How to write a filter that modifies existing Pandoc output for a given format

How to write a filter that modifies existing Pandoc output for a given format

[Guillermo Rodriguez Garcia]

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Hello all,

I have written simple Pandoc filters that do some modifications on Pandoc's 
AST -- for example, to generate certain LaTeX environments/constructs when 
a Div with certain classes is used in the input file. This worked fine.

However I am not sure how to write a filter that will only make some minor 
changes to the output for a given writer. Here's an example: I would like 
to write a filter so that if an image link contains a ".framed" class, then 
if generating LaTeX output, the \includegraphics command should include a 
[frame] option (this requires the adjustbox package to work).

I am not sure how to do this without replicating all the code Pandoc 
already uses to write out an image link in LaTeX.

Is there a way to do this?

Best, 

Guillermo


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How to write a filter that modifies existing Pandoc output for a given format

[Kolen Cheung]

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I think your filter need to output raw LaTeX, i.e. to convert that element into LaTeX code by your code, and put it in a raw element.

Or you can try to think of some LaTeX magic to do it. I think you still need a filter to capture the difference between a normal image to the one with class, and may be some LaTeX def in the preamble together with a command inserted by your filter could do it. After all, LaTeX is Turning Complete so it might be possible.

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Re: How to write a filter that modifies existing Pandoc output for a given format

[Guillermo Rodriguez Garcia]

Hello,

Thank you for your answer.

2017-01-27 9:02 GMT+01:00 Kolen Cheung <christian.kolen-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>:
> I think your filter need to output raw LaTeX, i.e. to convert that element into LaTeX code by your code, and put it in a raw element.

Yes. I know how to do that but then I need to replicate everything
Pandoc already does for image links (which is doable, but no the best
way)

Other cases are more difficult; for example tweaking the way tables
are generated just to add horizontal lines between rows would require
that I replicate all the code Pandoc already has for writing out
tables. That's quite a bit of work, and also a fragile approach.

>
> Or you can try to think of some LaTeX magic to do it. I think you still need a filter to capture the difference between a normal image to the one with class, and may be some LaTeX def in the preamble together with a command inserted by your filter could do it. After all, LaTeX is Turning Complete so it might be possible.

So I guess it is not possible to "modify" Pandoc output, only replace it.

It would be great if a filter could retrieve the output Pandoc intends
to generate for a given node, as a string, then modify that locally.

Best,

Guillermo Rodriguez Garcia
guille.rodriguez-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org

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Re: How to write a filter that modifies existing Pandoc output for a given format

[Kolen Cheung]

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Ah, yes, it is actually possible to do that. e.g. if you use panflute, it has a function convert_text that call pandoc to make the conversion. So you could convert that element from "panflute AST" to LaTeX, then use the re module to regex it to your liking. A potential issue is if a certain conversion is context-dependent (I can't name any example for the moment).

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Re: How to write a filter that modifies existing Pandoc output for a given format

[Guillermo Rodriguez Garcia]

I think the use case I described is a good example of a
context-dependent conversion.

Pandoc handles image links differently depending on whether they are
the only thing in a paragraph (in LaTeX this results in a float) or
whether they are inline in the text. This is (a simplified example of)
the kind of logic I don't want to duplicate...

2017-01-27 9:59 GMT+01:00 Kolen Cheung <christian.kolen-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>:
> Ah, yes, it is actually possible to do that. e.g. if you use panflute, it has a function convert_text that call pandoc to make the conversion. So you could convert that element from "panflute AST" to LaTeX, then use the re module to regex it to your liking. A potential issue is if a certain conversion is context-dependent (I can't name any example for the moment).

Guillermo Rodriguez Garcia
guille.rodriguez-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org

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Re: How to write a filter that modifies existing Pandoc output for a given format

[Kolen Cheung]

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I think you can sent the whole block to convert_text and it should know it is "in its own line" in the conversion.