MonadPlus reform proposal
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The MonadPlus class is ambiguous: while all instances satisfy Monoid and Left Zero, some such as [] satisfy Left Distribution, while others such as Maybe and IO satisfy Left Catch.
Proposal
It is proposed that MonadPlus be split like this:
MonadZero
class Monad m => MonadZero m where mzero :: m a
satisfying Left Zero:
mzero >>= k = mzero
MonadPlus
class MonadZero m => MonadPlus m where mplus :: m a -> m a -> m a
satisfying Monoid and Left Distribution:
mplus mzero b = b mplus a mzero = a mplus (mplus a b) c = mplus a (mplus b c) mplus a b >>= k = mplus (a >>= k) (b >>= k)
MonadOr
class MonadZero m => MonadOr m where morelse :: m a -> m a -> m a
satisfying Monoid and Left Catch:
morelse mzero b = b morelse a mzero = a morelse (morelse a b) c = morelse a (morelse b c) morelse (return a) b = return a
Instances of both
Some types could be made instances of both. For instance:
instance MonadOr [] where morelse [] b = b morelse a b = a