Unfortunately you are running in to strange behavior due to a lack of parenthesis. First of all, let's see what your original expression actually is: fmap (\x -> x) Just 2 = (fmap (\x -> x) Just) 2 So you can see that you are actually fmaping over Just, rather than Just 2. What does this mean? Well, let's ask GHC: :t fmap _ Just Found hole `_' with type: Maybe a -> b That is, when you try and fmap over the *function* Just, we have to provide a function that expects a Maybe a, rather than an a. In your first case, your providing the identity function, which fits the required type as Maybe a -> Maybe a. However, in your second example, you are trying to provide apply the (+ 2) function to a Maybe a value (because x :: Maybe a). You cannot in general add numbers to Maybe a, hence the error message. Your final expression works because $ is effectively providing parenthesis: fmap (\x -> x + 2) $ Just 2 = fmap (\x -> x + 2) (Just 2) The precedence of application in Haskell can be confusing at the start - I suggest liberally using parenthesis, and then using HLint to remove the ones that you don't need. Eventually, you'll build up the necessary intuition. Hope this helps, - Ollie On Mon, Apr 27, 2015 at 11:02 AM, Shishir Srivastava < shishir.srivastava@gmail.com> wrote:

Hi,

Please can someone explain why these two expression behave differently -

---- fmap (\x -> x) Just 2 Just 2

----- fmap (\x -> x+2) Just 2

*No instance for (Num (Maybe a0)) arising from a use of `it'* *In a stmt of an interactive GHCi command: print it*

---- The first expression evaluates fine whereas the second one fails. However if I do - ----

fmap (\x -> x+2) $ Just 2 Just 4 ----

Then the second expression also returns the Maybe value. Why is $ needed in second expression but not in the first one ?

Thanks, Shishir

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