[plt-scheme] Doubt in folding

On Thu, Oct 22, 2009 at 3:24 PM, Eduardo Bellani <ebellani at gmail.com> wrote:
> ;; my-fold-left : (X X -> X) X (listof X) -> (listof X)
> ;; applies operation to each element of lox, and
> ;; combines them in a new list. Works from left to right
> ;; in lox.
> (define (my-fold-left combinator initial lox)
>  (local [(define (folding local-lox accumulator)
>            (cond
>              [(empty? local-lox) accumulator]
>              [else
>               (folding (rest local-lox)
>                        (combinator (first local-lox)
>                                    accumulator))]))]
>    (folding lox initial)))
>
>
> ;; plt's built in foldl
> (foldl cons empty (list 1 2 3 4))         ; evals to (4 3 2 1)
>
> ;; mine
> (my-fold-left cons empty (list 1 2 3 4))  ; evals to (4 3 2 1) thanks
> for the hint!


This version also works for the limited test cases I ran:

(define (myfoldl comb init l)
  (if (empty? l)
      init
      (myfoldl comb
               (comb (car l) init)
	       (cdr l))))

Dave