[Haskell-cafe] Re: Would someone explain this code to me?

"Justin Bailey" writes:

I'm reading Chris Okasaki's "Purely Functional Data Structures", and some of his Haskell is confusing me. He defines the type Color and RedBlackSet as:

data Color = R | B data RedBlackSet a = E | T Color (RedBlackSet a) a (RedBlackSet a)

and then later he defines a function insertSet:

insertSet x s = T B a y b where ins E = T R E x E ... T _ a y b = ins s

What I don't understand is his use of the "T" constructor, both at

insertSet x s = T B a y b

Here it creates a new RedBlackSet

and in the where statement:

T _ a y b = ins s

Here it's a pattern match. So if ins s returns (T x a' y' b'), then a = a'; y = y'; b = b' are used in the expresion covered by the where clause. -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk