Re: Announcing the first actually stable release of guile-for-loops

[Zelphir Kaltstahl <zelphirkaltstahl@posteo.de>]

Hi Linus!

Although I just ported multiple usages of those kind of loops from a
previously Racket project to a Guile project, I think this is quite
cool! When I ported those usages in my code, it also resulted in some
named lets (I guess quite naturally, as an available looping construct),
so I can relate to that.

Thanks!

Regards,

Zelphir

On 1/23/20 6:00 PM, guile-user-request@gnu.org wrote:
> Message: 4
> Date: Thu, 23 Jan 2020 13:10:46 +0100
> From: Linus Björnstam <linus.internet@fastmail.se>
> To: guile-user@gnu.org
> Subject: Announcing the first actually stable release of
> 	guile-for-loops
> Message-ID: <f536f849-5266-40d6-a3dc-f815834a0d30@www.fastmail.com>
> Content-Type: text/plain;charset=utf-8
>
> Hiya everybody!
>
> I have spent some time implementing efficient for loops for guile, and they are baked and ready to go. I have worked the last weeks at implementing generalized support for non-tail-recursive loops and am happy to announce for/foldr. It is a generic right fold, with support for delaying it's arguments as either thunks or promises. 
>
> The syntax is more or less the same as racket's loops, and they are generally compatible. The code generated is for almost all cases as fast as hand-rolled code. They are all expressed as left or right folds, and are as such (apart from for/list, but read about that in the documentation) free of mutation. They are all converted to named lets. 
>
> Some examples:
>
> (for/list ((a (in-range 1 6)))
>   (* a a)) ;; => (1 4 9 16 25)
>
> (for*/list ((a (in-string "ab")) (b (in-range 1 3)))
>   (list a b)) 
> ;; => ((#\a 1) (#\a 2) (#\b 1) (#\b 2))
>
> There are many more looping constructs, among others: 
> for/sum, for/vector, for/or, for/and, for/first, for/last and a side-effecting simple for.
>
> Here is a sieve of erathostenes:
>
> (define (erathostenes n)
>   (define vec (make-vector n #t))
>   (for/list ([i (in-range 2 n)] #:when (vector-ref vec i))
>     (for ([j (in-range/incr (* 2 i) n i)])
>       (vector-set! vec j #f))
>     i))
>
> The code and documentation is available here: 
> https://hg.sr.ht/~bjoli/guile-for-loops
>
> A web-friendly documentation can be found here: 
> https://man.sr.ht/%7Ebjoli/for-loops-docs/for-loops.md
>
> The thing I had been waiting for is right fold. That allows us to write loops like guile's map: non-tail recursive:
> (for/foldr ((identity '())) ((a (in-list '(1 2 3))))
>   (cons (* a a) identity))
>
> becomes equivalent to:
>
> (let loop ((random-identifier '(1 2 3)))
>   (if (null? random-identifier)
>       '()
>       (let ((a (car random-identifier)))
>         (cons (* a a) (loop (cdr random-identifier))))))
>
> Happy hacking
> Linus Björnstam