tool for haskell-nix-overlay
Haskell Nix
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NOTE: Eelco recently explained that it is possible to depend on outputs of derivations. Thus the whole design of hack-nix should change: The solver should be rewritten in Haskell and be run. Still patching should be allowed.

Hack-nix is the tool used by haskell-nix-overley to get hackage contents putting them into a .nix file which is read by the cabal dependency resolver written in the nix language itself.


Build dependencies of hack-nix using the nix-haskell-overlay system:

make the first line in hack-nix-envs/nix/default.nix point to the nix-haskell-overlay.


nix-env -p hack-nix-envs/default -iA env -f hack-nix-envs/nix/default.nix --show-trace
source hack-nix-envs/default/source-me/haskell-env

ADVANCED USAGE (multiple configurations)

hack-nix --write-hack-nix-cabal-config

writes a configuration file. Adjust it to your needs. For example, for hack-nix this file looks like this:

# generated lines:

Because there are no flags there is only one way to build the package. If you had run this command within the darcs repo (8flags) you would have got 2 ** 8 lines. However you must be insane to test them all. But having some configurations can be useful too. Note that it references additional files called ways/base{3,4}.nix. This tells the nix system to add additional filters forcing using base3 or base4 only. In theory you don't have to use such filters. But in practise you can't feed the solver with all packages. This would be too slow.

So applying the filter

base = { lt = "4"; };

disregards base >= 4.

Also note that different haskellPackages are used selecting different ghc versions.

# real world .hack-nix-cabal-config example (Darcs)
base3:[("haskellPackages","haskellPackages"),("flags","curl -curl-pipelining http -static terminfo threaded -type-witnesses color mmap -test -hpc -deps-only"),("mergeWith","ways/base3.nix")]
base4:[("haskellPackages","haskellPackages"),("flags","curl -curl-pipelining http -static terminfo threaded -type-witnesses color mmap -test -hpc -deps-only"),("mergeWith","ways/base4.nix")]
682:[("haskellPackages","haskellPackages_ghc682"),("flags","curl -curl-pipelining http -static terminfo threaded -type-witnesses color mmap -test -hpc -deps-only"),("mergeWith","ways/base3.nix")]
683:[("haskellPackages","haskellPackages_ghc683"),("flags","curl -curl-pipelining http -static terminfo threaded -type-witnesses color mmap -test -hpc -deps-only"),("mergeWith","ways/base3.nix")]

in ways/base3.nix:

  filtersByName = {
    base = { lt = "4"; };
    # you can add additional dependencies here etc..
    # you can override everything you can think about.
    # configure once and have fun!

If you've customized these files you can now run


$ hack-nix --build-env base3
$ . hack-nix-envs/base3/source-me
$ ./Setup configure --distdir=distbase3
$ ./Setup build --builddir=distbase3


$ hack-nix --build-env base4
$ . hack-nix-envs/base4/source-me
$ ./Setup configure --builddir=distbase4

How to tell hack-nix about custom packages which are not yet on hackage?

You can always add additional packages to the pool of available packages by using a a way file like this:

in ways/way-definining-additional-dependency-wash:

args: {
  packageOverrides = args.packageOverrides ++ [ (import /pr/tasks/wash/WashNGo-2.12/dist/WashNGo.nix) ];

The dist/WashNGo.nix file is created by hack-nix --to-nix. This command also creates a dist source file for you which is picked up by nix

Fetching & Updating Darcs Repositories Automatically

See nix-repository-manager. Contact me if you have any questions.


I can't live without them. They make looking up implementation details so much faster!

Add this line to your ~/.hack-nix/config file:

create-haskell-tags TTVim

Put something like this in your .vimrc:

for t in split($TAG_FILES,":")
  exec "set tags+=".t

and be done. The soure-me script will define TAG_FILES for you. So you always have accurate tags. hasktags is used. So code which is generated by either CPP or template haskell is missed.


honor: condTestSuites


error: Setup: failed to parse output of 'ghc-pkg dump'

cause: After sourcing a env file the ghc or cabal version changed.

fix: rm Setup and recompile that file


  • Get more users to fix bugs and help mantain this all
  • support emacs tags
  • support haddock
  • make the ruby bindings library use shared libs.
  • implement multithread support: ./Setup build +RTS -N4 -RTS
  • Should this project be renamed to cabal2nix ?

Well, its already useful enough :)