OCaml Print statements

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?

3 Answers

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.

Since OCaml isn't a pure functional language, there are many ways to do this. Here is the way I write this kind of code, just for a concrete example.

let rec mylength list =
(* DEBUG *)
let () = Printf.printf "mylength here, null list: %b\n%!" (list = [])
in
(* DEBUG *) match list with | [] -> 0 | _ :: rest -> 1 + mylength rest

After it works you can remove the stuff inside the (* DEBUG *) comments.

Note the use of %! to flush the buffer. If you do a lot of debugging with printf (as I do), it's really useful to learn about this.

Here's a simple example that shows that expression sequences as mentioned in Ray Toal's answer don't necessarily need parentheses around them:

let get_a_string = let a_string = "a string" in (* The semicolon causes the return value of the statement to be discarded *) Printf.printf "Debug: %s\n" a_string; a_string
let () = Printf.printf "Result: %s\n" get_a_string

Another way to discard a function's return value is using ignore:

ignore (Printf.printf "Debug info");