Second match block not executing (OCaml)

August 3, 2022

Hey I’m new to ocaml and was trying out some of the exercises in a dune build. I think my issue is due to the semicolons but I’m not really sure. When running the following code:

let somelist = [1;2;3;4] in
  (* last element in a list *)
    match (last somelist) with
    | Some (v) -> 
      print_endline "last value of list:" ;
      print_int v;
    | None -> () ;
    
    print_newline ();

    (* last 2 elements in a list *)
    match (last_two somelist) with
    | Some (a::b) ->
      print_endline "last 2 elements in list: \n";
      print_int a;
      print_int (List.hd b);
    | None -> () ;
    | Some ([]) -> () ;;

print_newline () ;;

it only prints out:

last value of list: 
4

instead of continuing and printing out the last 2 elements in the list afterwards. I tested out the other 2 functions, last and last_two in utop and they seem to work.

I wanted it to print out:

last value of list:
4
last 2 elements in list: 
3 4

>Solution :

Each branch of a match contains a list of expressions separated by ; (or at least this is one way to describe the syntax). So your second match is contained within just one of the branches of the first match. I.e., it executes only if last somelist is None.

The usual way to solve this is by enclosing the first match in begin / end or parentheses. Here’s one way to do this:

begin
match (last somelist) with
| Some (v) -> 
  print_endline "last value of list:" ;
  print_int v;
| None -> ()
end;

print_newline ();

match (last_two somelist) with
.  .  .