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copied!<p>The essence of <code>call-with-concurrent-continuation</code>, or <code>call/cc</code> for short, is the ability to grab checkpoints, or continuations, during the execution of a program. Then, you can go back to those checkpoints by applying them like functions.</p> <p>Here's a simple example where the continuation isn't used:</p> <pre><code>> (call/cc (lambda (k) (+ 2 3))) 5 </code></pre> <p>If you don't use the continuation, it's hard to tell the difference. Here's a few where we actually use it:</p> <pre><code>> (call/cc (lambda (k) (+ 2 (k 3)))) 3 > (+ 4 (call/cc (lambda (k) (+ 2 3)))) 9 > (+ 4 (call/cc (lambda (k) (+ 2 (k 3))))) 7 </code></pre> <p>When the continuation is invoked, control flow jumps back to where the continuation was grabbed by <code>call/cc</code>. Think of the <code>call/cc</code> expression as a hole that gets filled by whatever gets passed to <code>k</code>.</p> <p><code>list-iter</code> is a substantially more complex use of <code>call/cc</code>, and might be a difficult place to begin using it. First, here's an example usage:</p> <pre><code>> (define i (list-iter '(a b c))) > (i) a > (i) b > (i) c > (i) list-ended > (i) list-ended </code></pre> <p>Here's a sketch of what's happening:</p> <ol> <li><code>list-iter</code> returns a procedure of no arguments <code>i</code>.</li> <li>When <code>i</code> is invoked, we grab a continuation immediately and pass it to <code>control-state</code>. When that continuation, bound to <code>return</code>, is invoked, we'll immediately return to whoever invoked <code>i</code>.</li> <li>For each element in the list, we grab a new continuation and overwrite the definition of <code>control-state</code> with that new continuation, meaning that we'll resume from there the next time step 2 comes along.</li> <li>After setting up <code>control-state</code> for the next time through, we pass the current element of the list back to the <code>return</code> continuation, yielding an element of the list.</li> <li>When <code>i</code> is invoked again, repeat from step 2 until the <code>for-each</code> has done its work for the whole list.</li> <li>Invoke the <code>return</code> continuation with <code>'list-ended</code>. Since <code>control-state</code> isn't updated, it will keep returning <code>'list-ended</code> every time <code>i</code> is invoked.</li> </ol> <p>As I said, this is a fairly complex use of <code>call/cc</code>, but I hope this is enough to get through this example. For a gentler introduction to continuations, I'd recommend picking up <a href="http://rads.stackoverflow.com/amzn/click/026256100X" rel="nofollow">The Seasoned Schemer</a>.</p>

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