Question

Is it possible to put print statements for debugging/testing in blocks of code?

For example, in Java you can use System.out.println("") in the middle of methods to check variables or other things.

But in OCaml, would a command like print_string work? Wouldn’t it return a value of type unit, thus causing an error, instead of allowing you to print it?

Answer 1

You will be fine if you embed the print in an expression sequence.

UPDATE

Here is a concrete example, because the answer above was rather short and, well, answers with links only are not good answers. It is the classic tail-recursive formulation of factorial (yeah, boring, but familiar):

# let fac n =
  let rec f i n acc =
    if i >= n
    then acc
    else (Printf.printf "%8d%8d%8d\n" i n acc; f (i+1) n ((i+1)*acc))
  in
    f 0 n 1;;
val fac : int -> int = <fun>
# fac 5;;
       0       5       1
       1       5       1
       2       5       2
       3       5       6
       4       5      24
- : int = 120

Here you can see the side-effects of the print, but the result still turned out to be 120 as expected.