Design pattern 2: Command line wrappers

So assume we have created a pipeline using the job ticket pattern, how do we get this started from the command line? It turned out that adding a “command wrapper step” made a lot of sense. Here is how this works:

Why is this useful? In XProc steps you’ve written can be easily re-used. So it makes sense to keep steps that implement application logic “pure”: as unaware of the environment they’re running in as possible. Sometimes you’re using them as part of a command line tool and, maybe, sometimes in other contexts. Making them aware of things like command line arguments severely hinders re-use. So best to keep the interface of the functional steps simple using ”straight” options (booleans, strings, numbers etc.).

What would such a command-line wrapper step look like? You can of course make this is complex as you like. I used a fairly simple setup that turned out to be sufficient for my purposes:

<p:declare-step  >

  <p:option name="commandLine" as="xs:string"/><p:variable name="commandParts" as="xs:string*" select="tokenize($commandLine, '\s+')[.]"/>
  <p:variable name="commandFlags" as="xs:string*" select="$commandParts[starts-with(., '-')]"/>
  <p:variable name="commandArguments" as="xs:string*" select="$commandParts[not(starts-with(., '-'))]"/>

    <p:when test="'-help' = $commandArguments">
      … (output a help text)