[racket] Looping with look-behind and look-forward

On Sun, May 27, 2012 at 8:46 AM, Harry Spier <vasishtha.spier at gmail.com> wrote:
> (define (f1 l)
>   (for/fold ([result-list '()])
>    ( [prior (in-list (append '() l))]
>      [x (in-list l)]
>      [next (in-list  (append (rest l) '()))])
>    ...
>    ... make new-result-element using x prior and next...
>   (cons new-result-element result-list))
>  (reverse result-list))

If you don't need result-list in the body of the for you can
use for/list which looks a bit simpler.

(define (f1 l)
  (for/list    ([prior (in-list (append '() l))]
                [x (in-list l)]
                [next (in-list  (append (rest l) '()))])
   ...
   ... make new-result-element using x prior and next...))


Cheers,
Sam