Oops...wrong list address...sorry ---------- Forwarded message ---------- From: Alexander Dunlap Date: Thu, Jul 2, 2009 at 8:54 PM Subject: Re: [Haskell-beginners] help with types and composition To: Britt Anderson , haskell-beginners@haskell.org On Thu, Jul 2, 2009 at 10:54 AM, Britt Anderson 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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Try (f2 .) . f1 . The type of (f2 .) is (a -> [Integer]) -> a -> Integer and the type of (.) is (b -> c) -> (a -> b) -> (a -> c) so for the second (.), b :: ([Integer] -> [Integer]), c :: ([Integer] -> Integer) and a :: [Integer] so the final type is [Integer] -> [Integer] -> Integer. (It might be easier to work out the types yourself if you can - my description is doubtless rather difficult to follow.) A word of caution, though - most Haskellers in my experience prefer to not do this type of composition; they find it clearer to specify one or both of the arguments explicitly. Hope that helps you, Alex