<HTML><BODY>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" <ianj@ccs.neu.edu>: <blockquote style="border-left:1px solid #0857A6; margin:10px; padding:0 0 0 10px;"> <div id="">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" <<a href="sentmsg?compose&To=kalimehtar@mail.ru">kalimehtar@mail.ru</a>> To: "J. Ian Johnson" <<a href="sentmsg?compose&To=ianj@ccs.neu.edu">ianj@ccs.neu.edu</a>> Cc: <a href="sentmsg?compose&To=users@racket%2dlang.org">users@racket-lang.org</a> 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" <<a href="sentmsg?compose&To=ianj@ccs.neu.edu">ianj@ccs.neu.edu</a>>: 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! <a href="https://github.com/ianj/nifty-macros/tree/master/for-acc" target="_blank">https://github.com/ianj/nifty-macros/tree/master/for-acc</a> -Ian ----- Original Message ----- From: Roman Klochkov < <a href="sentmsg?compose&To=kalimehtar@mail.ru">kalimehtar@mail.ru</a> > To: <a href="sentmsg?compose&To=users@racket%2dlang.org">users@racket-lang.org</a> 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))) <a href="http://common-lisp.net/project/iterate/doc/index.html" target="_blank">http://common-lisp.net/project/iterate/doc/index.html</a> Is there something similar? -- Roman Klochkov -- Roman Klochkov </div> </blockquote> -- Roman Klochkov </BODY></HTML>