XProc 3.0 - Connecting steps using ports

It's possible to define a default connection for an input port. A typical use case would be some conversion that in most cases uses a standard XSLT stylesheet. But sometimes this standard conversion needs to be overridden. Here 's how you could define such an input port:

<p:input name="special-stylesheet">
  <p:document href="default-stylesheet.xsl"/>
</p:input>

If you don't connect anything to the special-stylesheet port, it will connect to default-stylesheet.xsl. But if you do the default will be ignored.

The rules and syntax for defining a default connection are the same as for defining an explicit port connection. There's one exception: A default port connection cannot be dynamic (cannot depend on things happening at runtime). Therefore you cannot use <p:pipe> or the pipe attribute.

An output port of a pipeline needs to know what to deliver to the outside world. You specify this by connecting it to some output port of one of the steps that make up the pipeline.

For the primary output port this will usually be the last step of your pipeline. In that case you don't have to do anything, the implicit connection mechanism will take care of everything. But if you want to output something else you need to specify this. For example:

<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" …>

  <p:input port="source"/>
  <p:output port="result"/>
  <p:output port="extra" pipe="result@transform-step"/>
  …

  <p:xslt name="transform-step">
    <p:with-input port="stylesheet" href="do-something.xsl"/>
  </p:xslt>
  …

</p:declare-step>

The extra output port will always emit the result of the transformation named transform-step, even if the pipeline goes on after this.

When you invoke the outermost XProc pipeline using an XProc processor, you need to bind its input and output ports to something (usually files on disk). This binding is done through the command line. The same is true for setting the pipeline's options.

Unfortunately I cannot give you much guidance in this: How an XProc processor binds the pipeline's ports and sets options is implementation defined. So: read the processor specific documentation.

The introduction to XProc article already explained implicit binding. Here 's a rerun:

One input and one output port of a step can be designated as primary port (by convention called source and result). Primary ports of steps "auto-connect" when you place steps next to each other in XProc code. This is called implicit binding.

Not only does implicit binding connects steps, it also connects the primary input and output port of a pipeline to its constituent steps. Here's an example pipeline that chains two XSLT transformations:

Not all ports are primary ports and even for primary ports the previous step is not always the right one. So XProc provides you with means to explicitly bind ports. Explicit port binding is always done on input ports. You specify where an input port is connected to, not where an output port should deliver its results. Pull, not push.

Input ports must be connected (or have a default connection, see the section called “Input port defaults”). Any unconnected output ports (also the primary ones) silently discard their results.

You can connect an input port in several ways. Remember, the examples in this article only show the most common use, lots of details and settings have been left out. Please refer to the specification if you need more information.