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MonadPlus reform proposal

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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 ==
   
 
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
 
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