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Diffstat (limited to 'libraries/base/Data/Maybe.hs')
| -rw-r--r-- | libraries/base/Data/Maybe.hs | 16 |
1 files changed, 16 insertions, 0 deletions
diff --git a/libraries/base/Data/Maybe.hs b/libraries/base/Data/Maybe.hs index fe2a0abc1e..991a25cb12 100644 --- a/libraries/base/Data/Maybe.hs +++ b/libraries/base/Data/Maybe.hs @@ -49,10 +49,26 @@ import GHC.Base data Maybe a = Nothing | Just a deriving (Eq, Ord) +-- | Lift a semigroup into 'Maybe' forming a 'Monoid' according to +-- <http://en.wikipedia.org/wiki/Monoid>: \"Any semigroup @S@ may be +-- turned into a monoid simply by adjoining an element @e@ not in @S@ +-- and defining @e*e = e@ and @e*s = s = s*e@ for all @s ∈ S@.\" Since +-- there is no \"Semigroup\" typeclass providing just 'mappend', we +-- use 'Monoid' instead. +instance Monoid a => Monoid (Maybe a) where + mempty = Nothing + Nothing `mappend` m = m + m `mappend` Nothing = m + Just m1 `mappend` Just m2 = Just (m1 `mappend` m2) + instance Functor Maybe where fmap _ Nothing = Nothing fmap f (Just a) = Just (f a) +instance Applicative Maybe where + pure = return + (<*>) = liftA2 id + instance Monad Maybe where (Just x) >>= k = k x Nothing >>= _ = Nothing |