39 lines
1.4 KiB
Racket
39 lines
1.4 KiB
Racket
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#lang syndicate
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;; After the Fahrenheit-to-Celsius converter example in "Fabrik - A
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;; Visual Programming Environment", Ingalls, Wallace, Chow, Ludolph
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;; and Doyle, OOPSLA 1988.
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;;---------------------------------------------------------------------------
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(define (make-idempotent-field name initial-value [=? equal?])
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(local-require (submod syndicate/actor implementation-details))
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(define f (make-field name initial-value))
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(match-lambda*
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[(list) (f)]
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[(list new-value)
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(unless (=? (field-ref (field-handle-desc f)) new-value)
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(f new-value))]))
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(define-syntax-rule (idempotent-field [id init] ...)
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(begin (define id (make-idempotent-field 'id init)) ...))
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;;---------------------------------------------------------------------------
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(struct temperature (unit value) #:prefab)
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(struct set-temperature (unit value) #:prefab)
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(spawn (idempotent-field [C 0] [F 32])
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(begin/dataflow (F (+ (* (C) 9/5) 32)))
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(begin/dataflow (C (* (- (F) 32) 5/9)))
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(assert (temperature 'C (C)))
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(assert (temperature 'F (F)))
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(on (message (set-temperature 'C $v)) (C v))
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(on (message (set-temperature 'F $v)) (F v)))
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(spawn (on (asserted (temperature $unit $value))
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(printf "Temperature in ~a is ~a\n" unit (exact->inexact value))))
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(spawn (on (asserted (observe (set-temperature _ _)))
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(send! (set-temperature 'C 20))
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(send! (set-temperature 'F 90))))
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