I'd like to put a "me too" on that one. This part of the class hierarchy is currently a bit inexpressive. I've been annoyed by the fact that if I want to express in the type signature of a function that a monad has a failure mechanism, I'm forced to go all the way up to MonadPlus, which makes it look like something fully nondeterministic is happening, when really I'm just possibly failing. Also, mplus and morelse are quite different things, and it would be nice to have both even in a monad supporting mplus. For instance, in the list monad, mplus = (++) but morelse [] ys = ys morelse (x:xs) ys = (x:xs) - Cale On 21/03/06, Ashley Yakeley <ashley@semantic.org> wrote:

Does this come under the "standard libraries" topic? I would like to see the MonadPlus class disambiguated:

class Monad m => MonadZero m where mzero :: m a class MonadZero m => MonadPlus m where mplus :: m a -> m a -> m a class MonadZero m => MonadOr m where morelse :: m a -> m a -> m a

<http://haskell.org/haskellwiki/MonadPlus_reform_proposal>

-- Ashley Yakeley, Seattle WA WWED? http://www.cs.utexas.edu/users/EWD/

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