Haskell: Defining function types..

techguy

Hi There,

I'm pretty new to Haskell atm. I'm struggling with most things but one thing I can't get my head around is defining function types. I can define a function like so: sum a b = a+b and it works fine.

Why must we define/declare/type [??] the function beforehand like so:
sum :: Integer->Integer->Integer

This above example works with or without the type declaration but I am having trouble with making a function that takes in integers and returns a float.
How do I get the following to return a float?

sum :: Integer->Integer->???
sum a b = 2.0*(a+b)

If I change ??? to Integer I get the error "Instance of Fractional Integer required for definition of sum"

If I change it to Float I get the error "Type error in explicitly typed binding"

Any ideas?Thanks..!

ronivek

sum :: (Fractional a) => a -> a -> a

That should work fine; it just needed to be bumped up a level of abstraction basically. Now it will work for all numbers that can be represented fractionally.

techguy

Cool, thanks i'll give that a go