On Mon, Mar 25, 2013 at 2:41 PM, J. Ian Johnson <[email protected]> wrote:
> Let me know how you like my library. I'm thinking of adding some more
> ergonomics to it:
> 1) support right folds so a for/list-like comprehension has better allocation
> behavior.
> 2) add a keyword for-clause for starting an inner iteration so you don't have
> to do this:
> (for ([e0 (in-list l0)]
> [e1 (in-list l1)])
> (for ([e2 (in-list l2)]
> [e3 (in-list l3)])
> ...body...))
> but instead you can do this:
> (for ([e0 (in-list l0)]
> [e1 (in-list l1)]
> #:start-inner-loop ;; suggest a better name please
> [e2 (in-list l2)]
> [e3 (in-list l3)])
> ...body...))
IMHO the first form is clearer. I don't see any gain in the second one.
> I seem to remember that for* is not equivalent to successive nesting of for
> forms, but I can't remember how I reached this conclusion.
I have found, in one case, that performance of nested fors is
significantly better than a single for*, but I have not investigated
the cause. See the code fragment below.
Cheers
P.
(define (risolvi-2 sexomini)
(let ((tabella (crea-tabella)))
(let fansexomino ((sexomini sexomini))
(cond ((null? sexomini)
(eureka tabella))
((posizione-accettabile? tabella sexomini)
(let ((sexomino (unsafe-first sexomini)))
(for ((orientazione (in-list sexomino)))
(let ((forma (unsafe-first orientazione)))
(for ((posizione (in-list (unsafe-rest orientazione))))
(when (compatibile? forma posizione tabella)
(let* ((x (posizione-x posizione))
(y (posizione-y posizione))
(v (unsafe-vector-ref tabella x)))
(unsafe-vector-set! v y forma)
(fansexomino (unsafe-rest sexomini))
(unsafe-vector-set! v y #f))))))))))))
#|
;; risolvi-2 is faster
(define (risolvi-3 sexomini)
(let ((tabella (crea-tabella)))
(let fansexomino ((sexomini sexomini))
(cond ((null? sexomini)
(eureka tabella))
((posizione-accettabile? tabella sexomini)
(for* ((sexomino (in-value (first sexomini)))
(orientazione (in-list sexomino))
(forma (in-value (first orientazione)))
(posizione (in-list (rest orientazione)))
#:when (compatibile? forma posizione tabella)
(x (in-value (posizione-x posizione)))
(y (in-value (posizione-y posizione)))
(v (in-value (vector-ref tabella x))))
(vector-set! v y forma)
(fansexomino (rest sexomini))
(vector-set! v y #f)))))))
|#
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