Oneliners

What error code is that?

$ ocaml -I +unix unix.cma -e 'print_endline (Unix.error_message (EUNKNOWNERR 10))'
No child processes

Perl equivalent:

$ perl -le '$!=10; print $!'
No child processes

No longer a oneliner, but if you want the readable code:

#! /usr/bin/env -S thin-ocamlscript -package core,core_unix -linkpkg --

let () =
  let errno = int_of_string Sys.argv.(1) in
  let e = Core_unix.Error.of_system_int ~errno in
  let code = Core.Sexp.to_string_hum (Core_unix.Error.sexp_of_t e) in
  let msg = Core_unix.Error.message e in
  Printf.printf "%d = %s = %s\n" errno code msg

or, embracing Core instead of only grabbing its convenient derivation:

#! /usr/bin/env -S thin-ocamlscript -package core,core_unix -linkpkg --
open Core
open Core_unix

let () =
  let errno = Int.of_string (Sys.get_argv ()).(1) in
  let e = Error.of_system_int ~errno in
  let code = Sexp.to_string_hum (Error.sexp_of_t e) in
  let msg = Error.message e in
  Printf.printf "%d = %s = %s\n" errno code msg