Question

this is just a question regarding notation in OCaml.

I am trying to test the function

let rec add (x : 'a) (l : 'a set) : bool =
  begin 
    match l with
    | [] -> []
    | hd :: rest -> 
      if x = hd then rest 
      else (hd :: (add x rest))
  end

my test case is

let test () : bool =
add (3 [1; 2; 4]) = [1; 2; 3; 4]
;; run_test "add 3 [1; 2; 4]" test

I am getting an "this expression is not a function, cannot be applied" error

Is something wrong with my notation?

Answer 1

OCaml doesn’t have a built-in set type (instead there is a Set module, or library). So, there’s also no built-in notation for set constants.

For smallish sets, one often uses lists. And, in fact, List.mem is a function that operates on lists (not sets). The notation for a list is like this: [1; 2; 3; 4].

(As a side comment, your function named add doesn’t add anything. But maybe you’re just getting started.)