Passing parameters to Lua filters (and custom readers/writers)

[massifrg]

I'm looking at ways to pass parameters to Lua filters (and also to custom readers/writers).

The -M option works well, especially for boolean (-M myFlag) and string (-M myVar=test) values.
You just setup a filter like this:

local myFlag, myVar
local filter = {
  Meta = function(meta)
    myFlag = meta.myFlag -- true
    myVar  = meta.myVar  -- "test"
  end
}

I don't know how to read parameters passed with -V (--variable) and --variable-json options, except for the deprecated PANDOC_WRITER_OPTIONS.variables table.

The PANDOC_WRITER_OPTIONS global variable (a WriterOptions object) works in filters too, and it has a variables field with the variables passed through -V, but unfortunately its use is deprecated.

In custom writers, you get the WriterOptions object through the second argument of Writer and ByteStringWriter functions; so parameters passed with -V are available.

In filters, it looks like the only way to pass parameters is using the -M option (or environment variables).
Since you're adding metadata, you should take care of removing them, when they're just processing parameters, not to get them in the resulting document. Anyways, this is a minor problem.

In custom readers, there's no variables field in ReaderOptions, so it looks like the only way to pass parameters is using environment variables.

Is it really like this?

[jgm]

It says "Accessing this variable in custom writers is deprecated." Not sure this applies to filters. @tarleb ?