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)Read more about this topic: Tagged Union
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