+1

On Fri, Jan 24, 2020, 20:13 Stefan Israelsson Tampe <stefan.itampe@gmail.com>
wrote:

>
>
> ---------- 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
>>
>>