5.02 MonadError Broke My Code

[Ashley Yakeley]

MonadError seems to have been redefined in 5.02 to have a fundep:

5.00.2:

    class (Monad m) => MonadError e m

5.02:

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

Why? Perhaps the IO Monad can have only one kind or level of error, but 
why can't other Monads have more?

Anyway, because of GHC's naive instance overlapping checking, it broke my 
code:

    instance (JVMMonad m) => MonadError ThrowableRef m where
		      {
      		throwError = jvmThrow;
      		catchError = jvmCatch;
      		};

because GHC thinks it might overlap with the Prelude's

    instance MonadError Exception IO


-- 
Ashley Yakeley, Seattle WA