100 lines
2.3 KiB
Scheme
100 lines
2.3 KiB
Scheme
; -*- Mode: Scheme; Syntax: Scheme; Package: Scheme; -*-
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; Copyright (c) 1993-1999 by Richard Kelsey and Jonathan Rees. See file COPYING.
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; This is file util.scm.
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;;;; Utilities
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(define (unspecific) (if #f #f))
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(define (reduce cons nil list) ;used by length, append, etc.
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(if (null? list)
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nil
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(cons (car list) (reduce cons nil (cdr list)))))
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(define (filter pred lst)
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(reduce (lambda (x rest)
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(if (pred x) (cons x rest) rest))
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'()
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lst))
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; Position of an object within a list
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(define (pos pred)
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(lambda (thing l)
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(let loop ((i 0) (l l))
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(cond ((null? l) #f)
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((pred thing (car l)) i)
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(else (loop (+ i 1) (cdr l)))))))
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(define posq (pos eq?))
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(define posv (pos eqv?))
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(define position (pos equal?))
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; Is pred true of any element of l?
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(define (any pred l)
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;; (reduce or #f l), sort of
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(if (null? l)
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#f
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(or (pred (car l))
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(any pred (cdr l)))))
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; Is pred true of every element of l?
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(define (every pred l)
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;; (reduce and #t l), sort of
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(if (null? l)
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#t
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(and (pred (car l)) (every pred (cdr l)))))
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(define (sublist l start end)
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(if (> start 0)
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(sublist (cdr l) (- start 1) (- end 1))
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(let recur ((l l) (end end))
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(if (= end 0)
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'()
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(cons (car l) (recur (cdr l) (- end 1)))))))
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(define (last x)
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(if (null? (cdr x))
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(car x)
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(last (cdr x))))
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(define (insert x l <)
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(cond ((null? l) (list x))
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((< x (car l)) (cons x l))
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(else (cons (car l) (insert x (cdr l) <)))))
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;----------------
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; Variations on a theme.
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;
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; FOLD is a tail-recursive version of REDUCE.
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(define (fold folder list accumulator)
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(do ((list list (cdr list))
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(accum accumulator (folder (car list) accum)))
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((null? list)
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accum)))
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(define (fold->2 folder list acc0 acc1)
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(let loop ((list list) (acc0 acc0) (acc1 acc1))
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(if (null? list)
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(values acc0 acc1)
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(call-with-values
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(lambda ()
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(folder (car list) acc0 acc1))
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(lambda (acc0 acc1)
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(loop (cdr list) acc0 acc1))))))
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(define (fold->3 folder list acc0 acc1 acc2)
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(let loop ((list list) (acc0 acc0) (acc1 acc1) (acc2 acc2))
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(if (null? list)
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(values acc0 acc1 acc2)
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(call-with-values
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(lambda ()
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(folder (car list) acc0 acc1 acc2))
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(lambda (acc0 acc1 acc2)
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(loop (cdr list) acc0 acc1 acc2))))))
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