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

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From: "Glenn Michaels" <gmichaels <at> Safe-mail.net>
To: 24080 <at> debbugs.gnu.org
Subject: bug#24080: 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).

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