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

and in the where statement:

            T _ a y b = ins s

Is "T" being redefined, so the "T" called in the first line is a new function that happens to be called "T" which is defined in the where statement? If that was the case, I would expect this to work:

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

but it does not. If anyone can explain what's going on here, I'd appreciate it. Thank you!

Justin