MonadFix instance is needlessly strict

[gergoerdi]

As explained on http://stackoverflow.com/a/34287498/477476 , RevState's MonadFix instance is too strict and is practically unusable as a result. The fix is a trivial matter of using an irrefutable pattern in the instance definition; i.e. going from

instance MonadFix m => MonadFix (StateT s m) where
  mfix f = StateT $ \s -> mfix (\(x, _) -> runStateT (f x) s)

to

instance MonadFix m => MonadFix (StateT s m) where
  mfix f = StateT $ \s -> mfix (\ ~(x, _) -> runStateT (f x) s)

[DanBurton]

Thanks for the bugfix! I've whipped up a little test suite based on the example in that SO question.