[racket] Compile-time evaluation

 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 <ryanc at ccs.neu.edu>:
>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
>> < samth at ccs.neu.edu >:
>>
>> On Mon, Jul 29, 2013 at 1:53 PM, Vincent St-Amour
>> < stamourv at ccs.neu.edu
>> < https://e.mail.ru/messages/sentmsg?compose&To=stamourv@ccs.neu.edu >> 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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