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.

2 files changed, 41 insertions, 9 deletions

diff --git a/buildHaskell/default.nix b/buildHaskell/default.nix
new file mode 100644
index 000000000000..0b6a488b3168
--- /dev/null
+++ b/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}
+    '';
+  };
+}
diff --git a/default.nix b/default.nix
index bfc1d4015cf8..d6d32c45195c 100644
--- a/default.nix
+++ b/default.nix
@@ -23,13 +23,14 @@ let
     depot = depot;
   };
 in {
-  nixos       = readTree ./nixos;
-  utils       = readTree ./utils;
-  emacs       = readTree ./emacs;
-  website     = readTree ./website;
-  lisp        = readTree ./lisp;
-  gopkgs      = readTree ./gopkgs;
-  monzo_ynab  = readTree ./monzo_ynab;
-  third_party = readTree ./third_party;
-  tools       = readTree ./tools;
+  nixos        = readTree ./nixos;
+  utils        = readTree ./utils;
+  emacs        = readTree ./emacs;
+  website      = readTree ./website;
+  lisp         = readTree ./lisp;
+  gopkgs       = readTree ./gopkgs;
+  monzo_ynab   = readTree ./monzo_ynab;
+  third_party  = readTree ./third_party;
+  tools        = readTree ./tools;
+  buildHaskell = readTree ./buildHaskell;
 }