Ross Paterson wrote:

On Fri, Apr 01, 2011 at 08:51:52PM -0400, Job Vranish wrote:

...

Is there a particular reason that the transformers version of the MaybeT monad does not have a MonadFix instance?

I don't think so. The original MaybeT package had an instance

instance (MonadFix m) => MonadFix (MaybeT m) where -- I hope this is correct. At a minimum, it typechecks. mfix f = MaybeT (mfix (maybe (return Nothing) (runMaybeT . f)))

which would loop if f returned Nothing

In fact it loops regardless of f. Prelude Control.Monad.Maybe Control.Monad.Identity ghci> runIdentity $ runMaybeT $ mfix undefined

while your instance, which is equivalent to

instance (MonadFix m) => MonadFix (MaybeT m) where mfix f = MaybeT (mfix (runMaybeT . f . unJust)) where unJust = fromMaybe (error "mfix MaybeT: Nothing")

is compatible with the MonadFix instance for Maybe, so it should probably go in.

+1 (and I agree that the error can't happen) Bertram