Pandoc filter to extract only the code blocks. You might use this, for example, to extract sample code from a tutorial.

I originally developed this filter because I wanted something like literate Haskell, but using Markdown. I wanted to be able to write a Markdown file with Haskell code blocks, and then be able to compile and execute it, or use Pandoc to produce a PDF. Cabal doesn't support that, so one workaround is to pre-process the file using Pandoc with this filter and the --write=plain option to produce a source file that can be compiled.

I also use this when I'm updating a tutorial I wrote some time ago; I'll extract the code examples and verify that they still work with the latest software.

This package is available from Hackage, or as a Nix flake.

To install from Hackage, use cabal install. The package name is pandoc-select-code.

Note: Flakes must be enabled in your Nix or NixOS installation.

One way to use the Nix flake is to create a shell.nix with pandoc and this package, like so:

with (import <nixpkgs> {});
let
  pandoc-include-plus = (builtins.getFlake git+https://codeberg.org/mhwombat/pandoc-include-plus).packages.${builtins.currentSystem}.default;
in
mkShell {
  buildInputs = [
    pandoc
    pandoc-select-code
    # add any other software you want to use in the shell.
  ];
}

Enter the shell using nix-shell, and this package will be available for use.

Add this package to your flake.nix:

{
  inputs.pandoc-select-code.url = "github:mhwombat/pandoc-select-code";

  outputs = { self, pandoc-select-code }: {
    # Use in your outputs
  };
}

You can process your Markdown document either using pandoc directly, or through Hakyll.

Use this filter by adding --filter=pandoc-select-code to your pandoc command. For example:

pandoc --filter=pandoc-linear-table myfile.md --output=myfile.pdf

Use this filter as a transform in Hakyll. For example, you could modify site.hs, adding

import Text.Pandoc.Filters.SelectCode (transform)

and changing

pandocCompiler

to

pandocCompilerWithTransform defaultHakyllReaderOptions defaultHakyllWriterOptions transform