*fmap*<http://hackage.haskell.org/packages/archive/base/latest/doc/html/Prelude.html#v:fmap> :: Functor f => (a -> b) -> f a -> f b<http://hackage.haskell.org/packages/archive/base/latest/doc/html/Prelude.html#v:fmap> fmap f (Just a) = Just (f a) We wrap Just around (f a) because f return a value with type b instead (Just b). But in (*>>=*)<http://hackage.haskell.org/packages/archive/base/latest/doc/html/Prelude.html#v:-62--62--61-> :: Monad m => m a -> (a -> m b) -> m b<http://hackage.haskell.org/packages/archive/base/latest/doc/html/Prelude.html#v:-62--62--61-> Just x >>= f = f x We don't need to wrap Just around (f a) because f return (Just b). --Trung 2012/12/20 Miguel Negrao <miguel.negrao-lists@friendlyvirus.org>
A 20/12/2012, às 14:07, Trung Quang Nguyen escreveu:
Hi all,
I saw this
• instance Monad Maybe where • return x = Just x • Nothing >>= f = Nothing • Just x >>= f = f x • fail _ = Nothing
I am wondering about the implementation of function (>>=). Why don't it be Just x >>= f = Just (f x)?
Any body knows about this?
That would be the implementation of fmap for Maybe:
instance Functor Maybe where fmap _ Nothing = Nothing fmap f (Just a) = Just (f a)
so, different behavior.
best, Miguel _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners
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