Oh yes, I understand now. Just x >>= f = f x the output of f is actually (Monad value) like in this example (Just 3) >>= (\x -> Just $ x^2) At the first sight, I thought about (Monad (f x)), but it's wrong because it will be (Monad (Monad value)) when f return. Thanks a lot! --Trung 2012/12/20 Tom Davie <tom.davie@gmail.com>

On 20 Dec 2012, at 14:07, Trung Quang Nguyen <trungnq97@gmail.com> wrote:

Hi all,

I saw this

1. instance Monad Maybe where 2. return x = Just x 3. Nothing >>= f = Nothing 4. Just x >>= f = f x 5. 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?

The reason is in the type of bind:

(>>=) :: Monad m => m a -> (a -> m b) -> m b

The function f takes a non-in-a-monad value, and gives you an in-a-monad value.

Bob

-- *Trung Nguyen* Mobile: +45 50 11 10 63 LinkedIn: http://www.linkedin.com/pub/trung-nguyen/36/a44/187 View my blog at http://www.onextrabit.com/

*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

-- *Trung Nguyen* Mobile: +45 50 11 10 63 LinkedIn: http://www.linkedin.com/pub/trung-nguyen/36/a44/187 View my blog at http://www.onextrabit.com/