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:

(deftype pair (&key (car 't) (cdr 't))
  `(cons ,car ,cdr))