[racket] Typed Racket and macros expanding to for*:

Yes, the #:when clause has to be last. I got my reasoning backwards. 

Do try both and report back. I am curious how expensive the ec is compared to the #:when test. 


On Jul 25, 2012, at 4:37 AM, Jens Axel Søgaard wrote:

> 2012/7/24 Vincent St-Amour <stamourv at ccs.neu.edu>:
>> You could capture the continuation outside `for*' check the index in the
>> body and jump out when it gets out of range.
>> 
>>    (let/ec: done : Void
>>      (for*: (for:-clause ...)
>>        (unless (< i m*n) (done (void)))
>>        (define x (let () . defs+exprs))
>>        (vector-set! v i x)
>>        (set! i (assert (+ i 1) index?))))
> 
> That works.
> 
> Matthias:
>> But if Jens is happy, I am happy :-)
> 
> Only part happy :-)
> 
> Using let/ec feels like cheating in the context of the for-framework.
> I dug up the for for*/vector. It uses for*/fold with an #:when clause
> as the last (phew) clause:
> 
> (define-syntax (for*/vector stx)
>    (syntax-case stx ()
>      [(for*/vector (for-clause ...) body ...)
>       (syntax/loc stx
>         (list->vector
>          (for*/list (for-clause ...) body ...)))]
>      [(for*/vector #:length length-expr (for-clause ...) body ...)
>       (syntax/loc stx
>         (let ([len length-expr])
>           (unless (exact-nonnegative-integer? len)
>             (raise-argument-error 'for*/vector
> "exact-nonnegative-integer?" len))
>           (let ([v (make-vector len)])
>             (for*/fold ([i 0])
>                 (for-clause ... #:when (< i len))
>               (vector-set! v i (let () body ...))
>               (add1 i))
>             v)))]))
> 
> I'll try both the let/ec and the for*/fold solution and see what's faster.
> 
> /Jens Axel
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