MTL库需要哪些语言扩展？

Question

I'm trying to understand monad transformers by implementing my own tiny library based on the designs of existing ones.

What I'm stuck on is the language extensions. In MonadError , the only extension mentioned is UndecidableInstances . However, I can't get similar code to compile without also using FunctionalDependencies and FlexibleInstances .

Example that I believe requires FunDeps:

class (Monad m) => MonadError e m | m -> e where

And a flexible instance:

instance MonadError e m => MonadError e (MaybeT m) where

How does the MonadError code avoid the extra two extensions?

Answer 1

You don't have to spell out all the extensions in the source file itself. Common extensions that are used by all/most modules can be listed in the project's cabal file. In mtl's case :

extensions:
    MultiParamTypeClasses
    FunctionalDependencies
    FlexibleInstances