Environment not part of a continuation?

Is the environment not part of a continuation in scheme?

I have tested this with Chicken, Gauche, Racket and Gambit, and they all behave similarly:

(define kont #f)

(let ((a 1)
      (b 2))
  (call-with-current-continuation
   (lambda (k)
     (set! kont k)
     (display 'mutating)
     (newline)
     (set! a -1)
    开发者_运维问答 (set! b -2)))
  (display (+ a b))
  (newline))

I would expect -3 when the LET is evaluated, but +3 in the calls to kont (since I thought the program would remember the bindings of a and b before mutation):

(let ... ) ; <-- evaluating the LET above
; prints "mutating"
=> -3
(kont 100)
=> -3
(kont 100)
=> -3

So the continuation only affects control, and not the environment? In this case, why is it said that one of the ways to implement continuations is to "copy the stack" (are bindings are not on the stack?)

The continuation captures the bindings. However, as you surmise, these bindings are mutable.

You've been somewhat misled, here, by the "copies the stack" slogan. While this is a reasonable way to think about call/cc, it's not the whole story. For one thing, you really really wouldn't want a language feature that exposed whether or not local bindings were stack-allocated or not.

Instead, call/cc is defined using the notion of "program contexts". For a nice treatment of this, you might want to take a look at Shriram Krishnamurthi's (free, online) textbook PLAI, or at the (not-free, much more in-depth) book "Semantics Engineering with PLT Redex".

As an aside; your program doesn't really check what you wanted it to check, because you never invoked the captured continuation. I think you wanted to write something like this:

#lang racket

(define kont #f)

(let ([a 3])

  (let/cc k
    (set! kont k)
    (set! a 4))
  (printf "~s\n" a))

(kont)

... which shows pretty much the same behavior that you mention above.

You change the values of a and b in the environment with set!. So a and b is -1 and -2 in the continuation environment. You can not unroll side effects. There are no differences between a, b and kont in your continuation.