Difference between revisions of "MonadPlus reform proposal"
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BrettGiles (talk | contribs) m (Adding category) |
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=== MonadZero === | === MonadZero === | ||
− | + | <haskell> | |
− | + | class Monad m => MonadZero m where | |
+ | mzero :: m a | ||
+ | </haskell> | ||
satisfying '''Left Zero''': | satisfying '''Left Zero''': | ||
− | + | ||
+ | <haskell> | ||
+ | mzero >>= k = mzero | ||
+ | </haskell> | ||
=== MonadPlus === | === MonadPlus === | ||
− | + | <haskell> | |
− | + | class MonadZero m => MonadPlus m where | |
+ | mplus :: m a -> m a -> m a | ||
+ | </haskell> | ||
satisfying '''Monoid''' and '''Left Distribution''': | satisfying '''Monoid''' and '''Left Distribution''': | ||
− | + | ||
− | + | <haskell> | |
− | + | 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) | ||
+ | </haskell> | ||
=== MonadOr === | === MonadOr === | ||
− | + | <haskell> | |
− | + | class MonadZero m => MonadOr m where | |
+ | morelse :: m a -> m a -> m a | ||
+ | </haskell> | ||
satisfying '''Monoid''' and '''Left Catch''': | satisfying '''Monoid''' and '''Left Catch''': | ||
− | + | ||
− | + | <haskell> | |
− | + | morelse mzero b = b | |
− | + | morelse a mzero = a | |
+ | morelse (morelse a b) c = morelse a (morelse b c) | ||
+ | morelse (return a) b = return a | ||
+ | </haskell> | ||
== Instances of both == | == Instances of both == | ||
Line 39: | Line 54: | ||
Some types could be made instances of both. For instance: | Some types could be made instances of both. For instance: | ||
− | + | <haskell> | |
+ | instance MonadOr [] where | ||
morelse [] b = b | morelse [] b = b | ||
morelse a b = a | morelse a b = a | ||
+ | </haskell> | ||
[[Category:Proposals]] [[Category:Monad]] | [[Category:Proposals]] [[Category:Monad]] |
Revision as of 00:26, 4 January 2007
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