OCaml cons purpose

Question

I'm reading through the language manual for OCaml and came across the "cons" operator, denoted as

::

However, it's not explained at all what it is, and what its typically used for.

Answer 1

This is the fundamental list structuring operator. The list [1; 2; 3] is constructed with three applications of the :: operator:

$ ocaml
        OCaml version 4.01.0

# 1 :: 2 :: 3 :: [];;
- : int list = [1; 2; 3]

(This operation has been called cons since the early Lisp days of 50 years ago.)

The :: operator can also appear in patterns, to destructure a list:

let rec length l =
    match l with
    | [] -> 0
    | h :: t -> 1 + length t