Two ways to write lists in OCaml

The :: operator in OCaml is used to create head tail lists.

[1; 2; 3;]

Can also be written as:

1 :: (2 :: (3 :: []))

And in fact, the AST of the first example is a nested data structure joined with the :: operator.

You can read more about lists here

The first way is convenient, but the second way is important because lists are pattern matched with the :: operator.

let pat_match_func numbers =
  match numbers with
  | first_number :: rest_of_numbers ->
    # do something with first number

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