[Haskell-cafe] Usage of . and $
jeff.polakow at db.com
Tue Mar 6 15:14:39 EST 2007
haskell-cafe-bounces at haskell.org wrote on 03/06/2007 02:43:03 PM:
> Usually, I can do this, but today, my brain is weak, and I'm just trying
> get this piece of code out the door. My code looks like this:
> weight = sum (IntMap.elems (IntMap.intersectionWith
> (\x y -> x*y) queryVector rationalProjection))
> I know that this will work (ignoring indentation):
> sum $ IntMap.elems $ IntMap.intersectionWith (\x y -> x*y) queryVector
> But why won't this?:
> sum . IntMap.elems . IntMap.IntersectionWith ...
> Is there a difference between the "elegance" of function composition
I assume your really asking why something like:
(*) $ 2 $ 3
won't work? If so, the reason is that $ associates to the right. So you
((*) $ 2) $ 3
If not, could you give the full expression which doesn't work?
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