This tool download and cache the source code of packages in your local hackage,
it can then use this cache to generate
tags files aggregating the sources of all the dependencies of your cabal projects.
You basically do `codex update` in your cabal project directory and you'll get a file
codex.tags by default, or
TAGS when using emacs format) that you can use in your
favorite text editor.
Usage overview can be found in the README.