Add nix expression for reformatting sources of arbitrary packages

See the description in readme.

README.md

@@ -64,6 +64,18 @@ formatted output.
 $ ormolu --mode inplace Module.hs
 ```
 
+## Running on Hackage
+
+It's possible to try Ormolu on arbitrary packages from Hackage. For that
+execute:
+
+```console
+$ nix-build -A hackage.<package>
+```
+
+Then inspect `result/log.txt` for possible problems. The derivation will
+also contain formatted files for inspection.
+
 ## Contributing
 
 See [CONTRIBUTING.md](./CONTRIBUTING.md).

default.nix

@@ -1,18 +1,29 @@
 let
   pkgs = import ./nix/nixpkgs;
-  gitignoreSource = import ./nix/gitignore { inherit (pkgs) lib; };
   compiler = "ghc864";
-  source = gitignoreSource ./.;
+  source = pkgs.lib.sourceByRegex ./.[
+    "^.*\.md$"
+    "^app.*$"
+    "^data.*$"
+    "^ormolu.cabal$"
+    "^src.*$"
+    "^tests.*$"
+    ];
   haskellPackages = pkgs.haskell.packages.${compiler}.override {
     overrides = (self: super:
     super // {
       "ormolu" = super.callCabal2nix "ormolu" source { };
     });
   };
+  ormolize = import ./nix/ormolize {
+    inherit pkgs;
+    inherit haskellPackages;
+  };
 in {
   ormolu = haskellPackages.ormolu;
   ormolu-shell = haskellPackages.shellFor {
     packages = ps: [ ps.ormolu ];
     buildInputs = [ pkgs.cabal-install ];
   };
+  hackage = pkgs.lib.mapAttrs ormolize haskellPackages;
 }

nix/gitignore/default.nix

@@ -1,8 +0,0 @@
-{ lib }:
-
-let
-  rev = "ec5dd0536a5e4c3a99c797b86180f7261197c124";
-  sha256 = "0k2r8y21rn4kr5dmddd3906x0733fs3bb8hzfpabkdav3wcy3klv";
-  url = "https://github.com/hercules-ci/gitignore/archive/${rev}.tar.gz";
-  nixGitIgnore = builtins.fetchTarball { inherit url sha256; };
-in (import nixGitIgnore { inherit lib; }).gitignoreSource

nix/ormolize/default.nix

@@ -0,0 +1,19 @@
+{ pkgs, haskellPackages }: _: p:
+  pkgs.stdenv.mkDerivation {
+    name = p.name + "-ormolized";
+    src = p.src;
+    buildInputs = [
+      haskellPackages.cpphs
+      haskellPackages.ormolu
+      pkgs.glibcLocales
+    ];
+    LANG = "en_US.UTF-8";
+    buildPhase = ''
+      find . -name '*.hs' -exec bash ${./ormolize.sh} {} \; 2> log.txt
+    '';
+    installPhase = ''
+      mkdir "$out"
+      find . -name '*.hs' -exec cp --parents {} $out \;
+      cp log.txt $out/log.txt
+    '';
+  }

nix/ormolize/ormolize.sh

@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+# drop includes
+sed -i '/^#include/d' "$1"
+
+# drop CPP
+sed -i '/^{-# LANGUAGE CPP/d' "$1"
+
+# deal with CPP in a fairly straightforward way
+cpphs "$1" --noline > "$1-nocpp" 2> /dev/null
+
+# annoyingly, cpphs cannot modify files in place
+mv "$1-nocpp" "$1"
+
+# run ormolu
+ormolu -m inplace "$1"