* nix-org-babel
This is a quick and dirty implementation of =org-babel-tangle= in Nix.
It provides a function that takes an Org string and returns the contents of source blocks in a particular language.

[[https://github.com/talyz/fromElisp][talyz/fromElisp]] has a built-in Org support, but I created this because I needed more control on which parts to extract. It provides an option for excluding subtrees matching a particular heading pattern. For example, you can omit archived subtrees from the output.
** Usage
Import the flake.
*** Extracting source blocks from an Org file/string
**** tangleOrgBabel function
This function takes an Org string and returns its source blocks.

Example:

#+begin_src nix
  let
    tangle = lib.tangleOrgBabel {
      languages = [ "emacs-lisp" ];
    };
  in
  # Return a string
  tangle (builtins.readFile ./config.org)
#+end_src

Arguments:

1. An attribute set of options.
2. An Org input string.
**** tangleOrgBabelFile function
Similar to =tangleOrgBabel=, but this function takes a file as an argument and writes the output to a file.

Example:

#+begin_src nix
  # Write to a file
  let
    pkgs = import nixpkgs {
      inherit system;
      overlays = [
        org-babel.overlay
      ];
    };
  in
  pkgs.tangleOrgBabelFile "init.el" ./config.org {
    languages = [ "emacs-lisp" ];
  }
#+end_src

Note that this function is provided in the overlay of the flake.

Arguments:

1. A string for the derivation name.
2. An input file path.
3. An attribute set of options.
*** Options
**** Languages
This option is required.

Example:

#+begin_src nix
  {
    languages = [ "emacs-lisp" "elisp" ];
  }
#+end_src
**** Filtering subtrees
You can transform the input by specifying =processLines= option.
It must be a function that takes a list of strings and returns a list of strings.

This library also contains =excludeOrgSubtreesOnHeadlines= function which can be used to exclude subtrees according to a predicate on the headline text, so you can use it in the option.

Example:

#+begin_src nix
  {
    # Exclude archived subtrees
    processLines = excludeOrgSubtreesOnHeadlines (matchOrgTag "ARCHIVE");
  }
#+end_src

You can use the following predicates from this library:

- matchOrgTag :: Returns true if the headline matches a tag given as the argument. The argument must be a string.
- matchOrgHeadline :: Returns true if the headline matches the text given as the argument. The argument must be a string.
- matchOrgHeadlines :: Returns true if the headline matches any of the texts given as the argument. The argument must be a list of strings.