Fwd: Announcing the first stable release of guile-for-loops

[Stefan Israelsson Tampe <stefan.itampe@gmail.com>]

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---------- Forwarded message ---------
From: Stefan Israelsson Tampe <stefan.itampe@gmail.com>
Date: Fri, Jan 24, 2020 at 12:42 PM
Subject: Re: Announcing the first stable release of guile-for-loops
To: Linus Björnstam <linus.bjornstam@veryfast.biz>


Would be cool to have those implemented in guile, that would make my
guile-syntax-parse a bit leaner

Regards
Stefan

On Thu, Jan 23, 2020 at 3:03 PM Linus Björnstam <
linus.bjornstam@veryfast.biz> wrote:

> 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
>
>

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