GNU bug report logs - #24080
fold-left description in (rnrs lists) section of guile manual incorrect

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: "Glenn Michaels" <gmichaels <at> Safe-mail.net>
To: bug-guile <at> gnu.org
Subject: fold-left description in (rnrs lists) section of guile manual
 incorrect
Date: Tue, 26 Jul 2016 16:15:06 -0400

The section on the (rnrs lists) library in the guile manual contains the following:

Scheme Procedure: fold-left combine nil list1 list2 …
Scheme Procedure: fold-right combine nil list1 list2 …

    These procedures are identical to the fold and fold-right procedures provided by SRFI-1.

In the case of fold-left and SRFI-1 fold, this is not correct; the order in which the arguments are passed to the combine procedure is different. Compare:

(let ((f (lambda (a b) (format #f "(f ~a ~a)" a b))))
  (for-each
   (lambda (fold)
     (display (fold f "0" '(a b c d e)))
     (format #t " ; ~s\n" (procedure-name fold)) )
   (list (@ (rnrs lists) fold-left)
         (@ (rnrs lists) fold-right)    ;same as srfi-1 fold-right
         (@ (srfi srfi-1) fold)
         (@ (srfi srfi-1) fold-right))))

Output:

(f (f (f (f (f 0 a) b) c) d) e) ; fold-left
(f a (f b (f c (f d (f e 0))))) ; fold-right
(f e (f d (f c (f b (f a 0))))) ; fold
(f a (f b (f c (f d (f e 0))))) ; fold-right

In the single-list case, fold-left can be thought of as applying f left-associatively to the elements of the list.
Moreover, in this case (fold f nil l) == (fold-left (lambda (x y) (f y x)) nil l).

(In the case of fold-right, both versions are identical).

Message #10 received at 24080-done <at> debbugs.gnu.org (full text, mbox):

From: Andy Wingo <wingo <at> pobox.com>
To: "Glenn Michaels" <gmichaels <at> Safe-mail.net>
Cc: 24080-done <at> debbugs.gnu.org
Subject: Re: bug#24080: fold-left description in (rnrs lists) section of guile
 manual incorrect
Date: Thu, 04 Aug 2016 22:40:25 +0200

On Tue 26 Jul 2016 22:15, "Glenn Michaels" <gmichaels <at> Safe-mail.net> writes:

> The section on the (rnrs lists) library in the guile manual contains the following:
>
> Scheme Procedure: fold-left combine nil list1 list2 …
> Scheme Procedure: fold-right combine nil list1 list2 …
>
>     These procedures are identical to the fold and fold-right procedures provided by SRFI-1.

I believe this was fixed by

  commit 498cd58cb43e0dde024ad3c62983def16a0c7ca9
  Author: Andy Wingo <wingo <at> pobox.com>
  Date:   Thu Jun 23 21:39:23 2016 +0200

      Fix R6RS fold-left documentation
      
      * doc/ref/r6rs.texi (rnrs lists): Fix documentation of fold-left.

Should be out in 2.1.4.

Cheers,

Andy

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