[Haskell-cafe] Re: Very crazy
David Menendez
dave at zednenem.com
Tue Sep 25 22:22:39 EDT 2007
On 9/25/07, Andrew Coppin <andrewcoppin at btinternet.com> wrote:
> This is why I found it so surprising - and annoying - that you can't use
> a 2-argument function in a point-free expression.
[...]
> I can't figure out why map . map works, but sum . zipWith (*) doesn't
> work. As I say, the only reason I can see is that the type checker hates
> me and wants to force me to write everything the long way round...
I suspect you're getting confused by the way Haskell treats
multi-parameter functions. Specifically, as far as function
composition is concerned, all Haskell functions have one parameter.
It may help to fully parenthesize function applications.
For example,
\xs ys -> sum (zipWith f xs ys)
is
\xs ys -> sum (((zipWith f) xs) ys)
The rule is that you can replace "f (g x)" with "(f . g) x". If you
apply that above, you get
\xs ys -> (sum . ((zipWith f) xs)) ys
or
\xs -> sum . zipWith f xs
Removing 'xs' takes a little more work, because it's nested more
deeply. We can rewrite the above as,
\xs -> ((.) sum) ((zipWith f) xs)
Applying the rule f (g x) = (f . g) x gets us,
\xs -> (((.) sum) . (zipWith f)) xs
or
((.) sum) . zipWith f
or
(sum .) . zipWith f
The reason "map . map" is acceptible is that you can write "map (map f)".
--
Dave Menendez <dave at zednenem.com>
<http://www.eyrie.org/~zednenem/>
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