Call-with-current-continuation

In Scheme programming, the function call-with-current-continuation, abbreviated call/cc, is a control operator. It has been adopted by several other programming languages.

Taking a function f as its only argument, call/cc takes the current continuation (i.e., a "snapshot" of the current control context or control state of the program) as an object and applies f 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.

Making this type of implicit program state visible as an object is known in computer science as reification. (Note that Scheme does not syntactically distinguish continuation application from function application.)

With call/cc a programmer can implement a variety of complex control operators from other languages via a few lines of code, e.g., McCarthy's amb operator for non-deterministic choice, Prolog-style backtracking, Simula 67-style coroutines and generalizations thereof, Icon-style generators, or engines and threads or even the obscure COMEFROM.

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