Editing Scheme (programming language) (section)

===First-class continuations===
{{Main|Continuation}}
Continuations in Scheme are [[first-class object]]s. Scheme provides the procedure <code>[[call-with-current-continuation]]</code> (also known as <code>call/cc</code>) to capture the current continuation by packing it up as an escape procedure bound to a formal argument in a procedure provided by the programmer. (R5RS sec. 6.4)<ref name="r5rs"/> First-class continuations enable the programmer to create non-local [[Control flow|control constructs]] such as [[iterator]]s, [[coroutine]]s, and [[backtracking]].

Continuations can be used to emulate the behavior of [[return statement]]s in imperative programming languages. The following function <code>find-first</code>, given function <code>func</code> and list <code>lst</code>, returns the first element <code>x</code> in <code>lst</code> such that <code>(func x)</code> returns true.

<syntaxhighlight lang="scheme">
(define (find-first func lst)
  (call-with-current-continuation
   (lambda (return-immediately)
     (for-each (lambda (x)
		 (if (func x)
		     (return-immediately x)))
	  lst)
     #f)))

(find-first integer? '(1/2 3/4 5.6 7 8/9 10 11))
===> 7
(find-first zero? '(1 2 3 4))
===> #f
</syntaxhighlight>

The following example, a traditional programmer's puzzle, shows that Scheme can handle continuations as first-class objects, binding them to variables and passing them as arguments to procedures.

<syntaxhighlight lang="scheme">
(let* ((yin
         ((lambda (cc) (display "@") cc) (call-with-current-continuation (lambda (c) c))))
       (yang
         ((lambda (cc) (display "*") cc) (call-with-current-continuation (lambda (c) c)))))
    (yin yang))
</syntaxhighlight>
When executed this code displays a counting sequence: <code>@*@**@***@****@*****@******@*******@********...</code>
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