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).
