope bindings. Unless otherwise specified, the environment of a contained step is its 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. The standard modifications made to an inherited environment are: The declared inputs of the container are added to the readable ports. In other words, contained steps can see the inputs to their container. The union of all the declared outputs of all of the containers's 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 the primary input port of the container, if it has one, otherwise the default readable port is unchanged. The names and values from each p:variable present at the beginning of the container are added, in document order, to the in-scope bindings. A new binding replaces an old binding with the same name. See for the specification of variable evaluation. A step with no parent inherits the empty environment.

XProc uses XPath as an expression language. XPath expressions are evaluated by the XProc processor in several places: on compound steps, to compute the default values of options and the values of variables; on atomic steps, to compute the actual values of options and the values of parameters. XPath expressions are also passed to some steps. These expressions are evaluated by the implementations of the individual steps. This distinction can be seen in the following example: <p:variable name="home" select="'http://example.com/docs'"/> <p:load name="read-from-home"> <p:with-option name="href" select="concat($home,'/document.xml')"/> </p:load> <p:split-sequence name="select-chapters" test="@role='chapter'"> <p:input port="source" select="//section"/> </p:split-sequence> The href option of the p:load step is evaluated by the XProc processor. The actual href option received by the step is simply the string literal “http://example.com/docs/document.xml”. (The selection on the source input of the select-chapters step is also evaluated by the XProc processor.) The XPath expression “@role='chapter'” is passed literally to the test option on the p:split-sequence step. That's because the nature of the p:split-sequence is that it evaluates the expression. Only some options on some steps expect XPath expressions. The XProc processor evaluates all of the XPath expressions in select attributes on variables, options, parameters, and inputs, in match attributes on p:viewport, and in test attributes on p:when steps. An XProc implementation can use either or to evaluate these expressions. This is a compromise driven entirely by the timing of XProc development. During the development of this specification, the community indicated that it was too early to mandate that all implementations use XPath 2.0 and too late to mandate that all implementations use XPath 1.0. Many, many expressions that are likely to be used in XProc pipelines are the same in both versions (simple element tests, ancestor and descendant tests, string-based attribute tests, etc.). As an aid to interoperability, pipeline authors may indicate the version of XPath that they are using. The attribute xpath-version may be used on p:pipeline, p:declare-step, (or p:library) to identify the XPath version that should be used to evaluate XPath expressions on the pipeline(s). The attribute is lexically scoped, but see below. If an xpath-version is specified on a p:pipeline or p:declare-step, then that is the version of XPath that the step uses. If it does not specify a version, but a version is specified on one of its ancestors, the nearest ancestor version specified is the version that it uses. If no version is specified on the step or among its ancestors, then its XPath version is implementation-defined. The decision about which XPath version applies can be made dynamically. For example, if a pipeline explicitly labeled with xpath-version “1.0” imports a library that does not specify a version, the implementation may elect to make the implementation-defined XPath version of the steps in the library also “1.0”. If the same implementation imports that library into a pipeline explicitly labled with xpath-version “2.0”, it can make the implementation-defined version of those steps “2.0”. The following rules determine how the indicated version and the implementation's actual version interact: If the indicated version and the implementation version are the same, then that version is used. If the indicated version is 1.0 and the implementation uses XPath 2.0 (or later), the expression must be evaluated in XPath 1.0 compatibility mode. It is a static error if the processor does not support XPath 1.0 compatibility mode. If the indicated version is 2.0 (or later) and the implementation uses XPath 1.0, the implementation must not evaluate any expression that it cannot determine will give the same result in XPath 1.0 that it would have given if XPath 2.0 had been used. It is a static error if the processor cannot determine that the expression would yield the same result.

When the XProc processor evaluates an XPath expression using XPath 1.0, unless otherwise indicated by a particular step, it does so with the following context: context node The document node of a document. The document is either specified with a binding or is taken from the default readable port. It is a dynamic error if a document sequence appears where a document to be used as the context node is expected. If there is no binding and there is no default readable port then the context node is an empty document node. context position and context size The context position and context size are both “1”. variable bindings The union of the in-scope options and variables are available as variable bindings to the XPath processor. function library The core function library and the . in-scope namespaces The namespace bindings in-scope on the element where the expression occurred. When the XProc processor evaluates an XPath expression using XPath 2.0, unless otherwise indicated by a particular step, it does so with the following static context: XPath 1.0 compatibility mode Is true if the indicated XPath version is 1.0, false otherwise. Statically known namespaces The namespace declarations in-scope for the containing element or made available through p:namespaces. Default element/type namespace The null namespace. Default function namespace The function namespace. In-scope schema definitions None. In-scope variables The union of the in-scope options and variables are available as variable bindings to the XPath processor. Context item static type Document. Function signatures The signatures of the and the . Statically known collations Implementation defined but must include the Unicode codepoint collation. The version of Unicode supported is implementation-defined, but it is recommended that the most recent version of Unicode be used. Default collation Unicode codepoint collation. Base URI The base URI of the element on which the expression occurs. Statically known documents None. Statically known collections None. And the following dynamic context: context item The document node of a document. The document is either specified with a binding or is taken from the default readable port. It is a dynamic error if a document sequence appears where a document to be used as the context node is expected. If there is no binding and there is no default readable port then the context node is undefined. context position and context size The context position and context size are both “1”. Variable values The union of the in-scope options and variables are available as variable bindings to the XPath processor. Function implementations The and the . Current dateTime The point in time returned as the current dateTime is implementation-defined. Implicit timezone The implicit timezone is implementation defined. Available documents The set of available documents (those that may be retrieved with a URI) is implementation dependent. Available collections None. Default collection None.

When a step evaluates an XPath expression using XPath 1.0, it does so with the following context: context node The document node that appears on the primary input port of the step, unless otherwise specified by the step. context position and context size The position and size are both “1”, unless otherwise specified by the step. variable bindings None, unless otherwise specified by the step. function library The core function library, unless otherwise specified by the step. in-scope namespaces The set of namespace bindings provided by the XProc processor. The processor computes this set of bindings by taking a union of the bindings on the step element itself as well as the bindings on any of the options and parameters used in computing values for the step (see ). The results of computing the union of namespaces in the presence of conflicting declarations for a particular prefix are implementation-dependent. When a step evaluates an XPath expression using XPath 2.0, unless otherwise indicated by a particular step, it does so with the following static context: XPath 1.0 compatibility mode Is true if the indicated XPath version is 1.0, false otherwise. Statically known namespaces The namespace declarations in-scope for the containing element or made available through p:namespaces. Default element/type namespace The null namespace. Default function namespace The function namespace. In-scope schema definitions None. In-scope variables None, unless otherwise specified by the step. Context item static type Document. Function signatures The signatures of the . Statically known collations Implementation defined but must include the Unicode codepoint collation. Default collation Unicode codepoint collation. Base URI The base URI of the element on which the expression occurs. Statically known documents None. Statically known collections None. And the following dynamic context: context item The document node of the document that appears on the primary input of the step, unless otherwise specified by the step. context position and context size The context position and context size are both “1”, unless otherwise specified by the step. Variable values None, unless otherwise specified by the step. Function implementations The . Current dateTime An implementation defined point in time. Implicit timezone The implicit timezone is implementation defined. Available documents The set of available documents (those that may be retrieved with a URI) is implementation dependent. Available collections None. Default collection None.

The XProc processor must support a few additional functions in XPath expressions evaluated by the processor. In the following descriptions, the names of types (string, boolean, etc.) should be taken to mean the corresponding data types for an implementation that uses XPath 2.0 and as the most appropriate XPath 1.0 types for an XPath 1.0 implementation.

XPath expressions within a pipeline document can interrogate the processor for information about the current state of the pipeline. Various aspects of the processor are exposed through the p:system-property function in the pipeline namespace: 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. In other words, if a processor is run several times in succession, or if several processors are running simultaneously, each invocation of each processor should get a distinct value from p:episode. The unique identifier must consist of alphanumeric characters and must start with an alphabetic character. Thus, the string is syntactically an XML name. p:language Returns a string which identifies the current language, for example, for message localization purposes. The exact format of the language string is implementation defined but should be the same as the xml:lang attribute. p:product-name Returns a string containing the name of the implementation, as defined by the implementer. This should normally remain constant from one release of the product to the next. It should also be constant across platforms in cases where the same source code is used to produce compatible products for multiple execution platforms. p:product-version Returns a string identifying the version of the implementation, as defined by the implementer. This should normally vary from one release of the product to the next, and at the discretion of the implementer it may also vary across different execution platforms. p:vendor Returns a string which identifies the vendor of the processor. p:vendor-uri Returns a URI which identifies the vendor of the processor. Often, this is the URI of the vendor's web site. p:version Returns the version of XProc implemented by the processor; for processors implementing the version of XProc specified by this document, the value is “1.0”. The value of the version attribute is a token (i.e., an xs:token per ). p:xpath-version Returns the version of XPath implemented by the processor for evaluating XPath expressions on XProc elements.

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.

In the context of a p:for-each or a p:viewport, the p:iteration-position function reports the position of the document being processed in the sequence of documents that will be processed. In the context of other standard XProc compound steps, it returns 1. Function: integer p:iteration-position() In the context of an extension compound step, the value returned by p:iteration-position is implementation-defined.

In the context of a p:for-each or a p:viewport, the p:iteration-size function reports the number of documents in the sequence of documents that will be processed. In the context of other standard XProc compound steps, it returns 1. Function: integer p:iteration-size() In the context of an extension compound step, the value returned by p:iteration-size is implementation-defined.

Returns the base URI of the specified node, if it has one. The semantics of this function are the same as the semantics of the XPath 2.0 fn:base-uri() function. Function: string p:base-uri() Function: string p:base-uri(Node node) If no node is specified, the base URI of the context node is returned.

Resolves a relative URI with respect to a particular base URI. The semantics of this function are the same as the semantics of the XPath 2.0 fn:resolve-uri() function. Function: string p:resolve-uri(String relative) Function: string p:resolve-uri(String relative, String base) If no base is specified, the base URI of the context node is used.

It is implementation defined if the processor supports any other XPath extension functions.

Variables are name/value pairs. Pipeline authors can create variables to hold computed values. A variable 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 string value is computed and that string is used. Variables and options share the same scope and may shadow each other.

Some steps accept options. Options are name/value pairs, like variables. Unlike variables, the value of an option can be changed by the caller. 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 string value is computed and that string is used. The options declared on a step are its declared options. Option names are always expressed as literal values, pipelines cannot construct option names dynamically. The options on a step which have specified values, either because a p:with-option element specifies a value or because the declaration included a default value, are its specified options.

Some steps accept parameters. Parameters are name/value pairs, like variables and options. Unlike variables and 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 on parameter input ports. 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 string value is computed and that string is used. A parameter input port is a distinguished kind of input port which accepts (only) dynamically constructed parameter name/value pairs. See . Analogous to primary input ports, steps that have parameter inputs may designate at most one parameter input port as a primary parameter input port. If a step has a parameter input port which is explicitly marked “primary='true'”, or if it has exactly one parameter input port and that port is not explicitly marked “primary='false'”, then that parameter input port is the primary parameter input port of the step. If a step has a single parameter input port and that port is explicitly marked “primary='false'”, or if a step has more than one parameter input port and none is explicitly marked as the primary, then the primary parameter input port of that step is undefined. How an implementation maps parameters specified to the application, or through some API, to parameters accepted by the top level pipeline is implementation-defined.

An XProc pipeline may attempt to access arbitrary network resources: steps such as p:load and p:http-request can attempt to read from an arbitrary URI; steps such as p:store can attempt to write to an arbitrary location; p:exec can attempt to execute an arbitrary program. Note, also, that some steps, such as p:xslt and p:xquery, include extension mechanisms which may attempt to execute arbitrary code. In some environments, it may be inappropriate to provide the XProc pipeline with access to these resources. In a server environment, for example, it may be impractical to allow pipelines to store data. In environments where the pipeline cannot be trusted, allowing the pipeline to access arbitrary resources or execute arbitrary code may be a security risk. It is a dynamic error for a pipeline to attempt to access a resource for which it has insufficient privileges or perform a step which is forbidden. A conformant XProc processor may limit the resources available to any or all steps in a pipeline. A conformant implementation may raise dynamic errors, or take any other corrective action, for any security problems that it detects.

A pipeline author may identify the version of XProc against which a particular pipeline was authored by explicitly importing the library that identifies the steps defined by that version of XProc. For the version defined by this specification, the library is “http://www.w3.org/2008/xproc-1.0.xpl”. If the version is not explicitly identified, the implicit version should be the most recent version known to the processor. When a processor encounters a version it does not recognize, it proceeds in forwards-compatible mode. In forwards-compatible mode: The library that identifies the version of XProc is imported, see p:import. This provides the processor with declarations for any new step types. It is a dynamic error to attempt to evaluate a step type for which no implementation is known, but conditional processing and the step-available function can be used to write backwards-compatible pipelines. It is a static error if the signature of a known step in the version library has changed, except for new options. New options on known steps are ignored in the pipeline. As a consequence, future specifications must not change the semantics of existing step types without changing their names.

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. XProc is intended to work equally well with and . Unless otherwise noted, the term “XML” refers equally to both versions. Unless otherwise noted, the term Namespaces in XML refers equally to and . Support for pipeline documents written in XML 1.1 and pipeline inputs and outputs that use XML 1.1 is implementation-defined. Elements in a pipeline document represent the pipeline, the steps it contains, the connections between those steps, the steps and connections contained within them, and so on. Each step is represented by an element; a combination of elements and attributes specify how the inputs and outputs of each step are connected and how options 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 (both parameter and document), Output ports, Options and variables, and Parameters

There are four namespaces associated with XProc: http://www.w3.org/ns/xproc The namespace of the XProc XML vocabulary described by this specification; by convention, the namespace prefix “p:” is used for this namespace. http://www.w3.org/ns/xproc-step The namespace used for documents that are inputs to and outputs from several standard and optional steps described in this specification. Some steps, such as p:http-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/ns/xproc-error The namespace used for errors. The conventional prefix “err:” is used for this namespace.

Names are used to identify step types, steps, ports, options and variables, and parameters. Step types, options, variables, and parameters are named with QNames. Steps and ports are named with NCNames. The scope of a name is a measure of where it is available in a pipeline. If two names are in the same scope, we say that they are visible to each other. The scope of the names of the step types is the union of all the pipelines and pipeline libraries available directly or via p:import. In other words, the step types visible in a pipeline or library are: The standard, built-in types (p:pipeline, p:choose, etc.). Any implementation-provided types. The types visible in any library that is imported. The step types declared in the pipeline or library. The pipelines that are imported. For a pipeline, the pipeline itself. For a pipeline in a library, the types visible in the containing library. All the step types in a pipeline must have unique names: it is a static error if any step type name is built-in and/or declared or defined more than once in the same scope. The scope of the names of the steps themselves is determined by the environment of each step. In general, the name of a step, the names of its sibling steps, the names of any steps that it contains directly, the names of its ancestors, and the names of the siblings of its ancestors are all in a common scope. All steps in the same scope must have unique names: it is a static error if two steps with the same name appear in the same scope. The scope of an input or output port name is the step on which it is defined. The names of all the ports on any step must be unique. Taken together, these uniqueness constraints guarantee that the combination of a step name and a port name uniquely identifies exactly one port on exactly one in-scope step. The scope of option and variable names is determined by where they are declared. When an option is declared with p:option (or a variable with p:variable), unless otherwise specified, its scope consists of the sibling elements that follow its declaration and the descendants of those siblings. Parameter names are not scoped; they are distinct on each step.

When a relative URI appears in an option value, the base URI against which it must be made absolute is the base URI of the p:option element. If an option value is specified using a syntactic shortcut, the base URI of the step on which the shortcut attribute appears must be used. In general, whenever a relative URI appears, its base URI is the base URI of the nearest ancestor element. The pipeline author can control the base URIs of elements within the pipeline document with the xml:base attribute. The xml:base attribute may appear on any element in a pipeline and has the semantics outlined in .

A pipeline author can provide a globally unique identifier for any element in a pipeline with the xml:id attribute. The xml:id attribute may appear on any element in a pipeline and has the semantics outlined in .

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 an inline document, 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 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:output port="result"/> <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 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 A document is specified by source if it references a specific port on another step. The step and port attributes on the p:pipe element are used for this purpose. In this example, the “source” input to the p: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 if the port specified by a p:pipe is not in the readable ports of the environment. Specified inline 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 version="1.0"> ... </xsl:stylesheet> </p:inline> </p:input> </p:xslt> Inline documents are considered “quoted”. The pipeline processor passes them literally to the port, even if they contain elements from the XProc namespace or other namespaces that would have other semantics outside of the p:inline. Specified explicitly empty 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:xslt name="generate" version="2.0"> <p:input port="source"> <p:empty/> </p:input> <p:input port="stylesheet"> <p:inline> <xsl:stylesheet version="2.0"> ... </xsl:stylesheet> </p:inline> </p:input> <p:with-option name="template-name" select="'someName'"/> </p:xslt> 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. It is inconsistent with the specification to specify an empty binding as the context for evaluating an XPath expression. When an empty binding is specified for an XPath 1.0 expression, an empty document node must be used instead as the context node. 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.

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 p:pipeinfo.

Pipeline authors may add annotations to their pipeline documents with the p:pipeinfo element. The semantics of p:pipeinfo elements are implementation-defined. Processors should specify a way for their annotations to be identified, perhaps with extension attributes. Where p:documentation is intended for human consumption, p:pipeinfo elements are intended for processor consumption. A processor might, for example, use annotations to identify some particular aspect of an implementation, to request additional, perhaps non-standard features, to describe parallelism constraints, etc. When a p:pipeinfo appears as a descendant of p:inline, it has no special semantics; in that context it must be treated literally as part of the document to be provided on that port.

An element from the XProc namespace may have any attribute not from the XProc namespace, provided that the expanded-QName of the attribute has a non-null namespace URI. Such an attribute is called an extension attribute. The presence of an extension attribute must not cause the connections between steps to differ from the connections that would arise in the absence of the attribute. They must not cause the processor to fail to signal an error that would be signalled in the absence of the attribute. A processor which encounters an extension attribute that it does not recognize must behave as if the attribute was not present.

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: 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 . An xml:base attribute is allowed on any element. It has the semantics of . The p:documentation and p:pipeinfo elements are not shown, but they are allowed anywhere. Attributes that are syntactic shortcuts for option values are not shown. The types given for attributes should be understood as follows: ID, NCName, NMTOKEN, NMTOKENS, anyURI, boolean, integer, string: As per including whitespace normalization as appropriate. QName: With whitespace normalization as per and according to the following definition: In the context of XProc, a QName is almost always a QName in the Namespaces in XML sense. Note, however, that p:option and p:with-param values can get their namespace declarations in a non-standard way (with p:namespaces) and QNames that have no prefix are always in no-namespace, irrespective of the default namespace. PrefixList: As a list with item type NMTOKEN, per , including whitespace normalization. XPathExpression, XSLTMatchPattern: As a string per , including whitespace normalization, and the further requirement to be a conformant Expression per or , as appropriate, or Match pattern per or , as appropriate. A number of errors apply generally: It is a static error if any element in the XProc namespace has attributes not defined by this specification unless they are extension attributes. It is a static error if any required attribute is not provided. It is a static error if any attribute value does not satisfy the type required for that attribute. It is a static error if any string that must be interpreted as a QName uses a prefix for which there is not a namespace binding. It is a static error if any element in the XProc namespace or any step has element children other than those specified for it by this specification. It is a static error if any step directly contains text nodes that do not consist entirely of whitespace. It is a dynamic error if any option value does not satisfy the type required for that option. It is a static error if a compound step has no contained steps. If an XProc processor can determine statically that a dynamic error will always occur, it may report that error statically provided that the error does not occur among the descendants of a p:try. Errors inside a p:try must always be raised dynamically so that p:catch processing may be performed on them.

This section describes the core steps of XProc.

A p:pipeline declares a pipeline that can be evaluated by an XProc processor. It encapsulates the behavior of a subpipeline. Its children declare inputs, outputs, and options that the pipeline exposes and identify the steps in its subpipeline. (A p:pipeline is a simplified form of step declaration.) All p:pipeline pipelines have an implicit primary input port named “source” and an implicit primary output port named “result”. Any input or output ports that the p:pipeline declares explicitly are in addition to those ports and may not be declared primary. Viewed from the outside, a p:pipeline is a black box which performs some calculation on its inputs and produces its outputs. From the pipeline author's perspective, the computation performed by the pipeline is described in terms of contained steps which read the pipeline's inputs and produce the pipeline's outputs. If a pipeline does not have a type then that pipeline cannot be invoked as a step. The p:pipeline element is just a simplified form of step declaration. A document that reads: <p:pipeline some-attributes> some-content </p:pipeline> can be interpreted as if it read: <p:declare-step some-attributes> <p:input port='source' primary='true'/> <p:input port='paramters' kind='parameters' primary='true'/> <p:output port='result' primary='true'> some-content </p:declare-step> See p:declare-step for more details.

A pipeline might accept a document as input; perform XInclude, validation, and transformation; and produce the transformed document as its output. <p:pipeline xmlns:p="http://www.w3.org/ns/xproc"> <p:xinclude/> <p:validate-with-xml-schema> <p:input port="schema"> <p:document href="http://example.com/path/to/schema.xsd"/> </p:input> </p:validate-with-xml-schema> <p:xslt> <p:input port="stylesheet"> <p:document href="http://example.com/path/to/stylesheet.xsl"/> </p:input> </p:xslt> </p:pipeline>

A for-each is specified by the p:for-each element. It is a compound step that processes a sequence of documents, applying its subpipeline to each document in turn. When a pipeline needs to process a sequence of documents using a subpipeline that only processes a single document, the p:for-each construct can be used as a wrapper around that subpipeline. The p:for-each will apply that subpipeline to each document in the sequence in turn. The result of the p:for-each is a sequence of documents produced by processing each individual document in the input sequence. If the p:for-each has one or more output ports, what appears on each of those ports is the sequence of documents that is the concatenation of the sequence produced by each iteration of the loop on the port to which it is connected. If the iteration source for a p:for-each is an empty sequence, then the subpipeline is never run and an empty sequence is produced on all of the outputs. The p: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. 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 (explicit or supplied by default) 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 if the primary output port has no binding and the last step in the subpipeline does not have a primary output port. Note that outputs declared for a p:for-each serve a dual role. Inside the p:for-each, they are used to read results from the subpipeline. Outside the p:for-each, they provide the aggregated results. The sequence attribute on a p:output inside a p:for-each only applies inside the step. From the outside, all of the outputs produce sequences.

Within a p:for-each, the p:iteration-position and p:iteration-size are taken from the sequence of documents that will be processed by the p:for-each. The total number of documents is the p:iteration-size; the ordinal value of the current document (the document appearing on the current port) is the p:iteration-position. In the case where no XPath expression that must be evaluated by the processor makes any reference to p:iteration-size, its value does not actually have to be calculated (and the entire input sequence does not, therefore, need to be buffered so that its size can be calculated before processing begins).

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. <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.

A viewport is specified by the p:viewport element. It is a compound step that processes a single document, applying its subpipeline to one or more subtrees of the document. The result of the p:viewport is a copy of the original document where the selected subtrees have been replaced by the results of applying the subpipeline to them. The p:viewport-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. It is a dynamic error if the viewport source does not provide exactly one document. The match attribute specifies an XSLT match pattern. Each matching node in the source document is wrapped in a document node, as necessary, and provided, one at a time, to the viewport's subpipeline on a port named current. The base URI of the resulting document that is passed to the subpipeline is the base URI of the matched element or document. It is a dynamic error if the match expression on p:viewport does not match an element or document. After a match is found, the entire subtree rooted at that match is processed as a unit. No further attempts are made to match nodes among the descendants of any matched node. 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. The p:viewport must contain a single, primary output port explicit declared explicitly or supplied by default. If 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 if the primary output port has no binding and the last step in the subpipeline does not have a primary output port. What appears on the output from the p:viewport will be a copy of the input document where each matching node is replaced by the result of applying the subpipeline to the subtree rooted at that node. In other words, if the match pattern matches a particular element then that element is wrapped in a document node and provided on the current port, the subpipeline in the p:viewport is evaluated, and the result that appears on the output port replaces the matched element. If no documents appear on the output port, the matched element will effectively be deleted. If exactly one document appears, the contents of that document will replace the matched element. If a sequence of documents appears, then the contents of each document in that sequence (in the order it appears in the sequence) will replace the matched element. The output of the p:viewport itself is a single document that appears on a port named “result”. Note that the semantics of p:viewport are special. The output port in the p:viewport is used only to access the results of the subpipeline. The output of the step itself appears on a port with the fixed name “result” that is never explicitly declared.

Within a p:viewport, the p:iteration-position and p:iteration-size are taken from the sequence of documents that will be processed by the p:viewport. The total number of documents is the p:iteration-size; the ordinal value of the current document (the document appearing on the current port) is the p:iteration-position. In the case where no XPath expression that must be evaluated by the processor makes any reference to p:iteration-size, its value does not actually have to be calculated (and the entire input sequence does not, therefore, need to be buffered so that its size can be calculated before processing begins).

A p:viewport might accept an XHTML document as its input, add an hr element at the beginning of all div elements that have the class value “chapter”, and return an XHTML document that is the same as the original except for that change. <p:viewport match="h:div[@class='chapter']" xmlns:h="http://www.w3.org/1999/xhtml"> <p:insert position="first-child"> <p:input port="insertion"> <p:inline> <hr xmlns="http://www.w3.org/1999/xhtml"/> </p:inline> </p:input> </p:insert> </p:viewport> The nodes which match h:div[@class='chapter'] in the input document are selected. An hr is inserted as the first child of each h:div and the resulting version replaces the original h:div. The result of the whole step is a copy of the input document with a horizontal rule as the first child of each selected h:div.

A choose is specified by the p:choose element. It is a multi-container step that selects exactly one of a list of alternative subpipelines based on the evaluation of XPath expressions. A p:choose has no inputs. It contains an arbitrary number of alternative subpipelines, exactly one of which will be evaluated. The list of alternative subpipelines consists of zero or more subpipelines guarded by an XPath expression, followed optionally by a single default subpipeline. The p:choose considers each subpipeline in turn and selects the first (and only the first) subpipeline for which the guard expression evaluates to true in its context. If there are no subpipelines for which the expression evaluates to true, the default subpipeline, if it was specified, is selected. After a subpipeline is selected, it is evaluated as if only it had been present. The outputs of the p:choose are taken from the outputs of the selected subpipeline. The p:choose has the same number of outputs as the selected subpipeline with the same names. If the selected subpipeline has a primary output port, the port with the same name on the p:choose is also a primary output port. In order to ensure that the output of the p:choose is consistent irrespective of the subpipeline chosen, each subpipeline must declare the same number of outputs with the same names and the same settings with respect to sequences. 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 if two subpipelines in a p:choose declare different outputs. It is a dynamic error 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 expressions that occur on each branch are evaluated. The context node is specified as a binding for the p:xpath-context. If no binding is provided, the default p:xpath-context is the document on the default readable port. Each conditional subpipeline is represented by a p:when element. The default branch is represented by a p:otherwise element.

A p:xpath-context element specifies the context against which an XPath expression will be evaluated. When it appears in a p:when, it specifies the context for that p:when’s test attribute. When it appears in p:choose, it specifies the default context for all of the p:when elements in that p:choose. Only one binding is allowed and it works the same way that bindings work on a p:input. No select expression is allowed. It is a dynamic error if the xpath-context is bound to a sequence of documents. In an XPath 1.0 implementation, if the context node is bound to p:empty, or is unbound and the default readable port is undefined, an empty document node is used instead as the context. In an XPath 2.0 implementation, the context item is undefined.

A when specifies one subpipeline guarded by a test expression. Each p:when branch of the p:choose has a test attribute which must contain an XPath expression. That XPath expression's effective boolean value is the guard for the subpipeline contained within that p:when. The p:when can specify a context node against which its test expression is to be evaluated. That context node is specified as a binding for the p:xpath-context. If no context is specified on the p:when, the context of the p:choose is used.

An otherwise specifies the default branch; the subpipeline selected if no test expression on any preceding p:when evaluates to true.

A p:choose might test the version attribute of the document element and validate with an appropriate schema. <p:choose name="version"> <p:when test="/*[@version = 2]"> <p:validate-with-xml-schema> <p:input port="schema"> <p:document href="v2schema.xsd"/> </p:input> </p:validate-with-xml-schema> </p:when> <p:when test="/*[@version = 1]"> <p:validate-with-xml-schema> <p:input port="schema"> <p:document href="v1schema.xsd"/> </p:input> </p:validate-with-xml-schema> </p:when> <p:when test="/*[@version]"> <p:identity/> </p:when> <p:otherwise> <p:output port="result"> <!-- this output is necessary so that all the branches have the same outputs; it'll never really matter because we're just about to raise an error. --> <p:inline> <nop/> </p:inline> </p:output> <p:error code="NOVERSION"> <p:input port="source"> <p:inline> <message>Required version attribute missing.</message> </p:inline> </p:input> </p:error> </p:otherwise> </p:choose>

A group is specified by the p:group element. In a p:try, it is a non-step wrapper, everywhere else, it is a compound step. A group encapsulates the behavior of its subpipeline. A p:group is a convenience wrapper for a collection of steps.

<p:group> <p:variable name="db-key" select="'some-long-string-of-nearly-random-characters'"/> <p:choose> <p:when test="/config/output = 'fo'"> <p:xslt> <p:with-param name="key" select="$db-key"/> <p:input port="stylesheet"> <p:document href="fo.xsl"/> </p:input> </p:xslt> </p:when> <p:when test="/config/output = 'svg'"> <p:xslt> <p:with-param name="key" select="$db-key"/> <p:input port="stylesheet"> <p:document href="svg.xsl"/> </p:input> </p:xslt> </p:when> <p:otherwise> <p:xslt> <p:with-param name="key" select="$db-key"/> <p:input port="stylesheet"> <p:document href="html.xsl"/> </p:input> </p:xslt> </p:otherwise> </p:choose> </p:group>

A try/catch is specified by the p:try element. It is a multi-container step that isolates a subpipeline, preventing any dynamic errors that arise within it from being exposed to the rest of the pipeline. The p:group represents the initial subpipeline and the recovery (or “catch”) pipeline is identified with a p:catch element. The p:try step evaluates the initial subpipeline and, if no errors occur, the outputs of that pipeline are the outputs of the p:try step. However, if any errors occur, the p:try abandons the first subpipeline, discarding any output that it might have generated, and evaluates the recovery subpipeline. If the recovery subpipeline is evaluated, the outputs of the recovery subpipeline are the outputs of the p:try step. If the recovery subpipeline is evaluated and a step within that subpipeline fails, the p:try fails. The outputs of the p:try are taken from the outputs of the initial subpipeline or the recovery subpipeline if an error occurred in the initial subpipeline. The p:try has the same number of outputs as the selected subpipeline with the same names. If the selected subpipeline has a primary output port, the port with the same name on the p:try is also a primary output port. In order to ensure that the output of the p:try is consistent irrespective of whether the initial subpipeline provides its output or the recovery subpipeline does, both subpipelines must declare the same number of outputs with the same names and the same settings with respect to sequences. 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 if the p:group and p:catch subpipelines declare different outputs. A pipeline author can cause an error to occur with the p:error step. The recovery subpipeline of a p:try is identified with a p:catch: 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. What appears on the error output port is an error document. The error document may contain messages generated by steps that were part of the initial subpipeline. Not all messages that appear are indicative of errors; for example, it is common for all xsl:message output from the XSLT component to appear on the error output port. It is possible that the component which fails may not produce any messages at all. It is also possible that the failure of one component may cause others to fail so that there may be multiple failure messages in the document.

In general, it is very difficult to predict error behavior. Step failure may be catastrophic (programmer error), or it may be be the result of user error, resource failures, etc. Steps may detect more than one error, and the failure of one step may cause other steps to fail as well. The p:try/p:catch mechanism gives pipeline authors the opportunity to process the errors that caused the p:try to fail. In order to facilitate some modicum of interoperability among processors, errors that are reported on the error output port of a p:catch should conform to the format described here.

The error vocabulary consists of a root element, c:errors which contains zero or more c:error elements.

Each specific error is represented by an c:error element: The name and type attributes identify the name and type, respectively, of the step which failed. The code is a QName which identifies the error. For steps which have defined error codes, this is an opportunity for the step to identify the error in a machine-processable fashion. Many steps omit this because they do not include the concept of errors identified by QNames. If the error was caused by a specific document, or by the location of some erroneous construction in a specific document, the href, line, column, and offset attributes identify this location. Generally, the error location is identified either with line and column numbers or with an offset from the beginning of the document, but not usually both. The content of the c:error element is any well-formed XML. Specific steps, or specific implementations, may provide more detail about the format of the content of an error message.

Consider the following XSLT stylesheet: This styl