[Nix-dev] Haskell NG

[Peter Simons]

Hi guys,

in the spirit of "release early, release often", I'd like to inform everyone
with an interest in the Haskell part of Nixpkgs about the ongoing effort to
re-factor both cabal2nix and the code in Nixpkgs to achieve ...

 1) support for all of Hackage,

 2) death to the ghc-wrapper, and

 3) easier and more reliably customization of the package set.

The Nixpkgs code lives in the "haskell-ng" branch of my Github clone [1]. In
that branch, Haskell related code is in pkgs/development/haskell-modules:

 - hackage-packages.nix is generated automatically by the "hackage2nix" utility
   from the "v2.x" branch of the cabal2nix repository [2]. The file defines
   builds for the respective latest version of every Hackage package.
   hackage2nix can include some older package versions, too, if necessary. (I
   need to add code to hackage2nix to configure the choice of generated
   packages conveniently.)

   Note that a significant portion of those builds will not succeed because
   they depend on system libraries that we don't have. Finding those broken
   packages and fixing them (or adding meta.broken = true) will need a
   community effort once this setup has stabilized. The Hydra job [3] builds
   these packages to give us feedback on the state of affairs.

 - default.nix imports the contents from hackage-packages.nix, applies (a
   sequence of) configuration functions, and makes the result available as a
   "haskellPackages" set suitable for nix-env and friends. The purpose of those
   configurations is to adapt the package set for different compiler versions,
   i.e. you can think of this file as a simplified version of what used to be
   haskell-defaults.nix.

   In the new setup, it will no longer be possible to install Haskell libraries
   by name: you must access them by their attribute path. This is necessary for
   performance reasons, because nix-env won't like us to bring an additional
   7000+ packages into scope. The big advantage of this change is that we can
   drop all that freaky name-mangling as package names no longer need to be
   unique in the global scope.

 - generic-builder.nix is a vastly simplified build function for Haskell
   packages. It's main feature is that it works with plain GHC -- meaning that
   ghc-wrapper is no longer required anywhere in Nixpkgs. Furthermore, builds
   are simple non-recursive attribute sets now; there is no more of that
   recursive tying-the-knot stuff the old 'cabal' function used to require.

>From the user's point of view, there are two important changes:

 1) No more ghc-wrapper, i.e. we no longer support non-deterministic setups
    where people install random packages into random places and expect them to
    know about each other. Instead, 'ghcWithPackages' is the tool of choice to
    create a deterministic Haskell environment. I plan to add support to
    cabal2nix for generating appropriate build instructions for use with
    nix-shell. It might also be nice to have a tool "nix-cabal-shall" that
    combines cabal2nix and nix-shell into one.

 2) Haskell packages support 'deepOverride'. Suppose package 'foo' requires a
    non-default version of 'binary'. Then

      foo.deepOverride { binary = self.binary_0_4_0_1; };

    gives a derivation that uses this particular version to build 'foo' as well
    as all of foo's build inputs.

The current state of the repository is "work in progress". I don't use this
code for production yet, and neither should you. :-) I just wanted to share the
effort to keep everyone in the loop and to encourage you to provide feedback.

Best regards,
Peter


[1] https://github.com/peti/nixpkgs/tree/haskell-ng
[2] https://github.com/NixOS/cabal2nix/tree/v2.x
[3] http://hydra.nixos.org/jobset/nixpkgs/haskell-updates