Cabal2nix
=========

`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://nixos.org/nixpkgs/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

How to compile this package
---------------------------

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](http://hackage.haskell.org/package/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.