[Nix-dev] Haskell NG

[Shea Levy]

Not related to your central goals (which are awesome), but can we take this opportunity to dashify attribute names as well?

> On Dec 12, 2014, at 2:38 PM, Peter Simons <simons at cryp.to> wrote:
> 
> 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
> 
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