Question

I am evaluating a very simple piece of code in Ocaml toplevel:

let p5 () = print_int 5;;
p5 ();;

print_string "*************************";;

let p4 = print_int 4;;
p4;; 

And it returns:

val p5 : unit -> unit = <fun>
#   5- : unit = ()
#   *************************- : unit = ()
#   4val p4 : unit = ()
#   - : unit = ()

My questions are

1. What does () mean in let p5 () = print_int 5;;?
2. What do - and () mean in # 5- : unit = ()?
3. Is p4 a function?
4. Why is there a 4 in the beginning of # 4val p4 : unit = ()?
5. It seems that () could be used in Ocaml code to hide side effect, could anyone show me an example?

Answer 1

Here some answers:

1. () is the unit type value. The unit type is a type with only one value. This is usually used to produce functions which either return nothing meaningful or take nothing meaningful. Remember, that in OCaml all functions alway have to return something and take some arguments, so the unit type is used to get around this limitation. Think of this similar to the void type in C, C++ or Java.
2. There are two lines interleaved. The 5 is printed by the print_int function and not by the toplevel. The toplevel just returns - : unit = () without the 5. The toplevel is telling you that it did not create any new bindings - and that the last returned value is of type unit and has the value ().
3. No. It does not take any arguments, so it is not a function.
4. Again there are two lines interleaved. The 4 is printed by the print_int function. At this time, the toplevel is telling you, that it created a new binding p4, that this variable carries a value of type unit and that the stored value is ().
5. No, () is not used to hide side effects. It is usually used to create functions, which have side effects, and thus need not take any kind of argument.