The way I like to reason about these things
Prelude> :t (>>=)

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

Since the Monad m in this definition is Maybe, I'll just substitute it in like so:
Maybe a -> (a -> Maybe b) -> Maybe b

Then you notice that f is not (a -> b), it is (a -> Maybe b).

On Thu, Dec 20, 2012 at 9:07 AM, 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?

--Trung

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