53 lines
2.1 KiB
Scheme
53 lines
2.1 KiB
Scheme
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;;; setof.ss
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;;; Copyright (C) 1996 R. Kent Dybvig
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;;; from "The Scheme Programming Language, 2ed" by R. Kent Dybvig
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;;; Permission is hereby granted, free of charge, to any person obtaining a
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;;; copy of this software and associated documentation files (the "Software"),
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;;; to deal in the Software without restriction, including without limitation
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;;; the rights to use, copy, modify, merge, publish, distribute, sublicense,
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;;; and/or sell copies of the Software, and to permit persons to whom the
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;;; Software is furnished to do so, subject to the following conditions:
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;;;
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;;; The above copyright notice and this permission notice shall be included in
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;;; all copies or substantial portions of the Software.
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;;;
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;;; THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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;;; IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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;;; FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
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;;; THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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;;; LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
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;;; FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
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;;; DEALINGS IN THE SOFTWARE.
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;;; set-of uses helper syntactic extension set-of-help, passing it
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;;; an initial base expression of '()
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(define-syntax set-of
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(syntax-rules ()
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((_ e m ...)
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(set-of-help e '() m ...))))
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;;; set-of-help recognizes in, is, and predicate expressions and
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;;; changes them into nested named let, let, and if expressions.
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(define-syntax set-of-help
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(syntax-rules (in is)
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((_ e base)
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(set-cons e base))
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((_ e base (x in s) m ...)
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(let loop ((set s))
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(if (null? set)
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base
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(let ((x (car set)))
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(set-of-help e (loop (cdr set)) m ...)))))
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((_ e base (x is y) m ...)
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(let ((x y)) (set-of-help e base m ...)))
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((_ e base p m ...)
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(if p (set-of-help e base m ...) base))))
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;;; set-cons returns the original set y if x is already in y.
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(define set-cons
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(lambda (x y)
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(if (memv x y)
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y
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(cons x y))))
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