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
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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