What is Some in OCaml

Question

This Ocaml code traverses a list and outputs the last element.

I dont understand the second condition where we output Some x

let rec last = function 
| [] -> None 
| x::[] -> Some x
| _ :: t -> last t ;;

• So if the list is empty we return null.
• If x is the last element we return Some x (* what is Some x in this context? *)
• If x is not the last element we go further in the list.

Answer 1

Some is a constructor for the option type. None is the other constructor. Consider the following definition of option.

type 'a option = None | Some of 'a

The net effect of this is to provide for functions to have an option to return a value, or a value representing nothing. Imagine I want to search for the index of an item in a list. What should I return if the value isn't in the list?

let find_index value lst = 
  let rec aux value lst idx =
    match lst with
    | [] -> None
    | x::_ when x = value -> Some idx 
    | _::xs -> aux value xs (idx + 1)
  in
  aux value lst 0

# find_index 4 [1; 8; 2; 5; 4; 10];;
- : int option = Some 4
# find_index 4 [1; 8; 2; 5; 7; 10];;
- : int option = None

Both values have type int option so OCaml's type system is happy.

In your example, an empty list doesn't have a last element, so you return None.

We can then pattern match on this to handle the two situations:

let some_list = [2; 3; 4]

let () =
  match last some_list with
  | None -> print_endline "This list doesn't have a last item."
  | Some item -> print_endline ("The last item found was " ^ string_of_int item)

You may have run into languages where this type of situations is handled by returning a special error value. We could return -1 for instance. Or we could throw a ValueNotFound exception.