On 5 May 2009, at 11:27, z_axis wrote:
The following code snippets is from xmonad:
-- Given a window, find the screen it is located on, and compute
-- the geometry of that window wrt. that screen.
floatLocation :: Window -> X (ScreenId, W.RationalRect)
--.......
rr <- snd `fmap` floatLocation w
Prelude> :i fmap
class Functor f where fmap :: (a -> b) -> f a -> f b
It seems it is different from the definition of fmap ?
sincerely!
As the type signature of fmap explains, it transforms a function. Specifically, it starts with a function (a -> b), and it transforms it to accept an 'a' inside a functor instead of just an a, and return a 'b' inside the same functor instead of just a b. In other words, fmap applies functions inside containers.
We can see from floatLocation that it returns a pair inside a container - specifically, an X container. Fmap takes snd, and transforms it to work on values inside the X.
So, snd has type (a,b) -> b, thus fmap snd has type f (a,b) -> f b. In this case, the type it's being applied to is X (ScreenId, W.RationalRect), so f unifies with X, a with ScreenID and b with W.RationalRect. Making snd `fmap` floatLocation w hav the type X W.RationalRect.
Finally, the bind into rr there takes it out of the X monad all together, getting you a W.RationalRect.
You may want to read this article which explains some of Haskell's abstraciton mechanisms:
Bob