gistfile1.txt

{
    packageOverrides = pkgs:
        let
            haskellPackages = pkgs.haskell-ng.packages;
            dontCheck = pkgs.haskell-ng.lib.dontCheck;
            mapper = (p: dontCheck p);
        in {
            haskellEnv = haskellPackages.ghcHEAD.ghcWithPackages (p:
                (map mapper [ p.lens ]) ++ [ p.cabal2nix ]
            );
        };
}

This is a working Haskell Nix environment that installs lens and whatever else you want without their testing dependencies --- which seems like it should be the default; I want to use wreq without installing Snap to run its tests!

Some things to note:

• Lambdas are very weird in this language: x: y: z: expr
• Lets have semicolons: let x = expr; y = expr; in expr
• The only types are objects (curly braces representing a list of named attributes) and lists and numbers and booleans? It seems like a functional JSON. Haven't explored much
• Modules are implemented with objects, which is weird! You can now pass modules around. I wish Haskell had first-class modules.
• It's not obvious what any objects are named. I had to constantly flip between the nixpkgs repo on GitHub in order to type these long names, like pkgs.haskell-ng.lib.dontCheck.
• Ultimately I couldn't get GMP to compile due to its using -rpath on OS X Yosemite. It looks like OS X needs some love in the Nix world. I'll try again on Linux in the future.
• This seems strictly more powerful, like a virtualenv that you can program. Nix: ★★★★★