Parametric Polymorphism
type variables


Task

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.




f[a_] := Join[a, a]
f[{1, 2, 3}]
f[{"1", "2", "3"}]
f[{1.1, 2.1, 3.1}]
f[G[1, "a", Pi]]
g[x_] := x^2
g[2]
g[3.5]
g[Pi]
g["a"]