36 lines
968 B
Scheme
36 lines
968 B
Scheme
(define-library (list-error)
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(import (scheme base))
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(export make-list-error
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list-error-list
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list-error-tail
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list-error-message
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list-error?
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circular-list-error?
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dotted-list-error?)
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(begin
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(define-record-type <list-error>
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(make-list-error list tail)
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list-error?
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(list list-error-list)
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(tail list-error-tail))
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(define (list-error-message err)
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(let ((tail (list-error-tail obj)))
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(cond ((null? tail)
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"Proper list")
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((pair? tail)
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"Circular list")
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((eq? tail (list-error-list obj))
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"Not a list")
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(else
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"Dotted list"))))
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(define (circular-list-error? obj)
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(and (list-error? obj)
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(pair? (list-error-tail obj))))
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(define (dotted-list-error? obj)
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(and (list-error? obj)
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(not (null-or-pair? (list-error-tail obj)))))))
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