Example of an Fmap to distinguish from map?

Question

My understanding is that the difference between map and fmap is that the latter can return a function?

I'm studying the functors section of this http://learnyouahaskell.com and some of the explanation is a bit unclear.

Map and fmap behave identically in the following:

let exMap = map (+1) [1..5]
let exFMap = fmap (+1) [1..5]

What is a good example of an fmap returning a function?

Answer 1

No, the difference is that fmap applies to any functor. For instance:

readLine :: IO String           -- read a line
fmap length readLine :: IO Int  -- read a line and count its length

Just 4 :: Maybe Int
fmap (+10) (Just 4) :: Maybe Int  -- apply (+10) underneath Just
                                  -- returns (Just 14)

map turns a -> b into a function [] a -> [] b (usually written as [a] -> [b]).

fmap turns a -> b into a function f a -> f bfor any functor f, not only for f = []. The examples above chose f = IO and f = Maybe.