[Haskell-beginners] help with types and composition

[Geoffrey Marchant]

I usually go with
> (f1 .) . f2

Which makes the solution for the particular example:

> sumOfProducts = (sum .) . zipWith (*)

But if you want the operator that achieves this effect for arbitrary
functions,

> ((.) (.) (.))

does the trick. Or, if SKI is your thing, you might also try

> (ap (const ap) (ap (const const) (ap (const ap) const)))

but I don't usually do it that way unless I'm feeling particularly perverse.


On Thu, Jul 2, 2009 at 11:54 AM, Britt Anderson <britt at uwaterloo.ca> wrote:

>  I would like to write a function that takes two lists of numbers and
> returns a number. For simplicity, let's assume an integer, i.e.
> myfunc:: [Integer]->[Integer]->Integer
> But I can't quite get things right when trying to
> define myfunc = ...
>  and making the arguments implicit.
>
> My difficulty might be seen with the following two functions,
> let,
>
> f1 = (zipWith ($)) . (map  (*))
>
> f2 = sum
>
>
> if I set the types such that f1 returns [Integer] and sum accepts [Integer]
> it seems that somehow I should be able to compose them in to a single
> function that takes two lists and returns the number, but f1 . f2 and
> numerous variations don't work. I can curry some list l and f1 . (f2 l)
> works, but is there a way to get f1 and f2 combined into a function that
> accepts two lists?
>
>
> My problem is not really about writing the function but about composition.
> I can write the function with arguments or a local lambda  that suits my
> practical purpose. But the fact that I think I should be able to compose f1
> . f2 somehow reveals a conceptual misunderstanding that I hope someone will
> help me clear up.
>
> Thank you,
> Britt Anderson
>
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