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; Part of Scheme 48 1.9. See file COPYING for notices and license.
; Authors: Richard Kelsey, Jonathan Rees, Mike Sperber
; Nondeterminism, Prolog, or whatever you want to call it. This is
; depth-first search implemented using call/cc.
; The fluid variable $FAIL is bound to a thunk to be called in case of failure.
(define $fail
(make-fluid (make-cell
(lambda ()
(assertion-violation 'fail
"call to FAIL outside WITH-NONDETERMINISM")))))
(define (with-nondeterminism thunk)
(let-fluid $fail
(make-cell (lambda ()
(assertion-violation 'with-nondeterminism
"nondeterminism ran out of choices")))
thunk))
; Call the current failure function.
(define (fail)
((fluid-cell-ref $fail)))
; For the alternation operator, Icon's a | b or McCarthy's (amb a b),
; we write (either a b).
(define-syntax either
(syntax-rules ()
((either) (fail))
((either x) x)
((either x y ...)
(%either (lambda () x) (lambda () (either y ...))))))
; 1. Save the current failure procedure and continuation.
; 2. Install a new failure procedure that restores the old failure procedure
; and continuation and then calls THUNK2.
; 3. Call THUNK1.
(define (%either thunk1 thunk2)
(let ((save (fluid-cell-ref $fail)))
((call-with-current-continuation
(lambda (k)
(fluid-cell-set! $fail
(lambda ()
(fluid-cell-set! $fail save)
(k thunk2)))
thunk1)))))
; (one-value x) is Prolog's CUT operator. X is allowed to return only once.
(define-syntax one-value
(syntax-rules ()
((one-value x) (%one-value (lambda () x)))))
(define (%one-value thunk)
(let ((save (fluid-cell-ref $fail)))
(call-with-values thunk
(lambda args
(fluid-cell-set! $fail save)
(apply values args)))))
; (all-values a) returns a list of all the possible values of the
; expression a. Prolog calls this "bagof"; I forget what Icon calls it.
(define-syntax all-values
(syntax-rules ()
((all-values x) (%all-values (lambda () x)))))
(define (%all-values thunk)
(let ((results '()))
(either (let ((new-result (thunk)))
(set! results (cons new-result results))
(fail))
(reverse results))))
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