* nix-org-babel
This is an implementation of =org-babel-tangle= in Nix.
Given an Org string, it extracts the contents of source blocks in a particular language.
** Rationale
[[https://github.com/talyz/fromElisp][talyz/fromElisp]] provides support for Org, and it is also part of [[https://github.com/nix-community/emacs-overlay][emacs-overlay]], which apparently is used by many people for their own configs.
However, I wanted to experiment with more advanced features such as excluding archived entries, and I wasn't sure if I could mix that with their code base.
I needed the feature in [[https://github.com/akirak/emacs-twist][my own project]], and I wanted to put it under my control, so I wrote it from scratch.
** 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.overlays.default
      ];
    };
  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
Example:

#+begin_src nix
  {
    languages = [ "emacs-lisp" "elisp" ];
  }
#+end_src

Default: =[ "emacs-lisp" "elisp" ]=
**** 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 =excludeHeadlines= 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 = excludeHeadlines (matchOrgTag "ARCHIVE");
  }
#+end_src

You can use the following predicates from this library:

- tag :: Returns true if the headline matches a tag given as the argument. The argument must be a string.
- headlineText :: Returns true if the headline matches the text given as the argument. The argument must be a string.