Ross Paterson wrote:
Several functions on containers used to have types like
lookup :: (Ord k) => k -> Map k a -> Maybe a
but these were generalized to
lookup :: (Monad m, Ord k) => k -> Map k a -> m a
The proposal is to change these signatures back, specializing them to Maybe.
I'm all for Maybe in the question Monad VS Maybe. Concerning MonadZero VS Maybe, the first observation is of course that both approaches have the same expressiveness, lift :: MonadZero m => Maybe a -> m a lift = maybe mzero return unlift :: (forall m . MonadZero m => m a) -> Maybe a unlift = id are inverse to each other. In other words, Maybe has a universal property (this sounds much fancier than it really is). So, the only decision here is whether the library should factor things via Maybe or include an automatic lift for extra convenience. I'm not sure whether this convenience is actually used, can someone dig up statistics from hackage? Also, it's not entirely clear whether MonadZero is the most general abstraction. Is ApplicativeZero better? Or something else? In any case, I prefer Maybe here as well because it's straightforward and simple. Every other abstraction is just a lift away. Regards, apfelmus