# MonadPlus reform proposal

### From HaskellWiki

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The [[MonadPlus]] class is ambiguous: while all instances satisfy '''Monoid''' and '''Left Zero''', some such as <tt>[]</tt> satisfy '''Left Distribution''', while others such as <tt>Maybe</tt> and <tt>IO</tt> satisfy '''Left Catch'''. |
The [[MonadPlus]] class is ambiguous: while all instances satisfy '''Monoid''' and '''Left Zero''', some such as <tt>[]</tt> satisfy '''Left Distribution''', while others such as <tt>Maybe</tt> and <tt>IO</tt> satisfy '''Left Catch'''. |
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+ | == Proposal == |
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It is proposed that MonadPlus be split like this: |
It is proposed that MonadPlus be split like this: |
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morelse [] b = b |
morelse [] b = b |
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morelse a b = a |
morelse a b = a |
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+ | [[Category:Proposals]] |

## Revision as of 23:36, 15 January 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**.

## Contents |

## 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