15 Dec
2010
15 Dec
'10
6:51 a.m.
On 15/12/2010 11:39, Lennart Augustsson wrote:
Any refutable pattern match in do would force MonadFail (or MonadPlus if you prefer). So 1. (MonadFail m) => a -> m a, \ a -> return a 2. (MonadFail m) => m a, mfail "..." 3. (MonadFail m) => Maybe a -> m a, \ a -> case a of Nothing -> mfail "..."; Just x -> return x 4. (Monad m) => a -> b -> m a, \ a b -> return a 5. (Monad m) => (a, b) -> m a, \ (a, b) -> return a
As far as type inference and desugaring goes, it seems very little would have to be changed in an implementation.
Is there a need for a MonadFail, as distinct from mzero? fail always seems to be defined as error in ordinary monads, and as mzero in MonadPlus (or left at the default error).