54 lines
1.3 KiB
Scheme
54 lines
1.3 KiB
Scheme
#lang scheme/base
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(provide first second third fourth fifth sixth seventh eighth ninth tenth
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last
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rest
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cons?
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empty
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empty?)
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(define (first x)
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(if (and (pair? x) (list? x))
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(car x)
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(raise-type-error 'first "non-empty list" x)))
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(define-syntax define-lgetter
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(syntax-rules ()
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[(_ name npos)
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(define (name l0)
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(if (list? l0)
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(let loop ([l l0] [pos npos])
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(if (pair? l)
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(if (eq? pos 1) (car l) (loop (cdr l) (sub1 pos)))
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(raise-type-error
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'name (format "list with ~a or more items" npos) l0)))
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(raise-type-error 'name "list" l0)))]))
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(define-lgetter second 2)
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(define-lgetter third 3)
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(define-lgetter fourth 4)
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(define-lgetter fifth 5)
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(define-lgetter sixth 6)
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(define-lgetter seventh 7)
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(define-lgetter eighth 8)
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(define-lgetter ninth 9)
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(define-lgetter tenth 10)
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(define (last l)
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(if (and (pair? l) (list? l))
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(let loop ([l l])
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(if (pair? (cdr l))
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(loop (cdr l))
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(car l)))
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(raise-type-error 'last "non-empty list" l)))
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(define (rest l)
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(if (and (pair? l) (list? l))
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(cdr l)
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(raise-type-error 'rest "non-empty list" l)))
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(define cons? (lambda (l) (pair? l)))
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(define empty? (lambda (l) (null? l)))
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(define empty '())
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