Yes, I think there should be a MonadFail distinct from MonadPlus.
Some types, like IO, are not in MonadPlus, but have a special implementation
of the fail method.
Personally, I think fail should just be removed, but that would break
existing code.
The fail method was introduced for the wrong reasons (better error messages
was the excuse).
-- Lennart
On Wed, Dec 15, 2010 at 11:51 AM, John Smith
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).
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