How call/cc Enables Creation of Control Structures

The call/cc function saves the stack at the moment it’s called, handing you a (magic) function (called k here) that provides you with the ability to do two things:

• Set the return value of call/cc
• Rewind the stack to what it looked like at the point in time when call/cc was invoked.

The following code uses it to implement a loop.

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#lang racket
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(let-values ([(loop n) (call/cc (λ (k) (values k 0)))])
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  (displayln n)
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  (when (< n 5)
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    (loop loop (+ n 1))))

We bind loop and n to be the spacetime traveler and loop counter respectively, and then we write the body: (displayln n). The last bit is the condition, which uses the spacetime traveler to rewind back to where loop and n were bound, and provide new values for them; loop is passed unchanged, and n is passed incremented.

I expect to be writing some full length posts on continuations, CPS, and the ContT monad transformer soon. Stay tuned!