# Difference between revisions of "MonadPlus reform proposal"

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− | [[Category:Proposals]] |
+ | [[Category:Proposals]] [[Category:Monad]] |

## Revision as of 18:39, 7 October 2006

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