Support buildHaskell

Use Nix and Briefcase to easily create Haskell executables.

I'd eventually like to support something like: briefcase.buildHaskell.project
that let me define a few top-level values (e.g. ghcExtensions) and would give me
a ghci environment with the proper `.ghci` settings so that my development
environment mirrored my build environment... baby steps, though.

buildHaskell/default.nix

@@ -0,0 +1,31 @@
+{ pkgs, ... }:
+
+{
+  # Create a nix-shell for Haskell development.
+  shell = { deps }: let
+    ghc = pkgs.haskellPackages.ghcWithPackages (hpkgs: deps hpkgs);
+  in pkgs.mkShell {
+    buildInputs = [ghc];
+  };
+
+  # Build a Haskell executable. This assumes a project directory with a
+  # top-level Main.hs. It also applies a few commonly used language extensions.
+  # Here is an overview of the arguments:
+  # - `name`: You can find the result at ./result/$name
+  # - `srcs`: Will be passed to `srcs` field of `pkgs.stdenv.mkDerivation`.
+  # - `deps`: A function that accepts `hpkgs` and returns a list of Haskell
+  #   dependencies.
+  program = { name, srcs, deps, ghcExtensions }: let
+    ghc = pkgs.haskellPackages.ghcWithPackages (hpkgs: deps hpkgs);
+  in pkgs.stdenv.mkDerivation {
+    name = name;
+    buildInputs = [];
+    srcs = srcs;
+    buildPhase = ''
+      ${ghc}/bin/ghc -Wall Main.hs ${pkgs.lib.concatMapStrings (x: "-X${x} ") ghcExtensions}
+    '';
+    installPhase = ''
+      mkdir -p $out && mv Main $out/${name}
+    '';
+  };
+}

default.nix

@@ -32,4 +32,5 @@ in {
   monzo_ynab   = readTree ./monzo_ynab;
   third_party  = readTree ./third_party;
   tools        = readTree ./tools;
+  buildHaskell = readTree ./buildHaskell;
 }