Editing Call-with-current-continuation (section)

{{short description|Control flow operator in functional programming}}
{{lowercase title}}
In the [[Scheme (programming language)|Scheme]] [[computer programming]] language, the [[procedure (computer science)|procedure]] '''call-with-current-continuation''', abbreviated '''call/cc''', is used as a [[control flow]] operator. It has been adopted by several other programming languages.

Taking a function <code>f</code> as its only argument, <code>(call/cc f)</code> within an expression is applied to the current [[continuation]] of the expression.
For example <code>((call/cc f) e2)</code> is equivalent to applying <code>f</code> to the current continuation of the expression. The current continuation is given by replacing <code>(call/cc f)</code> by a variable <code>c</code> bound by a lambda abstraction, so the current continuation is <code>(lambda (c) (c e2))</code>. Applying the function <code>f</code> to it gives the final result <code>(f (lambda (c) (c e2)))</code>. 

As a complementary example, in an expression <code>(e1 (call/cc f))</code>, the continuation for the sub-expression <code>(call/cc f)</code> is  <code>(lambda (c) (e1 c))</code>, so the whole expression is equivalent to <code>(f (lambda (c) (e1 c)))</code>.
In other words it takes a "snapshot" of the current control context or control state of the program as an object and applies <code>f</code> to it.  
The continuation object is a [[first-class value]] and is represented as a function, with [[function application]] as its only operation.  When a continuation object is applied to an argument, the existing continuation is eliminated and the applied continuation is restored in its place, so that the program flow will continue at the point at which the continuation was captured and ''the argument of the continuation'' then becomes the "return value" of the call/cc invocation. Continuations created with call/cc may be called more than once, and even from outside the dynamic extent of the call/cc application.

In computer science, making this type of implicit program state visible as an object is termed ''[[Reification (computer science)|reification]]''. ([[Scheme (programming language)|Scheme]] does not syntactically distinguish between applying continuations or functions.)

With call/cc a variety of complex control operators can be implemented from other languages via a few lines of code, e.g., [[John McCarthy (computer scientist)|McCarthy]]'s <code>amb</code> [[Operator (computer programming)|operator]] for [[Nondeterministic programming|nondeterministic choice]], [[Prolog]]-style [[backtracking]], [[Simula]] 67-style [[coroutine]]s and generalizations thereof, [[Icon (programming language)|Icon]]-style [[Generator (computer programming)|generator]]s, or [[Engine (computer science)|engine]]s and [[Thread (computing)|thread]]s or even the obscure [[COMEFROM]]{{cn|date=December 2021}}.