; is this the only case where walk returns a non-atom?
[(null? l) '()]
[(atom? (car l)) (begin
- (let/cc k2
+ (let/cc k2 (begin
(set! resume k2) ; now what will happen when resume is called?
; when the next line is executed, what will yield be bound to?
- (yield (car l)))
+ (yield (car l))))
; when will the next line be executed?
(walk (cdr l)))]
[else (begin
(walk (car l))
(walk (cdr l)))]))]
[next (lambda () ; next is a thunk
- (let/cc k3
+ (let/cc k3 (begin
(set! yield k3) ; now what will happen when yield is called?
; when the next line is executed, what will resume be bound to?
- (resume 'blah)))]
+ (resume 'blah))))]
[check (lambda (prev)
(let ([n (next)])
(cond
; when will n fail to be an atom?
[else #f])))])
(lambda (lst)
- (let ([fst (let/cc k1
+ (let ([fst (let/cc k1 (begin
(set! yield k1) ; now what will happen when yield is called?
(walk lst)
; when will the next line be executed?
- (yield '()))])
+ (yield '())))])
(cond
[(atom? fst) (check fst)]
; when will fst fail to be an atom?
Delimited control operators
===========================
-Here again is the CPS for `callcc`:
+Here again is the CPS transform for `callcc`:
[callcc (\k. body)] = \outk. (\k. [body] outk) (\v localk. outk v)
-`callcc` is what's known as an *undelimited control operator*. That is, the continuations `outk` that get bound into our `k`s include all the code from the `call/cc ...` out to *and including* the end of the program. Calling such a continuation will never return any value to the call site. (See the technique employed in the `delta` example above, with the `(begin (let/cc k2 ...) ...)`, for a work-around.)
+`callcc` is what's known as an *undelimited control operator*. That is, the continuations `outk` that get bound into our `k`s include all the code from the `call/cc ...` out to *and including* the end of the program. Calling such a continuation will never return any value to the call site.
+
+(See the technique employed in the `delta` example above, with the `(begin (let/cc k2 ...) ...)`, for a work-around. Also. if you've got a copy of *The Seasoned Schemer*, see the comparison of let/cc vs. "collector-using" (that is, partly CPS) functions at pp. 155-164.)
Often times it's more useful to use a different pattern, where we instead capture only the code from the invocation of our control operator out to a certain boundary, not including the end of the program. These are called *delimited control operators*. A variety of these have been formulated. The most well-behaved from where we're coming from is the pair `reset` and `shift`. `reset` sets the boundary, and `shift` binds the continuation from the position where it's invoked out to that boundary.
You use these like so:
-* [reset m] is `reset M` where `M` is [m]
-* [shift k M] is `shift (\k. M)` where `M` is [m]
-* and [abort M] is `abort M` where `M` is [m]
+* [reset body] is `reset BODY` where `BODY` is [body]
+* [shift k body] is `shift (\k. BODY)` where `BODY` is [body]
+* and [abort value] is `abort VALUE` where `VALUE` is [value]
There are also `reset` and `shift` and `abort` operations in the Continuation monad in our OCaml [[monad library]]. You can check the code for details.