MonadPlus reform proposal

Categories: Proposals | Monad

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.

Contents 1 Proposal 1.1 MonadZero 1.2 MonadPlus 1.3 MonadOr 2 Instances of both 3 Discussion

1 Proposal

It is proposed that MonadPlus be split like this:

1.1 MonadZero

class Monad m => MonadZero m where
   mzero :: m a

satisfying Left Zero:

mzero >>= k = mzero

1.2 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)

1.3 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

2 Instances of both

Some types could be made instances of both. For instance:

instance MonadOr [] where
   morelse [] b = b
   morelse a b = a

3 Discussion

Given that Control.Applicative(Alternative) now defines a class which seems innately bound to Left Catch, at least in spirit, it seems to make sense to clean up MonadPlus such that all instances obey Left Distribution? --sclv

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