This is a first step toward moving to `transformers-0.4.*` and `ExceptT`. We need to: - add a type (not `String`) for errors - replace calls to `strMsg` with the appropriate constructor everywhere. - update `monad-unify` to provide an appropriate error type, and handle that error type in `TypeChecker.Monad`.
This is a first step toward moving to
transformers-0.4.*andExceptT. We need to:String) for errorsstrMsgwith the appropriate constructor everywhere.monad-unifyto provide an appropriate error type, and handle that error type inTypeChecker.Monad.