callCabal2nix

Brian Hicks, September 22, 2020

When you're setting up a Haskell project in Nix, most advice says "call cabal init and then cabal2nix . > default.nix to get a buildable Haskell project."

But it turns out you don't have to run cabal2nix by hand to keep it up to date! pkgs.haskellPackages.callCabal2nix does the same thing as cabal2nix, but without writing a file. The nixpkgs manual doesn't mention it since it uses import-from-derivation, which can cause some problems with Hydra but I've found it super helpful so I'm writing it up! If you need more detail than what I've written here, diving into the source is probably your best bet.

The basic usage: call pkgs.haskellPackages.callCabal2nix (note the lowercase n in nix) with...

the project name
the path to the project source
the set of options you'd normally provide to a call to the stuff cabal2nix generates

If you want something similar to the default.nix produced by the cabal2nix instructions in the nixpkgs manual, put this in default.nix:

{ pkgs ? import <nixpkgs> { }, ... }:
pkgs.haskellPackages.callCabal2nix "name-of-your-haskell-project" ./. { }

callCabal2nix is always a sibling of ghcWithPackages, so you can pin your compiler version in the same way:

{ pkgs ? import <nixpkgs> { }, ... }:
pkgs.haskell.packages.ghc865.callCabal2nix "name-of-your-haskell-project" ./. { }

If you rely on getting a build environment with cabal2nix, the output callCabal2nix has a .env which you can provide to a mkShell's inputsFrom stanza:

{ pkgs ? import <nixpkgs> { }, ... }:
pkgs.mkShell {
  inputsFrom = [ (pkgs.haskellPackages.callCabal2nix "project" ./. { }).env ];
}

If you were previously calling cabal2nix with command-line options, you can instead use callCabal2nixWithOptions (again, note the lowercase n in nix.) This form adds a string before the final argument which is eventually used literally in a call to cabal2nix.

That's it! Enjoy not having to write make rules to call cabal2nix!

Oh and one final note: all the code links in this post are links to specific lines the repo as of the time of writing. If you are trying to find more information in the future, you may want to switch to the master branch of nixpkgs and sniff around for callCabal2nix to get the most up-to-date view.