Printf with automatic '\n'


I’d like to have a printf-like function, that prints a newline after each message.

Here is one of my attempts (which doesn’t even compile):

let warn fmt = Printf.eprintf (fmt ^ "\n")

let () =
  let fname = "abc" in
  let lnum = 123 in
  warn "%s:%d: hello world" fname lnum

Any suggestions how to accomplish this?

Just replace ^ (string concatenation) by ^^ (format string concatenation): let warn fmt = Printf.eprintf (fmt ^^ "\n").


Thanks a lot! (I don’t even dare to tell you how much time I spent trying to implement this).

Is it possible to also exit the program after printing the message?

This makes it exit, but doesn’t print anything:

let fail fmt = Printf.eprintf (fmt ^^ "\n"); exit 1
let () =
  fail "giving up after %d errors" 123

(I’m aware of exit preventing the partial function application here, but don’t know how fix it).

You should use kprintf for these cases:

let fail fmt = Printf.kfprintf (fun _ -> exit 1) stderr (fmt ^^ "\n%!")

Similarly, for Format (although no need to play with format strings):

let fail fmt = 
    (fun fmt -> Format.pp_print_flush fmt (); exit 1) 
    Format.err_formatter fmt

Thanks a lot!

PS: Where did you guys learn all these nifty things?

1 Like

Lost in the mists of history? Seems like this stuff has been in OCaml since forever. (It was one of the first things I noticed when I was learning the language back in 2001.)