[racket] Building an indexing function for lists-of-lists

Thanks a lot Dan and Matthias. Your explanations are very helpful.

Best,
Rian


On Mon, Jan 13, 2014 at 7:06 PM, Daniel Prager <daniel.a.prager at gmail.com>wrote:

> Hi Rian
>
> > are there are any built-in functions that I can take advantage of to
> make this function cleaner?
>
> I'm not aware of a built-in function to help, but sometimes it's quite
> easy to abstract upwards and write your own.
>
> In this case a generalization to map for trees that preserves structure
> looks about right:
>
> ;; Apply f to every atom in 'tree', preserving the list-y structure.
> ;;
> (define (tree-map f tree)
>   (define (T t)
>     (cond [(empty? t) empty]
>           [(atom? (first t)) (cons (f (first t)) (T (rest t)))]
>           [else (cons (T (first t)) (T (rest t)))]))
>   (T tree))
>
> > (tree-map (λ (x) '*) '(+ (- .1 .2) .3))
> '(* (* * *) *)
>
> Then instead of mapping to an asterisk, you can use an incrementing
> counter:
>
> (define (make-counter from)
>   (let ([index from])
>     (λ ()
>       (let ([result index])
>         (set! index (add1 index))
>         result))))
>
> > (define c (make-counter 1))
> > (c)
> 1
> > (c)
> 2
>
> Now you can write your indexing function succinctly:
>
> (define (index-s-exp s-exp index)
>   (let ([c (make-counter index)])
>     (tree-map (λ (x) (c)) s-exp)))
>
>
> Aside: This is a less pure solution compared to Matthias's, since it makes
> use of state in the counter.
>
>
> Dan
>



-- 
Rian Shams
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.racket-lang.org/users/archive/attachments/20140114/70daf860/attachment.html>