On Tue, Jul 13, 2010 at 11:28 AM, Shlomi Vaknin
Thank you Bob, your example clarified how actually using such data type would appear in haskell. I naively thought it would be as simple as defining a regular list, but i see it is slightly more strict than that. I appreciate your help! Vadali
Well it _is_ as simple as defining a regular list, which would be (fictionally since (:) and [] are reserved identifiers) :
data [] a = [] | a : [a]
Which is the same as :
data List a = Empty | Cons a (List a)
You can then handle lists with pattern matching :
map f [] = [] map f (x:xs) = f x : map f xs
Or for our List type :
map f Empty = Empty map f (Cons x xs) = Cons (f x) (map f xs)
His definition of a tree :
data Tree a = Leaf | Branch a [Tree a]
follows the same idea and is as easy to handle with pattern matching :
treeMap f Leaf = Leaf treeMap f (Branch x xs) = Branch (f x) (map (treeMap f) xs)
As you see, an user defined type is manipulated with the same mechanism as a "primitive" type, this uniformity is part of the power of Haskell in that it encourages users to create their types and allows seamless integration of external library types with "primitive" types. -- Jedaï