Tagged Union - Examples

Examples

Say we wanted to build a binary tree of integers. In ML, we would do this by creating a datatype like this:

datatype tree = Leaf | Node of (int * tree * tree)

This is a tagged union with two cases: one, the leaf, is used to terminate a path of the tree, and functions much like a null value would in imperative languages. The other branch holds a node, which contains an integer and a left and right subtree. Leaf and Node are the constructors, which enable us to actually produce a particular tree, such as:

Node(5, Node(1,Leaf,Leaf), Node(3, Leaf, Node(4, Leaf, Leaf)))

which corresponds to this tree:

Now we can easily write a typesafe function that, say, counts the number of nodes in the tree:

fun countNodes(Leaf) = 0 | countNodes(Node(int,left,right)) = 1 + countNodes(left) + countNodes(right)