MonadError section in “All about monads”

问题

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) => Monaderror e m | m -> e where
  throwError :: e -> m a
  catchError :: m a -> (e -> m a) -> m a

And one of the instance is Either e.

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

Here is what I don't understand. The MonadError class take two type parameters, and (Either e) takes one, how is this instantiation work? Is this because the functional dependencies? I still don't get it.

AND! I've runed this code in GHCi(with -XFunctionalDependencies, -XMultiParamTypeClasses) didn't compile! What's is this piece of code, anyway?

回答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.

来源：https://stackoverflow.com/questions/12955415/monaderror-section-in-all-about-monads