CODEWITHSUNDEEP
schemelisp
Tryer outer
Tryer outer

Reputation: 47

Call a procedure in Scheme after waiting a few seconds?

Is it possible to call a procedure in Scheme after waiting a few seconds?

For example:

(define (facs)
(fac 3)
;wait 5 seconds
(fac 7))

Upvotes: 0

Views: 396

Answers (2)

user5920214
user5920214

Reputation:

Not in portable Scheme as far as I know. Many implementations will have ways of doing this, of course. For instance in Racket:

(define (sleepy s)
  (values (current-seconds)
          (begin
            (sleep s)
            (current-seconds))))

> (sleepy 2)
1612809817
1612809819

(Of course, current-seconds is also not part of standard Scheme.)

Upvotes: 1

ad absurdum
ad absurdum

Reputation: 21367

There is no provision for a sleep procedure in Standard Scheme; you will need to rely on implementation-specific features.

If you are using Guile Scheme, you can use sleep to sleep for a number of seconds, or usleep to sleep for a number of microseconds:

(define (delayed-5s msg)
  (display "Waiting 5s...")
  (force-output)
  (sleep 5)
  (newline)
  (display msg)
  (newline))

Note that you should flush the output to be sure that messages are seen when you want them seen. Typically, (but not always) printing a newline will flush the output buffer, so you could move (newline) before (sleep 5) and reasonably expect that the display will work as expected; but it is better to flush the output explicitly to ensure that messages are flushed from the output buffer when you want them to be. In the above code, both force-output and sleep are Guile-specific procedures.

If you are using Chez Scheme, you can do this:

(define (delayed-5s msg)
  (display "Waiting 5s...")
  (flush-output-port (current-output-port))
  (sleep (make-time 'time-duration 0 5))
  (newline)
  (display msg)
  (newline))

In Chez Scheme, sleep takes a time object, and make-time takes three arguments, returning a time object. The first of those is a time-type argument, the second is the number of nanoseconds, and the third is the number of seconds. Again, you should flush the output buffer, and flush-output-port does that, but it needs an output-port, which the procedure current-output-port provides. In the above code, sleep and make-time are Chez-specific procedures, but flush-output-port and current-output-port are standard R6RS Scheme library procedures.

You can see that using the sleep facility provided by Chez Scheme is a little bit more involved than the one provided by Guile Scheme. Other implementations are likely to have similar provisions, but they are not required to.

Upvotes: 2

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