OCaml try with issue

I don't anderstand the behaviour of the try ... with ... feature of OCaml.

here is a little sample to reproduce my problem :

let () =
  try int_of_string "4" with
  | Failure -> -1
  | n -> n

I compile with

ocamlc test.ml

Then I get this error:

File "test.ml", line 2, characters 6-23:  
2 |   try int_of_string "4" with  
 
Error: This expression has type int but an expression was expected of type unit

How can I modify my little code sample to make it work?

Your code can be read as

let () = (* I expect an unit result *)
 try int_of_string "4"
    (* if the call to `int_of_string` succeeds returns this result,
       which is an int *)
 with (* otherwise if an exception was raised* *)
 | Failure ->
   (* returns 1 if the raised exception was the `Failure` exception *)
   -1 
 | n -> n (* otherwise returns the exception `n` *)

which fails due to a lot of small paper cuts. The corrected version would be

let n =
  try int_of_string "4" with 
  | Failure _  (* Failure contains a error message as an argument *)  -> - 1

Nevertheless, I would suggest to use the match ... with exception ... -> ... construct which may be closer to your intuition

let n = match int_of_string "4" with
| exception Failure _msg -> -1
| x -> x (* if there was no exception, returns the result *)

but you can also avoid the exception with

let n = match int_of_string_opt "4" with
| None -> -1
| Some x -> x

or even

let n = Option.value ~default:(-1) (int_of_string_opt "4")