91 lines
2.1 KiB
Scheme
91 lines
2.1 KiB
Scheme
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; Copyright (c) 1993, 1994 Richard Kelsey and Jonathan Rees. See file COPYING.
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; Internal variable representing the failure stack.
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(define (fail) (*fail*))
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(define *fail* (lambda () (error "You didn't do (init).")))
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; For the alternation operator, Icon's a | b or McCarthy's (amb a b),
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; we write (either a b).
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(define-syntax either
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(syntax-rules ()
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((either) (fail))
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((either x) x)
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((either x y ...)
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(%either (lambda () x) (lambda () (either y ...))))))
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(define (%either thunk1 thunk2) ;Macro auxiliary
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(saving-failure-state
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(lambda (restore)
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((call-with-current-continuation
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(lambda (k)
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(set! *fail*
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(lambda ()
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(restore)
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(k thunk2)))
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thunk1))))))
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(define (saving-failure-state proc)
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(let ((save *fail*))
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(proc (lambda () (set! *fail* save)))))
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; (one-value x) is Prolog's CUT operator
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(define-syntax one-value
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(syntax-rules ()
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((one-value x) (%one-value (lambda () x)))))
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(define (%one-value thunk)
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(saving-failure-state
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(lambda (restore)
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(let ((value (thunk)))
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(restore)
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value))))
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; (all-values a) returns a list of all the possible values of the
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; expression a. Prolog calls this "bagof"; I forget what Icon calls it.
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(define-syntax all-values
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(syntax-rules ()
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((all-values x) (%all-values (lambda () x)))))
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(define (%all-values thunk)
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(let ((results '()))
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(either (let ((new-result (thunk)))
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(set! results (cons new-result results))
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(fail))
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(reverse results))))
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; Generate all the members of list l. E.g.
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; (all-values (+ (member-of '(10 20 30)) (member-of '(1 2 3))))
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; => '(11 12 13 21 22 23 31 32 33)
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(define (member-of l)
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(if (null? l)
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(fail)
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(either (car l) (member-of (cdr l)))))
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; Crufty initialization hack that allows you to type failing
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; expressions at the R-E-P loop (if there is an R-E-P loop). E.g. try
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; evaluating the sequence
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; (either 1 2)
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; (fail)
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; (+ (fail) 10)
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(define (init)
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(set! *fail* #f) ;for GC purposes
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(either 'initialized
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(let loop ()
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(either 'failed (loop)))))
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(display "Type (init) at the read-eval-print loop.")
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(newline)
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