This expression is of type a * b * c, but type int was expected

Question

I am new to OCaml and I would like to write a function that returns the sum of all multiples of 3 and 5 below z.

Here is what I did:

let rec problemone x y z = match x with
  | x when x > z -> y
  | x when x mod 3 = 0 && x mod 5 = 0 -> problemone(x+1,y+x,z)
  | x when x mod 3 = 0 && x mod 5 <> 0 -> problemone(x+1,y+x,z)
  | x when x mod 3 <> 0 && x mod 5 = 0 -> problemone(x+1,y+x,z)
  ;;

Unfortunately, this does not work, and he tells me that:

Error: This expression has type 'a * 'b * 'c
       but an expression was expected of type int

Answer 1

The syntax for applying functions in OCaml is problemone (x+1) (y+x) z, rather than problemone(x+1,y+x,z).

(x+1,y+x,z)is interpreted as a tuple, so the error is of the tuple type 'a * 'b * 'c. The tuple is passed as the first argument, which is expected to be int. And since the functions are in OCaml, the compiler will not consider the application of only one argument to a function that expects multiple errors. Therefore, he only complains about the inconsistency of the type of the first argument.

Answer 2

(x+1, y+x, z) ( 3-) x ( y z).

problemone (x+1) (y+x) z.