Cabal2nix
=========

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

    $ cabal2nix cabal://mtl
    { mkDerivation, base, lib, transformers }:
    mkDerivation {
      pname = "mtl";
      version = "2.2.1";
      sha256 = "1icdbj2rshzn0m1zz5wa7v3xvkf6qw811p4s7jgqwvx1ydwrvrfa";
      libraryHaskellDepends = [ base transformers ];
      homepage = "http://github.com/ekmett/mtl";
      description = "Monad classes, using functional dependencies";
      license = 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 still causes network traffic. Run the utility
with `--help` to see the complete list of supported command-line flags.

Detailed instructions on how to use those generated files with Nix can be found at
https://haskell4nix.readthedocs.io/nixpkgs-users-guide.html#how-to-create-nix-builds-for-your-own-private-haskell-packages.

`cabal2nix` can also build derivations for projects from other sources than
Hackage. You only need to provide a URI that points to a cabal project. The
most common use-case 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, lib, transformers }:
    mkDerivation {
      pname = "mtl";
      version = "2.2.1";
      src = ./.;
      libraryHaskellDepends = [ base transformers ];
      homepage = "http://github.com/ekmett/mtl";
      description = "Monad classes, using functional dependencies";
      license = 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 repository types:

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

## `hackage2nix`

This repository also contains, in the [`hackage2nix/`](./hackage2nix) directory,
the tool to update the Haskell packages in
[nixpkgs](https://github.com/NixOS/nixpkgs). It has its own README there.

## Building

`cabal2nix` is built using [`cabal-install`](https://www.haskell.org/cabal/),
like you'd expect, and you are free to use your favourite way of setting up
a Haskell development environment.

Since `cabal2nix` is quite intertwined with the packages `distribution-nixpkgs`
and `hackage-db`, we recommend setting up a shared development environment
for the three packages like so:

```console
$ mkdir /path/to/cabal2nix-root && cd /path/to/cabal2nix-root
$ # clone repositories, note that you may need to checkout an
$ # older tag for some depending on breaking changes on master.
$ git clone https://github.com/NixOS/cabal2nix.git
$ git clone https://github.com/NixOS/hackage-db.git
$ git clone https://github.com/NixOS/distribution-nixpkgs.git
$ # setup development environment with shellFor
$ cat > shell.nix << EOF
# assumes nix{os,pkgs}-unstable
{ pkgs ? import <nixpkgs> {} }:

pkgs.haskellPackages.shellFor {
  packages = p: [
    p.cabal2nix-unstable
    p.distribution-nixpkgs
    p.hackage-db
  ];

  # for running doctests locally
  nativeBuildInputs = [
    pkgs.haskellPackages.doctest
  ];

  # set environment variable, so the development version of
  # distribution-nixpkgs finds derivation-attr-paths.nix
  distribution_nixpkgs_datadir = toString ./distribution-nixpkgs;
}
EOF
$ # tell cabal about local packages
$ cat > cabal.project << EOF
packages: ./*/*.cabal
EOF
$ # test our new development environment
$ nix-shell --run "cabal v2-build exe:cabal2nix exe:hackage2nix"
```