65 lines
1.7 KiB
Racket
65 lines
1.7 KiB
Racket
#lang planet dyoo/whalesong/base
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Why on earth are these here?
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Because first, etc. don't work on cyclic lists:
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(define web-colors
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(shared ([W (cons "white" G)]
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[G (cons "grey" W)])
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W))
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(first web-colors)
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fails with expected argument of type <list>.
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But car/cdr still do the trick per email from mflatt, 10/20/2011.
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So we suppress the built-in functions from lang/htdp-advanced
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and provide these instead.
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|#
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(require (for-syntax racket/base))
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(provide first second third fourth fifth sixth seventh eighth ninth
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rest)
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(define (rest x)
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(cond
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[(pair? x)
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(cdr x)]
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[else
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(raise-type-error 'rest
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"list with at least one element"
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x)]))
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(define-syntax (define-list-selectors stx)
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(syntax-case stx ()
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[(_ [(name ordinal) ...])
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(with-syntax ([(offset ...)
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(build-list (length (syntax->list #'(name ...)))
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(lambda (i) i))])
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#'(begin
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(define (name p)
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(pair-ref p offset 'name 'ordinal p))
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...))]))
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(define (pair-ref x offset name ordinal original)
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(cond
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[(pair? x)
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(cond
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[(= offset 0)
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(car x)]
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[else
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(pair-ref (cdr x) (sub1 offset) name ordinal original)])]
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[else
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(raise-type-error name
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(format "list with ~a elements" ordinal)
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original)]))
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(define-list-selectors [[first one]
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[second two]
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[third three]
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[fourth four]
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[fifth five]
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[sixth six]
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[seventh seven]
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[eighth eight]
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[ninth nine]])
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