37 lines
1000 B
Racket
37 lines
1000 B
Racket
#lang racket
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(require racket/serialize)
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(define-serializable-struct soft-state-record (thnk))
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(define-struct some (value))
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(define *soft-state-cache*
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(make-weak-hash))
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(define (make-soft-state thnk)
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(make-soft-state-record thnk))
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(define (soft-state-ref ss)
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(match ss
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[(struct soft-state-record (thnk))
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(define the-weak-box
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(hash-ref! *soft-state-cache* ss (lambda () (make-weak-box (make-some (thnk))))))
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(define the-val
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(weak-box-value the-weak-box))
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(if (some? the-val)
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(some-value the-val)
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(local [(define real-val (thnk))]
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(hash-set! *soft-state-cache* ss (make-weak-box (make-some real-val)))
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real-val))]))
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(define soft-state? soft-state-record?)
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(define-syntax-rule (soft-state expr ...)
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(make-soft-state (lambda () expr ...)))
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(provide
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soft-state)
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(provide/contract
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[soft-state? (any/c . -> . boolean?)]
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[make-soft-state ((-> any/c) . -> . soft-state?)]
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[soft-state-ref (soft-state? . -> . any/c)])
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