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
<voldermort@hotmail.com> wrote:
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.