Add a timeout function to the standard library

[vicuna]

Original bug ID: 5908
Reporter: smimram
Status: resolved (set by @xavierleroy on 2013-01-28T08:56:37Z)
Resolution: suspended
Priority: normal
Severity: feature
Category: standard library
Monitored by: @dbuenzli

Bug description

Hi,

I have tried to program a "timeout" function in a portable way but did not succeed with the current standard library. Since this is quite a commonly used idiom, it might be worth adding such a function to the standard library.

I wanted a "timeout" function of type:

float -> ('a -> 'b) -> 'a -> 'b option

which takes a maximum number n of seconds to run, a function f, an argument x, and returns Some (f x) if the computation ends before n seconds and None otherwise. I have tried the following:

• the simple implementation using Unix.setitimer / Unix.alarm does not work under windows because of signals implementation
• the caml-list suggested using Gc.create_alarm which is portable, but really imprecise (we cannot predict when a gc will occur)
• I could have used threads and Thread.kill, but unfortunately Thread.kill is not implemented with the native compiler
• this could be achieved with monadic threads, but I would rather not change the structure of my whole program

Thanks!

[vicuna]

Comment author: @xavierleroy

The bottom line of the discussion on caml-list is that such a "timeout" function cannot be implemented correctly on non-Unix platforms, esp. Windows, but also JS-of-OCaml. So, probably, what you're asking for is impossible.

There might be a way to partially emulate Unix.setitimer under Windows, creating a separate thread that posts pseudo-signals periodically. However, such pseudo-signals will not interrupt blocking I/O operations and will only be processed when the OCaml code polls for pending signals. This might be good enough for some applications, but certainly not all.