Why is there a Ocaml syntax error on if statement?

Question

As many of you already know, Ocaml's error messaging is really bad and I am stuck. I tried to search the problem, but the error message was too vague to search for. The following code is supposed to filter out all of the numbers that are above a certain threshold using recursion.

let rec list_above thresh lst =
  if lst = [] then 
    printf("Herewego");
  else 
    begin 
  if (List.hd lst) >= thresh then 
    (((List.hd lst)::(list_above thresh List.tl lst)))
  else if (List.hd lst) < thresh then 
    ((list_above thresh List.tl lst));
end
;;

It keeps saying that there is a syntax error on line 53, which is the line with the first else, but I can't see anything wrong with my if and else statement.

Answer 1

The ; operator in OCaml is used to separate two values. But there is no second value after the first ; in your code.

You may be used to ; in other languages like C, where ; is used to terminate all expression statements.

To fix the syntax error, remove the first ; in this code.

After fixing this error, you have a type error. There are two values returned (in different cases) by your function. One is returned by the printf call. It returns a value of type unit (because it doesn't really compute a value). The other is returned by the list expression (((List.hd lst)::(list_above thresh List.tl lst))) . This value is some sort of list.

Because unit is not a list type, this code can't be correct. A function has to return values of the same type in all cases.