Add ormolu to dev environment

Closes #221

default.nix

@@ -147,7 +147,10 @@ with rec
   # version from `package.yaml` and create a dummy module that we inject in the
   # `ghci` command.
   niv-devshell = haskellPackages.shellFor {
-    buildInputs = [ pkgs.nixpkgs-fmt ];
+    buildInputs = [
+      pkgs.nixpkgs-fmt
+      pkgs.ormolu
+    ];
     packages = ps: [ ps.niv ];
     shellHook = ''
       repl_for() {

nix/default.nix

@@ -1,9 +1,14 @@
 { sources ? import ./sources.nix }:
+let
+  ormoluCompiler = "ghc865";
+in
 import sources.nixpkgs {
   overlays = [
     (_: pkgs: { inherit sources; })
     (_: pkgs: { nixpkgs-fmt = import pkgs.sources.nixpkgs-fmt {}; })
     (_: pkgs: { termtosvg = pkgs.callPackage ./termtosvg.nix {}; })
+    (import ./haskell-overlay.nix { inherit ormoluCompiler; })
+    (_: pkgs: { ormolu = pkgs.haskell.packages."${ormoluCompiler}".ormolu; })
   ];
   config = {};
 }

nix/haskell-overlay.nix

@@ -0,0 +1,14 @@
+{ ormoluCompiler }: self: super:
+
+let
+  ormolu = import super.sources.ormolu { pkgs = self; inherit ormoluCompiler; };
+in
+{
+  haskell = super.haskell // {
+    packages = super.haskell.packages // {
+      "${ormolu.ormoluCompiler}" = super.haskell.packages.${ormolu.ormoluCompiler}.override {
+        overrides = ormolu.ormoluOverlay;
+      };
+    };
+  };
+}

nix/sources.json

@@ -23,6 +23,18 @@
         "url": "https://github.com/nix-community/nixpkgs-fmt/archive/2a64f4b6a597d0d62004ffcec3ae4fd83cd1efc8.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
+    "ormolu": {
+        "branch": "master",
+        "description": "A formatter for Haskell source code",
+        "homepage": null,
+        "owner": "tweag",
+        "repo": "ormolu",
+        "rev": "22839ed74c9964578ae3b2af5c693f605ac8a687",
+        "sha256": "055z8p8kl80fr50crcvy2djk3qg2x34p8pa7kihxlcj9wrhsijyy",
+        "type": "tarball",
+        "url": "https://github.com/tweag/ormolu/archive/22839ed74c9964578ae3b2af5c693f605ac8a687.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    },
     "termtosvg": {
         "branch": "develop",
         "description": "Record terminal sessions as SVG animations",