[Haskell-cafe] Re: Very crazy
andrewcoppin at btinternet.com
Tue Sep 25 07:24:13 EDT 2007
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
f3 (f2 (f1 x y z))
there's nothing you can do except leave point-free.
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