At 2002-01-18 00:06, Cagdas Ozgenc wrote:
Why does Haskell let you write functions that are not a part of type class?
Well, what classes should such functions as const, id and (.) be members of? const :: a -> b -> a; const x y = x; id :: a -> a; id x = x; (.) :: (b -> c) -> (a -> b) -> (a -> c); (.) f g x = f (g x);
For example instead of
elem :: Eq a => a -> [a] -> Bool map :: (a -> b) -> [a] -> [b]
class Container a where elem :: Eq b => b -> a b -> Bool map :: (b -> c) -> a b -> a c
would define methods that can work on all containers,
Actually this would only work on container type-constructors. If you had a type that was some kind of container only of Char, you could not make it an instance of your Container.
and create a discipline to write more reusable and generic functions.
There's already a generalised version of map, called fmap, that works on any Functor type. Picking the correct generalisation is often not obvious... -- Ashley Yakeley, Seattle WA