Re: progv in scheme

[Andrew Gwozdziewycz <apgwoz@gmail.com>]

On Tue, Sep 13, 2011 at 10:25 AM, Andrew Gwozdziewycz <apgwoz@gmail.com> wrote:
> On Tue, Sep 13, 2011 at 9:54 AM, Panicz Maciej Godek
> <godek.maciek@gmail.com> wrote:
>> Hello,
>> Is there any clever way of binding values to the list of unknown
>> symbols in scheme?
>>
>> In common lisp there is a form "progv" that takes the list of symbols
>> and their corresponding values and binds them within the body of
>> progv.
>>
>> It is possible to do it using eval, like this:
>> (define (bind-and-eval symbols values body)
>> (eval `((lambda ,symbols ,body) . ,values)
>>      (interaction-environment)))
>> (define-syntax let-symbols
>> (syntax-rules ()
>>  ((_ symbols values (body ...))
>>   (bind-and-eval symbols values (quote (body ...))))))
>>
>> but using eval for this just seems too heavy. Is there any way of
>> doing it that would be more legal?
>>
>> Best regards,
>> M.
>
> Seems likely that you could use `syntax-case' to create a `let` out of these:
>
> (define-syntax progv
>  (lambda (stx)
>    (define (create-bindings syms vals)
>      (datum->syntax stx (zip (syntax->datum syms) (syntax->datum vals))))
>    (syntax-case stx ()
>      ((_ symbols values body ...)
>       (with-syntax ((bindings (create-bindings #'symbols #'values)))
>          #'(let bindings
>               (begin body ...)))))))
>
>
>
> ;; usage
> (progv (foo bar baz) (1 2 3)
>   (format (current-output-port) "Values: ~a ~a ~a\n" foo bar baz))
> ;; output: "Values: 1 2 3"
>
> Hope this helps!
>
> Andrew

I should point out that I'm using zip in srfi-1 (use-modules (srfi srfi-1))



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