[Nix-dev] Full Stackage Support in Nixpkgs "master"

[Peter Simons]

Fellow Haskell Hackers,

commit 2aaa587b71 in Nixpkgs "master" [1] added full support for
Stackage [2] in the form of 51(!) new packages sets called

    haskell.packages.lts-x_y

where x_y stands for LTS Haskell version x.y, obviously. If you have a
cabal2nix-generated "shell.nix" file for your project, then you can
switch your build environment to the latest version of LTS Haskell, 3.5,
by running:

    nix-shell --argstr compiler lts-3_5

The 3.x package sets have been tested fairly well (on x64_64-linux) and
they should work 100%. Older versions of LTS Haskell received some
testing and probably work okay'ish, but don't be surprised if a few
Stackage packages in those older versions don't compile at the moment.
Packages that are not part of Stackage are available in those package
sets for the sake of convenience, but haven't been tested.

Hydra.nixos.org has no binaries for LTS Haskell. Hydra.cryp.to has
x86_64-linux binaries for most of LTS 3.x and all of LTS 3.5.
Personally, I'd like to have hydra.nixos.org build and distribute at
least some subset LTS Haskell -- say, the respective latest version in
every major release --, but our resources are scarce and we'll have to
investigate how much more is feasible.

Furthermore, our default package set haskell.packages.ghc783 a.k.a
haskellPackages now tracks Stackage Nightly. This change causes some
downgrades. For example, we used to ship version 0.11.x of the "vector"
library, but Stackage says we ought to distribute version 0.10.12.3. In
such cases, where Stackage hasn't accepted the latest available version
of a package yet, that package is always available in a version'ed
attribute, i.e. we have haskell.packages.xyz.vector_0_11_0_0. Run

    nix-env -f "<nixpkgs>" -qaP -A haskellPackages

to discover the available versions, and feel free to create your own
personal package set by overriding these default choices to your liking.
The Nixpkgs Haskell Manual [3] contains plenty of helpful information to
that end, and I'll also update it soon to reflect the new state of
affairs.

The "nixos-unstable" channel doesn't have these improvements yet.
They'll show up in a few days, but until then the pleasure of early
adopting is limited to the wild men and women who live on Git "master".

Best regards,
Peter



[1] https://github.com/NixOS/nixpkgs/commit/2aaa587b71e19ff105fc482414f9dbc3060d57b6
[2] http://www.stackage.org/
[3] http://nixos.org/nixpkgs/manual/#users-guide-to-the-haskell-infrastructure