[Haskell-cafe] Re: Very crazy

[Claus Reinke]

> This is why I found it so surprising - and annoying - that you can't use 
> a 2-argument function in a point-free expression.
> 
> For example, "zipWith (*)" expects two arguments, and yet
> 
>  sum . zipWith (*)
> 
> fails to type-check. You just instead write
> 
>  \xs ys -> sum $ zipWith(*) xs ys
> 
> which works as expected.

    Prelude> :t \xs ys->sum $ zipWith (*) xs ys
    \xs ys->sum $ zipWith (*) xs ys :: (Num a) => [a] -> [a] -> a

    Prelude> :t \xs->sum . zipWith (*) xs
    \xs->sum . zipWith (*) xs :: (Num a) => [a] -> [a] -> a

    Prelude> :t (sum .) . zipWith (*)
    (sum .) . zipWith (*) :: (Num a) => [a] -> [a] -> a

    Prelude> :t (\g->sum . g) . zipWith (*)
    (\g->sum . g) . zipWith (*) :: (Num a) => [a] -> [a] -> a

(.) composes single-parameter functions, so in (f . g) x y,
g only gets the first parameter, x. but by adding further
levels of composition to f, we can let g consume more 
parameters.

claus