I think, that in semantic of for/..., I mean (for/... ... (values ...)), it is impossible.
Possible API could be (for/collect ([a list] [b sum]) ([i (in-list '(1 2 3 4)]) (cond [(oddp? i) (a i) (b i)] [(< (b) 10) (b 1)])) Returns (values '(2 4) 8) Inside body of (for/collect ([id option ...] ...) body) (id val) = Add val to collection (id) = return current collection value For for/fold-like one may make (define-collector fold [fld #:bind v #:inner v]) Then (for/fold ([sum 0]) ([i (in-list '(1 2 3 4)]) ... (values (+ sum i))) Become (for/collect ([sum fold :initial 0]) ([i (in-list '(1 2 3 4)]) ... (sum (+ (sum) i)) ...) But I cannot imagine, how to make this without closures and set!. Суббота, 18 января 2014, 13:51 -05:00 от "J. Ian Johnson" <[email protected]>: >Suffice it to say, my package does not have that expressive power. I'd be >happy to collaborate with you to design a good extension to my package so that >this kind of accumulation is easier. >-Ian >----- Original Message ----- >From: "Roman Klochkov" < [email protected] > >To: "J. Ian Johnson" < [email protected] > >Cc: [email protected] >Sent: Saturday, January 18, 2014 1:13:38 PM GMT -05:00 US/Canada Eastern >Subject: Re[2]: [racket] Style. for/fold for/list for/lists > >It is great generalization for for/fold > >But still missed something like this: > >(iter (for i in l) >(if (oddp i) (collect i into a) (collect i into b)) >(when (> i 10) >(collect i into a)) >(finally (return a b))) > >Or in your for/acc >(for/acc ([:type append] [:type append]) >([i (in-list l)]) >(if (odd? i) (values (if (> i 10) (list i i) (list i)) null) >(values (if (> i 10) (list i) null) (list i)))) > >I had to double condition and the result is not so transparent. > >Simple solution is: > >(define (make-collector init add post) >(let ([a init]) >(case-lambda >[(val) (set! a (add val a)] >[() a]))) > >(define (make-list-collector [init '()]) (make-collector init cons reverse)) > >(let ([a (make-list-collector)] >[b (make-list-collector)]) >(for ([i (in-list l)]) >(if (odd? i) (a i) (b i)) >(when (> i 10) (a i))) >(values (a) (b))) > >But they say, that purity is the best and one should avoid set!... so this way >is the dark way, isn't it? > > >Суббота, 18 января 2014, 8:02 -05:00 от "J. Ian Johnson" < [email protected] >: > > > > > > >You have hit limitations of the for/fold macros. I wrote an extension for >controlling the way values get accumulated, along with post-processing like >list reversal. Let me know what you think! > >https://github.com/ianj/nifty-macros/tree/master/for-acc >-Ian >----- Original Message ----- >From: Roman Klochkov < [email protected] > >To: [email protected] >Sent: Sat, 18 Jan 2014 06:25:10 -0500 (EST) >Subject: [racket] Style. for/fold for/list for/lists > >Where can I find style guide for these > >for/lists: > >I have to duplicate values names: > >(define-values ([(desc-args lengths add-data) >(for/lists (desc-args lengths add-data) ([arg (in-list %args)]) >....)) is it ok? Or better >(define-values ([(desc-args lengths add-data) >(for/lists (a b c) (...) ...)) > >? > >for/fold has no "return" clause. So I have to do >(call-with-values >(for/fold (acc1 acc2 sum) (...) ...) >(lambda (acc1 acc2 sum) (reverse acc1) (reverse acc2) sum)) > >And for/list doesn't allow to add several items to resulting list in one >round. >Even with for/fold it gives > >(reverse >(for/fold (acc) ([item (in-list list)]) >(values (cons (get-val1 item) >(cons (get-val2 item) acc))))) > >Awful, especially for reading. > >It is clumsy and unobvious. >In Common Lisp iterate I could do >(iter (...) >(collect (foo) into acc1) >(collect (bar) into acc2) >(summing ... into sum) >(finally (return acc1 acc2 sum))) > >http://common-lisp.net/project/iterate/doc/index.html > >Is there something similar? > >-- >Roman Klochkov > > >-- >Roman Klochkov -- Roman Klochkov
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