152 lines
4.5 KiB
Scheme
152 lines
4.5 KiB
Scheme
;;;
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;;; <selector.ss> ---- List selectors
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;;; Time-stamp: <02/02/27 12:49:44 noel>
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;;;
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;;; Copyright (C) 2002 by Noel Welsh.
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;;;
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;;; This file is part of SRFI-1.
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;;; SRFI-1 is free software; you can redistribute it and/or
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;;; modify it under the terms of the GNU Lesser General Public
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;;; License as published by the Free Software Foundation; either
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;;; version 2.1 of the License, or (at your option) any later version.
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;;; SRFI-1 is distributed in the hope that it will be useful,
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;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
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;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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;;; Lesser General Public License for more details.
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;;; You should have received a copy of the GNU Lesser General Public
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;;; License along with SRFI-1; if not, write to the Free Software
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;;; Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
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;;; Author: Noel Welsh <noelwelsh@yahoo.com>
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;;
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;;
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;; Commentary:
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;; Based on the reference implementation by Olin Shiver and hence:
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;; Copyright (c) 1998, 1999 by Olin Shivers. You may do as you please with
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;; this code as long as you do not remove this copyright notice or
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;; hold me liable for its use. Please send bug reports to shivers@ai.mit.edu.
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;; -Olin
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#lang scheme/base
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(require srfi/optional
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(only-in scheme/list take drop take-right drop-right split-at))
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(provide first second
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third fourth
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fifth sixth
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seventh eighth
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ninth tenth
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car+cdr
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take drop
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take-right drop-right
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(rename-out [take take!]) (rename-out [drop-right drop-right!])
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split-at (rename-out [split-at split-at!])
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last
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last-pair)
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(define first car)
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(define second cadr)
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(define third caddr)
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(define fourth cadddr)
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(define (fifth x) (car (cddddr x)))
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(define (sixth x) (cadr (cddddr x)))
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(define (seventh x) (caddr (cddddr x)))
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(define (eighth x) (cadddr (cddddr x)))
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(define (ninth x) (car (cddddr (cddddr x))))
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(define (tenth x) (cadr (cddddr (cddddr x))))
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(define (car+cdr pair) (values (car pair) (cdr pair)))
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;; take & drop
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#; ; provided by scheme/list
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(define (take lis k)
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(check-arg integer? k 'take)
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(let recur ((lis lis) (k k))
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(if (zero? k) '()
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(cons (car lis)
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(recur (cdr lis) (- k 1))))))
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#; ; provided by scheme/list
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(define (drop lis k)
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(check-arg integer? k 'drop)
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(let iter ((lis lis) (k k))
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(if (zero? k) lis (iter (cdr lis) (- k 1)))))
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#; ; lists are immutable
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(define (take! lis k)
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(check-arg integer? k 'take!)
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(if (zero? k) '()
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(begin (set-cdr! (drop lis (- k 1)) '())
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lis)))
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;; TAKE-RIGHT and DROP-RIGHT work by getting two pointers into the list,
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;; off by K, then chasing down the list until the lead pointer falls off
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;; the end.
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#; ; provided by scheme/list
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(define (take-right lis k)
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(check-arg integer? k 'take-right)
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(let lp ((lag lis) (lead (drop lis k)))
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(if (pair? lead)
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(lp (cdr lag) (cdr lead))
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lag)))
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#; ; provided by scheme/list
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(define (drop-right lis k)
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(check-arg integer? k 'drop-right)
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(let recur ((lag lis) (lead (drop lis k)))
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(if (pair? lead)
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(cons (car lag) (recur (cdr lag) (cdr lead)))
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'())))
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;; In this function, LEAD is actually K+1 ahead of LAG. This lets
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;; us stop LAG one step early, in time to smash its cdr to ().
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#; ; lists are immutable
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(define (drop-right! lis k)
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(check-arg integer? k 'drop-right!)
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(let ((lead (drop lis k)))
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(if (pair? lead)
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(let lp ((lag lis) (lead (cdr lead))) ; Standard case
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(if (pair? lead)
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(lp (cdr lag) (cdr lead))
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(begin (set-cdr! lag '())
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lis)))
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'()))) ; Special case dropping everything -- no cons to side-effect.
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#; ; provided by scheme/list
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(define (split-at x k)
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(check-arg integer? k 'split-at)
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(let recur ((lis x) (k k))
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(if (zero? k) (values '() lis)
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(let-values ([(prefix suffix) (recur (cdr lis) (- k 1))])
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(values (cons (car lis) prefix) suffix)))))
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#; ; lists are immutable
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(define (split-at! x k)
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(check-arg integer? k 'split-at!)
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(if (zero? k) (values '() x)
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(let* ((prev (drop x (- k 1)))
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(suffix (cdr prev)))
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(set-cdr! prev '())
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(values x suffix))))
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;; these could be reprovided from scheme/base, but they don't raise an
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;; error on improper lists
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(define (last lis) (car (last-pair lis)))
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(define (last-pair lis)
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(check-arg pair? lis 'last-pair)
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(let lp ((lis lis))
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(let ((tail (cdr lis)))
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(if (pair? tail) (lp tail) lis))))
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;;; selector.ss ends here
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