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copied!<p>A heads up, this example is not concise nor exceptionally clear. This is a demonstration of a powerful application of continuations. As a VB/ASP/C# programmer, you may not be familiar with the concept of a system stack or saving state, so the goal of this answer is a demonstration and not an explanation.</p> <p>Continuations are extremely versatile and are a way to save execution state and resume it later. Here is a small example of a cooperative multithreading environment using continuations in Scheme:</p> <p>(Assume that the operations enqueue and dequeue work as expected on a global queue not defined here)</p> <pre><code>(define (fork) (display "forking\n") (call-with-current-continuation (lambda (cc) (enqueue (lambda () (cc #f))) (cc #t)))) (define (context-switch) (display "context switching\n") (call-with-current-continuation (lambda (cc) (enqueue (lambda () (cc 'nothing))) ((dequeue))))) (define (end-process) (display "ending process\n") (let ((proc (dequeue))) (if (eq? proc 'queue-empty) (display "all processes terminated\n") (proc)))) </code></pre> <p>This provides three verbs that a function can use - fork, context-switch, and end-process. The fork operation forks the thread and returns #t in one instance and #f in another. The context-switch operation switches between threads, and end-process terminates a thread.</p> <p>Here's an example of their use:</p> <pre><code>(define (test-cs) (display "entering test\n") (cond ((fork) (cond ((fork) (display "process 1\n") (context-switch) (display "process 1 again\n")) (else (display "process 2\n") (end-process) (display "you shouldn't see this (2)")))) (else (cond ((fork) (display "process 3\n") (display "process 3 again\n") (context-switch)) (else (display "process 4\n"))))) (context-switch) (display "ending process\n") (end-process) (display "process ended (should only see this once)\n")) </code></pre> <p>The output should be</p> <pre><code>entering test forking forking process 1 context switching forking process 3 process 3 again context switching process 2 ending process process 1 again context switching process 4 context switching context switching ending process ending process ending process ending process ending process ending process all processes terminated process ended (should only see this once) </code></pre> <p>Those that have studied forking and threading in a class are often given examples similar to this. The purpose of this post is to demonstrate that with continuations you can achieve similar results within a single thread by saving and restoring its state - its continuation - manually.</p> <p>P.S. - I think I remember something similar to this in On Lisp, so if you'd like to see professional code you should check the book out.</p>

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