Polymorphic functions

The syntactically simplest function is the identity function let id x = x.
What is its type? We can enter it into the toplevel to find out:

# let id x = x;;
id : 'a -> 'a = <fun>

The 'a is a type variable: it stands for an unknown type, just like a regular variable stands for an unknown value. Type variables always begin with a single quote. Commonly used type variables include 'a, 'b, and 'c, which OCaml programmers typically pronounce in Greek: alpha, beta, and gamma.

We can apply that identity function to any type of value we like:

# id 42;;
- : int = 42

# id true;;
- : bool = true

# id "bigred";;
- : string = "bigred"

Because you can apply that function to many types of values, it is a polymorphic function. (poly = many, morph = form)