15 Semicolon

Now the semicolon operator, also known as the sequence operator, becomes more useful.

• Syntax: e1; e2
• Evaluation
  • Evaluate e1 to a value v1
  • Throw away that value (note that e1 can have side effects)
  • Evaluate e2 to a value v2
  • return v2
• Type checking
  • If e1 : unit
  • and e2 : t
  • then e1; e2 : t

The semicolon is almost just syntactic sugar

e1; e2
(* is almost the same as *)
let () = e1 in e2

Except... it's not true that e1 : unit

• let syntax: type error
• semicolon syntax: type warning, because you probably don't want to discard e1's value
  • if you do, use ignore : 'a -> unit.

let print_and_add x y = print_int (x + y); print_newline (); x + y;;

val print_and_add : int -> int -> int = <fun>

Here, our function prints adds integers, and prints and returns the result.

print_and_add 0 1;;

1

- : int = 1

print_and_add 1 2;;

3

- : int = 3

This works because print_int and print_newline both return unit

print_int;;
print_newline;;

- : int -> unit = <fun>

- : unit -> unit = <fun>

But what if we wanted to chain functions that don't return units?

print_and_add 0 1; print_and_add 1 2;;

File "[5]", line 1, characters 0-17:
1 | print_and_add 0 1; print_and_add 1 2;;
    ^^^^^^^^^^^^^^^^^
Warning 10: this expression should have type unit.
File "[5]", line 1, characters 0-17:
1 | print_and_add 0 1; print_and_add 1 2;;
    ^^^^^^^^^^^^^^^^^
Warning 10: this expression should have type unit.

- : int = 3

1
3

We get a warning! Let's use the ignore function to fix this

ignore @@ print_and_add 0 1; print_and_add 1 2;;

1
3

- : int = 3

That's better!