Hacking on GHC Has Never Been Easier!

At ZuriHac 2019 Matthew Pickering talked about tools for working on GHC. There’s an associated blog post which is also well worth reading.

I wanted to focus on a small part of his presentation, which is about loading GHC into GHCi and using ghcid to automatically reload GHC on changes. This has been possible for almost a year now, as described in this blog post by Michael Sloan. It has since been improved to be even easier to use!

Putting this together with Alp Mestanogullari’s ghc.nix, it’s now possible to provision a GHC development environment from scratch with just a few commands, assuming that you have nix-shell, cabal-install, and ghcid installed. Here they are:

$ git clone --recursive https://gitlab.haskell.org/ghc/ghc/
$ cd ghc
$ git clone https://github.com/alpmestan/ghc.nix
$ cabal update
$ nix-shell ghc.nix/ --run './boot && ./configure && ghcid'

This will take a while the first time, but ghcid will cache generated artifacts under ./hadrian_ghci so even quitting and reloading will be significantly faster.

Congratulations, you are now a GHC developer!