[Haskell-cafe] Usage of . and $

[Jeff Polakow]

Jefferson Heard <jeff at renci.org> wrote on 03/06/2007 03:18:40 PM:

> Nope, I'm asking why 
> 
> um . IntMap.elems . IntMap.IntersectionWith (\x y -> x*y) queryVector 
> rationalProjection
> 
> won't work.
> 
We have (simplifying away some typeclass details):
 
    sum . elems :: IntMap a -> a

and:

    intersectionWith (*) :: IntMap a -> IntMap a -> IntMap a

but:

    ((sum . elems) .) :: (IntMap a -> IntMap a) -> IntMap a -> a

so you need something like: 

    sum . elems . uncurry (intersectionWith (*)) $ (queryVector, 
rationalProjection)

-Jeff


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