[Haskell-cafe] Re: Very crazy

[Andrew Coppin]

Chaddaï Fouché wrote:
> 2007/9/25, Andrew Coppin <andrewcoppin at btinternet.com>:
>   
>> 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
>>
>>     
>
> (sum . zipWith (*)) xs ys
> == (sum (zipWith (*) xs)) ys
>
> so you try to apply sum :: [a] -> Int to a function (zipWith (*) xs)
> :: [a] -> [b], it can't work !
>
> (sum.) . zipWith (*)
> works, but isn't the most pretty expression I have seen.
>   

I'm still puzzled as to why this breaks with my example, but works 
perfectly with other people's examples...

So you're saying that

  (f3 . f2 . f1) x y z ==> f3 (f2 (f1 x) y) z

? In that case, that would mean that

  (map . map) f xss ==> map (map f) xss

which *just happens* to be what we want. But in the general case where 
you want

  f3 (f2 (f1 x y z))

there's nothing you can do except leave point-free.