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
