Only regenerate parser if grammar has changed (#158)

Previously, `slime_parser.erl` was regenerated on each compilation,
which casued any file depending on it to be recompiled, even if nothing
changed.

This continues to generate the grammar each time, but if it doesn't
differ from what's on disk, skip regenerating the parser.

tasks/compile.peg.exs

@@ -13,8 +13,27 @@ defmodule Mix.Tasks.Compile.Peg do
       :slime, :attr_list_delims, %{"[" => "]", "(" => ")", "{" => "}"}
     )
     grammar = EEx.eval_file("src/slime_parser.peg.eex", attr_list_delims: attr_list_delims)
-    File.write!("src/slime_parser.peg", grammar)
-    peg = "src/slime_parser.peg" |> Path.expand |> String.to_charlist
+
+    if contents_changed?("src/slime_parser.peg", grammar) do
+      compile_grammar("src/slime_parser.peg", grammar)
+    else
+      :ok
+    end
+  end
+
+  defp contents_changed?(file, expected) do
+    case File.read(file) do
+      {:ok, contents} ->
+        contents != expected
+
+      _ ->
+        true
+    end
+  end
+
+  defp compile_grammar(file, grammar) do
+    File.write!(file, grammar)
+    peg = file |> Path.expand |> String.to_charlist
     case :neotoma.file(peg, transform_module: :slime_parser_transform) do
       :ok -> :ok
       {:error, reason} ->