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37 lines
850 B
Racket
37 lines
850 B
Racket
#lang racket/base
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(require (for-syntax racket/base))
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(provide (all-defined-out))
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(define-syntax-rule (until COND EXPR ...)
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(let loop ()
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(unless COND
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EXPR ...
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(loop))))
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(define-syntax-rule (while COND EXPR ...)
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(let loop ()
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(when COND
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EXPR ...
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(loop))))
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(define-syntax (forever stx)
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(syntax-case stx ()
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[(_ . EXPRS)
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;; todo: would be better with a syntax parameter
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(with-syntax ([stop (datum->syntax #'EXPRS 'stop)])
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#'(let/ec stop
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(while #t
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. EXPRS)))]))
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(module+ test
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(require rackunit)
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(check-equal? (let ([x 5])
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(until (zero? x)
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(set! x (- x 1)))
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x) 0)
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(check-equal? (let ([x 5])
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(while (positive? x)
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(set! x (- x 1)))
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x) 0))
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