On Apr 1, Matthias Felleisen wrote:
>
> > (define (fence s n)
> > (define is
> > (shared ((x (append (range 1 n) (range (- n 1) 2) x))) x))
> > (define wv
> > (for/list ((c s)) (begin0 (list c (car is)) (set! is (cdr is)))))
> > (map first (sort2 wv)))
>
>
> These four lines demonstrate how a locally delimited use of mutation
> achieves the same purpose.
>
> FEATURE REQUEST: could for just treat something like is as a
> sequence? I have seen this trick I just used time and again over my
> Scheme career and we shouldn't have to expose the set! statement at
> all.
It could -- but you're using the generic form of the sequence, so it's
trying to guess what you want to iterate over. This works:
(define wv (for/list ([c s] [i (in-list is)]) (list c i)))
> (BTW, I don't think I can use the built-ins to turn this cyclic form
> of data into a 'sequence'.)
...but I think that the real need here is for more sequence
combinators. In my Swindle iterator macros, I had this iteration
specification:
(x <- something <- something-else)
which would iterate over the first thing and then over the second
one. It works even with another sequence that is broken differently:
=> (list-of (list x y) (x <- 1 .. 4 <- 3 .. 1 and y <- '(x x) <- 1 ..))
((1 x) (2 x) (3 1) (4 2) (3 3) (2 4) (1 5))
Given that `for' uses a new kind of sequene values, it would be nice
to have some way to concatenate them
(for/list ([x (seq+ (range 1 4) (range 3 1))]) x)
and if the parts of the `seq+' expressions are lazy, you'd be able to
do iterate with:
(define waves
(seq+ (range 1 n) (range (- n 1) 2) waves))
(define wv
(for/list ([c s] [i waves]) (list c i)))
--
((lambda (x) (x x)) (lambda (x) (x x))) Eli Barzilay:
http://www.barzilay.org/ Maze is Life!
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