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|
;;;; Fun with Continuations
#| Adapted from MIT/GNU Scheme Reference Manual [Section 12.4]:
(call-with-current-continuation
(lambda (exit)
(for-each (lambda (x)
(if (negative? x)
(exit x)))
'(54 0 37 -3 245 -19)) ; **
#t))
;Value: -3
|#
;;; Continuations as Non-Local Exits
(define (funco:first-negative list-of-numbers)
(call-with-current-continuation
(lambda (k_exit)
(or (call-with-current-continuation
(lambda (k_shortcut)
(for-each (lambda (n)
(cond ((not (number? n))
(pp `(not-a-number: ,n))
(k_exit #f))
((negative? n)
(k_shortcut n))
(else
':keep-looking)))
list-of-numbers)
#f ;; Fall-through sentinel: no negatives found.
))
':no-negatives-found))))
#|
(funco:first-negative '(54 0 37 -3 245 -19))
;Value: -3
(funco:first-negative '(54 0 37 3 245 19))
;Value: :no-negatives-found
(funco:first-negative '(54 0 37 no 245 boo))
(not-a-number: no)
;Value: #f
|#
;;; Continuations for Proceeding (Suspend/Resume Backtracking)
(define (funco:first-negative-n-proceed list-of-numbers) ;;; **
(call-with-current-continuation
(lambda (k_exit)
(or (call-with-current-continuation
(lambda (k_shortcut)
(for-each (lambda (n)
(pp ;;; **
(call-with-current-continuation ;;; **
(lambda (k_proceed) ;;; **
(cond ((not (number? n))
(pp `(not-a-number: ,n))
(k_exit
(cons n k_proceed))) ;;; **
((negative? n)
(k_shortcut
(cons n k_proceed))) ;;; **
(else
':keep-looking)))
))) ;;; **
list-of-numbers)
#f ;; Fall-through sentinel: no negatives found.
))
':no-negatives-found))))
(define (funco:first-negative-n-proceed-more? smore) (pair? smore))
(define (funco:first-negative-n-proceed-more/found smore) (car smore))
(define (funco:first-negative-n-proceed-more/k smore) (cdr smore))
(define (funco:first-negative-n-proceed-more/next smore)
((funco:first-negative-n-proceed-more/k smore)
(funco:first-negative-n-proceed-more/found smore)))
#|
;;; ------------
(define funco:first-of-two
(funco:first-negative-n-proceed '(54 0 37 -3 245 -19)))
:keep-looking
:keep-looking
:keep-looking
;Value: funco:first-of-two
(funco:first-negative-n-proceed-more? funco:first-of-two)
;Value: #t
(funco:first-negative-n-proceed-more/found funco:first-of-two)
;Value: -3
(funco:first-negative-n-proceed-more/next funco:first-of-two)
-3
:keep-looking
;Value: funco:first-of-two
(funco:first-negative-n-proceed-more? funco:first-of-two)
;Value: #t
(funco:first-negative-n-proceed-more/found funco:first-of-two)
;Value: -19
(funco:first-negative-n-proceed-more/next funco:first-of-two)
-19
;Value: funco:first-of-two
(funco:first-negative-n-proceed-more? funco:first-of-two)
;Value: #f
funco:first-of-two
;Value: :no-negatives-found
|#
#|
;;; ----
(define funco:nada
(funco:first-negative-n-proceed '(54 0 37 3 245 19)))
:keep-looking
:keep-looking
:keep-looking
:keep-looking
:keep-looking
:keep-looking
;Value: funco:nada
(funco:first-negative-n-proceed-more? funco:nada)
;Value: #f
funco:nada
;Value: :no-negatives-found
|#
#|
;;; ----
(define funco:nans
(funco:first-negative-n-proceed '(54 0 37 no 245 boo)))
:keep-looking
:keep-looking
:keep-looking
(not-a-number: no)
;Value: funco:nans
(funco:first-negative-n-proceed-more? funco:nans)
;Value: #t
(funco:first-negative-n-proceed-more/found funco:nans)
;Value: no
(funco:first-negative-n-proceed-more/next funco:nans)
no
:keep-looking
(not-a-number: boo)
;Value: funco:nans
(funco:first-negative-n-proceed-more? funco:nans)
;Value: #t
(funco:first-negative-n-proceed-more/next funco:nans)
boo
;Value: funco:nans
(funco:first-negative-n-proceed-more? funco:nans)
;Value: #f
funco:nans
;Value: :no-negatives-found
|#
;;; Continuations for Backtracking (Re-entrant 1st-Class Continuations)
(define *k_re-funco*)
(define funco)
#|
(begin
(set! funco (+ 2 (call-with-current-continuation
(lambda (k_re-funco)
(set! *k_re-funco* k_re-funco)
3))))
':ok)
;Value: :ok
funco
;Value: 5
(*k_re-funco* 4)
;Value: :ok
funco
;Value: 6
(*k_re-funco* 5)
;Value: :ok
funco
;Value: 7
|#
;;; Dynamic Contexts and Within-Continuation
(define (funco:test-k-thunk k-thunk)
(let ((*foo* 2)) ;----------------------.
(define (foo-thunk) *foo*) ; *foo* is 2 out here. :
(call-with-current-continuation ; :
(lambda (k) ; :
(fluid-let ((*foo* 3)) ;---------------------. :
(k-thunk k foo-thunk) ; *foo* is 3 in here. : :
) ;---------------------' :
)) ; *foo* is 2 out here. :
)) ;----------------------'
#|
(funco:test-k-thunk (lambda (k thunk)
(k (thunk))))
;Value: 3
(funco:test-k-thunk (lambda (k thunk)
(within-continuation k thunk)))
;Value: 2
|#
|
