Timeouts in Scheme

Timeouts are important for programs that connect to external resources or that otherwise need to bound execution time. Implementing timeouts in Scheme can be achieved using threads and synchronization primitives.

(import (rnrs)
        (rnrs threads)
        (srfi :18))

(define (main)
  ; For our example, suppose we're executing an external
  ; call that returns its result on a channel after 2s.
  ; In Scheme, we'll use a mailbox to simulate this behavior.
  (define c1 (make-mailbox))
  (thread-start!
   (make-thread
    (lambda ()
      (thread-sleep! 2)
      (mailbox-send c1 "result 1"))))

  ; Here's the select implementing a timeout.
  ; We'll use mailbox-receive-timeout! to wait for the result
  ; with a timeout of 1 second.
  (let ((result (mailbox-receive-timeout! c1 1)))
    (if result
        (display (string-append result "\n"))
        (display "timeout 1\n")))

  ; If we allow a longer timeout of 3s, then the receive
  ; from c2 will succeed and we'll print the result.
  (define c2 (make-mailbox))
  (thread-start!
   (make-thread
    (lambda ()
      (thread-sleep! 2)
      (mailbox-send c2 "result 2"))))

  (let ((result (mailbox-receive-timeout! c2 3)))
    (if result
        (display (string-append result "\n"))
        (display "timeout 2\n"))))

(main)

Running this program shows the first operation timing out and the second succeeding.

$ scheme --script timeouts.scm
timeout 1
result 2

In this Scheme implementation:

1. We use the (rnrs threads) library for thread operations and (srfi :18) for additional threading utilities.

2. Instead of Go’s channels, we use Scheme’s mailboxes, which provide similar functionality for inter-thread communication.

3. The thread-start! function is used to create and start new threads, similar to Go’s goroutines.

4. We use thread-sleep! to simulate time-consuming operations.

5. The mailbox-receive-timeout! function is used to implement the timeout mechanism. It waits for a message from the mailbox for a specified duration and returns #f if the timeout is reached.

6. The select statement in Go is replaced with a simple if statement that checks whether a result was received before the timeout.

This Scheme code demonstrates how to implement timeouts for operations that might take too long to complete, allowing your program to move on or take alternative actions when necessary.