[Haskell-cafe] [Haskell] Functor => Applicative => Monad
voldermort at hotmail.com
Wed Dec 15 12:51:22 CET 2010
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).
More information about the Haskell-Cafe