srfi-1 circular-list

srfi-1 circular-list

[Kevin Ryde]

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        * srfi-1.scm (circular-list): Rewrite using set-cdr!, no need to copy
        parameter list.


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--- srfi-1.scm.~1.31.~	2003-12-03 07:14:46.000000000 +1000
+++ srfi-1.scm	2004-03-18 14:59:56.000000000 +1000
@@ -1,6 +1,6 @@
 ;;; srfi-1.scm --- List Library
 
-;; 	Copyright (C) 2001, 2002, 2003 Free Software Foundation, Inc.
+;; 	Copyright (C) 2001, 2002, 2003, 2004 Free Software Foundation, Inc.
 ;;
 ;; This library is free software; you can redistribute it and/or
 ;; modify it under the terms of the GNU Lesser General Public
@@ -247,16 +247,10 @@
       acc
       (lp (- n 1) (cons (init-proc (- n 1)) acc)))))
 
-(define (circular-list elt1 . rest)
-  (let ((start (cons elt1 '())))
-    (let lp ((r rest) (p start))
-      (if (null? r)
-	(begin
-	  (set-cdr! p start)
-	  start)
-	(begin
-	  (set-cdr! p (cons (car r) '()))
-	  (lp (cdr r) (cdr p)))))))
+(define (circular-list . lst)
+  (if (not (null? lst))
+      (set-cdr! (last-pair lst) lst))
+  lst)
 
 (define (iota count . rest)
   (check-arg-type non-negative-integer? count "iota")

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Re: srfi-1 circular-list

[Paul Jarc]

Kevin Ryde <user42@zip.com.au> wrote:
> +(define (circular-list . lst)
> +  (if (not (null? lst))
> +      (set-cdr! (last-pair lst) lst))
> +  lst)

Or, if we want to preserve the error when no elements are given:
(define (circular-list elt1 . rest)
  (define lst (cons elt1 rest))
  (set-cdr! (last-pair lst) lst)
  lst)


paul


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Re: srfi-1 circular-list

[Kevin Ryde]

prj@po.cwru.edu (Paul Jarc) writes:
>
> Or, if we want to preserve the error when no elements are given:

Oh, yes.  I don't know if it's supposed to be an error.  Perhaps it
should be left that way.


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