Re: [Haskell-cafe] Newbie question

2008/1/21 Alexander Seliverstov :

Hi, I try to undestand why this code dosen't work

f :: (Num a)=>Integer->a

f i = i

Integer is an instance of Num, so why does this code produce error: "Couldn't match expected type 'a' againsta inferred type 'Integer' ..."

But the type of this function says that it can return *any* instance of Num -- that is, the caller gets to choose which particular instance of Num they want. This function can only ever return an Integer. There is actually a function of this type, however; it's called fromIntegral. It works because it is a member of the Num type class. -Brent