The identity function is the function that simply returns its input:
# let id x = x;; id : 'a -> 'a = <fun>
'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
OCaml programmers typically pronounce in Greek: alpha, beta, and gamma.
We can apply the 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
id to many types of values, it is a polymorphic
function: it can be applied to many (poly) forms (morph).