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
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