Section 3.3. Continuations

Continuation是什么？
During the evaluation of a Scheme expression, the implementation must keep track of two things: (1) what to evaluate and (2) what to do with the value. Consider the evaluation of (null? x) within the expression below.

(if (null? x) (quote ()) (cdr x))

The implementation must first evaluate (null? x) and, based on its value, evaluate either (quote ()) or (cdr x). "What to evaluate" is (null? x), and "what to do with the value" is to make the decision which of (quote ()) and (cdr x) to evaluate and to do so. We call "what to do with the value" the continuation of a computation.

(call/cc
  (lambda (k)
    (* 5 4)))  ;; => 20 

(call/cc
  (lambda (k)
    (* 5 (k 4)))) ;; => 4


在传递给`call/cc`的函数return之后调用该函数的continuation
(let ([x (call/cc (lambda (k) k))])
  (x (lambda (ignore) "hi"))) 
;; => "hi"

(((call/cc (lambda (k) k)) (lambda (x) x)) "HEY!")
;; => "HEY!"