ocaml-win-error: manipulate Windows system errors

If your application is likely to run on Windows, simply replace uses of Unix.error_message with Win_error.error_message. When the application runs on Windows, Windows system errors will be decoded; when the application runs elsewhere then the existing Unix.error_message function is called transparently.

A simple example:

try
  let fd = Unix.openfile "/tmp/foo" [ Unix.O_RDONLY ] 0 in
  Unix.close fd
with
| Unix.Unix_error(e, _, _) ->
  Printf.fprintf stderr "Caught: %s\n%!" (Win_error.error_message e)

What's going on?

Errors are typically first thrown by the OCaml standard library as Unix.Unix_error exceptions. In some cases these are mapped onto their Unix equivalents on such as Unix.ENOENT, but when the code isn't recognised OCaml will raise Unix.EUNKNOWNERR (-code). This library is able to convert these codes back into human-readable strings.