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NixOS
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Description

Generate Nix build instructions from a Cabal file

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Cabal2nix

hackage release stackage LTS package stackage Nightly package Continous Integration

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/
directory, the tool to update the Haskell packages in nixpkgs. It has its own README there.

Building

cabal2nix
is built using
cabal-install
, 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:
$ 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  {} }:

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"

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