cabal2nix converts a single Cabal file into a single Nix build expression. For example:

$ cabal2nix cabal://mtl
{ mkDerivation, base, stdenv, transformers }:
mkDerivation {
  pname = "mtl";
  version = "2.2.1";
  sha256 = "1icdbj2rshzn0m1zz5wa7v3xvkf6qw811p4s7jgqwvx1ydwrvrfa";
  buildDepends = [ base transformers ];
  homepage = "http://github.com/ekmett/mtl";
  description = "Monad classes, using functional dependencies";
  license = stdenv.lib.licenses.bsd3;
}

Cabal files can be referred to using the magic URL cabal://NAME-VERSION, which will automatically download the file from Hackage. Alternatively, a direct http://host/path/pkg.cabal URL can be provided, as well as a file:///local/path/pkg.cabal URI that doesn't depend on network access. However, if the source hash is not already in cabal2nix's cache or provided using the --sha256 option, cabal2nix still needs to download the source code to compute the hash, which obviously still causes network traffic. Run the utility with --help to see the complete list of supported command line flags.

Detailed instructions how to use those generated files with Nix can be found at http://hydra.nixos.org/job/nixpkgs/trunk/manual/latest/download-by-type/doc/manual#users-guide-to-the-haskell-infrastructure.

cabal2nix can also build derivations for projects from other sources than hackage. You only need to provide an URI that points to a cabal project. The most common usecase for this is probably to generate a derivation for a project on the local file system:

$ cabal get mtl-2.2.1 && cd mtl-2.2.1
$ cabal2nix .
{ mkDerivation, base, stdenv, transformers }:
mkDerivation {
  pname = "mtl";
  version = "2.2.1";
  src = ./.;
  buildDepends = [ base transformers ];
  homepage = "http://github.com/ekmett/mtl";
  description = "Monad classes, using functional dependencies";
  license = stdenv.lib.licenses.bsd3;
}

This derivation will not fetch from hackage, but instead use the directory which contains the derivation as the source repository.

cabal2nix currently supports the following respository types:

• directory
• source archive (zip, tar.gz, ...) from http or https URL or local file.
• git, mercurial, svn or bazaar repository

The cabal2nix.cabal file for this package is automatically generated by the generate-cabal-file.hs program. The easiest way to accomplish that is to run

nix-shell release.nix -A cabal2nix.ghc7102.x86_64-linux.env --run "runhaskell generate-cabal-file.hs >cabal2nix.cabal"

where x86_64-linux should be replaced with whatever system ID is appropriate for your local machine. Basically, generate-cabal-file.hs requires the cartel library and git to run.

With the Cabal file in place, this is a normal Haskell project that can be compiled with cabal-install.

Users of Nix can build this package by running

nix-build release.nix -A cabal2nix.ghc7102.x86_64-linux

where, again, x86_64-linux should be replaced with the appropriate value. This gives you an executable at result/bin/cabal2nix. If you'd like to install the latest release version, then

nix-env -f "<nixpkgs>" -iA cabal2nix

is the way to go.