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.
If you're wondering how the compiler tells the difference, have a look at section 2.4 of the Haskell 98 Report (Identifiers and Operators). Roughly, an identifier beginning with a lowercase letter or underscore must be a variable identifier, while an identifier beginning with an uppercase letter must be a constructor identifier. In other words, the second example above cannot be the definition of a function called "T", because "T" cannot be the name of a function.