MonadError section in “All about monads”

Question

I\'m now really confused about the Error monad in which \"All about monads\" describes.

It claims the definition of Error monad as

class (Monad m) =>          


        

Answer 1

It's just a typo, the instance should be

instance MonadError e (Either e) where
  throwError = Left
  (Left e) `catchError` handler = handler e
  a        `catchError` _       = a

with two type parameters, as you expected.

Either e is the monad, and e is the corresponding error type.