Great! Thank you! (with- ...) is the best approach for such in-place substitutions. I missed this opportunity.
Вторник, 30 июля 2013, 0:40 -04:00 от Ryan Culpepper <[email protected]>: >I don't think read-time computation is the right approach for Racket. >Here's a compile/expand-time alternative that lets you use unsyntax as a >compile-time escape. > >(require (for-syntax racket/syntax)) > >(define-syntax (with-compile-time-unsyntax stx) > (syntax-case stx () > [(wctu form ...) > (syntax-local-introduce > (syntax-local-eval #'(quasisyntax (begin form ...))))])) > >Here's an example use: > >(begin-for-syntax > (define the-symbol 'apple)) > >(with-compile-time-unsyntax > (define (f x) > (case x > ((#,the-symbol) 'yes) > (else 'no)))) > >(f 'apple) ;; => 'yes >(f 'pear) ;; => 'no > >If you wanted, you could make a custom language (see #%module-begin) >that implicitly wrapped the whole module body with >with-compile-time-unsyntax automatically. > >You might find it difficult to use quasisyntax and unsyntax within such >a module. I haven't tried it or thought through how it would interact >with quasisyntax "levels". You could of course implement your own >quasisyntax-like form with a different escape binding. > >Ryan > > >On 07/29/2013 10:07 PM, Roman Klochkov wrote: >> Yes. Really it is read-time. >> >> In CL every top level expression read-compile-load or simply load if it >> is compiled. >> >> So in Racket would be natural to have (for-syntax ...) environment in #. >> I hope, that Racket also read toplevel form, loads it, then reads next >> (not reads all at once, then load first form , second form and so on). >> >> Can you invent such read-macro? >> >> >> Понедельник, 29 июля 2013, 21:44 -04:00 от Sam Tobin-Hochstadt >> < [email protected] >: >> >> On Mon, Jul 29, 2013 at 1:53 PM, Vincent St-Amour >> < [email protected] >> < https://e.mail.ru/messages/sentmsg?compose&[email protected] >> >> wrote: >> > Here's a quick solution: >> > >> > #lang racket >> > >> > (define-syntax-rule (hash-dot e) >> > (let-syntax ([computed-e (lambda (stx) (datum->syntax #'e e))]) >> > (computed-e))) >> > >> > (hash-dot (+ 2 3)) >> > >> >> In CL, #. is read-time, not compile-time, evaluation, so this isn't >> quite the same. It would be easy to create a read-table that enabled >> this, though. You'd have to make some decision about what environment >> the read-time expression sees -- I don't know what CL does here. >> >> Sam >> >> >> >> -- >> Roman Klochkov >> >> >> >> ____________________ >> Racket Users list: >> http://lists.racket-lang.org/users >> > -- Roman Klochkov
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