Question

I am trying to understand for myself, which form of polymorhism does OCaml language have.

I was provided by an example

let id x = x

Isn’t this example equivalent to C++ template function

template<class A> A id(A x) { return x; }

If so then my question is: are there any other forms of polymorphism in OCaml? This notion is called “generic algorithm” in the world of imperative languages, not “polymorphism”.

Answer 1

There are basically three language features that are sometimes called polymorphism:

• Parametric polymorphism (i.e. “generics”)
• Subtype polymorphism, this is the ability of a subtype of a type to offer a more specific version of an operation than the supertype, i.e. the ability to override methods (and the ability of the the runtime system to call the correct implementation of a method based on the runtime type of an object). In OO languages this is often simply referred to as “polymorphism”.
• So-called ad-hoc polymorphism, i.e. the ability to overload functions/methods.

As you already discovered, OCaml has parametric polymorphism. It also has subtype polymorphism. It does not have ad-hoc polymorphism.

Since in your title you’ve asked for examples, here’s an example of subtype polymorphism in OCaml:

class c = object
    method m x = x+1
end

class d = object
    inherit c
    method m x = x+2
end

let main = 
    let o:c = new d in
    print_int (o#m 2)

This will print 4.