Problem Statement

Parametric Polymorphism is a way to define types or functions that are generic over other types. The genericity can be expressed by using type variables for the parameter type, and by a mechanism to explicitly or implicitly replace the type variables with concrete types when necessary.

Write a small example for a type declaration that is parametric over another type, together with a short bit of code (and its type signature) that uses it.

A good example is a container type, let's say a binary tree, together with some function that traverses the tree, say, a map-function that operates on every element of the tree. This language feature only applies to statically-typed languages.

Let's start with the solution:

procedure main()
    bTree := [1, [2, [4, [7]], [5]], [3, [6, [8], [9]]]]
    mapTree(bTree, write)
    bTree := [1, ["two", ["four", [7]], [5]], [3, ["six", ["eight"], [9]]]]
    mapTree(bTree, write)
end

procedure mapTree(tree, f)
    every f(\tree[1]) | mapTree(!tree[2:0], f)
end