Distribution.Compat.Error

To re-iterate an old suggestion: how about using a custom data type like data ParseResult a = ParseFailed PError | ParseOk a instead of Either for parse results, thus removing the dependency on mtl, which will help with bootstrapping. I know there is Distribution.Compat.Error, but that defines a Monad instance for Either, so if one uses that there'll be trouble later if some program includes both Cabal and mtl.