[plt-scheme] syntax-local-context?

From: Jens Axel Søgaard (jensaxel at soegaard.net)
Date: Wed Feb 14 09:14:43 EST 2007

jos koot skrev:
> The following confuses me:
> (define-syntax (show-local-context stx)
> (syntax-case stx ()
>  ((show-local-context x)
>   (let ((x (syntax x)))
>    (printf "~s ~s.~n" (syntax-object->datum x) (syntax-local-context))
>     x))))
> (let ((d (show-local-context (add1 2)))) d) ; --> 3
> ; displayed: (add1 2) top-level.
> I expected display: (add1 2) expression. Do I misunderstand the manual 
> or (less probably) is this a bug?

   (show-local-context (add1 2))
is expanded the transformer associated with
show-local-context is called.

The return value of the transformer is
a syntax object containing (add1 2).

The expander then expands (add1 2), and it
becomes (#%app add1 2).

Now the expanded result is evaluated, and the
result of evaluating (#%app add1 2) is 3.

To get what you expected, you need to let the
transformer return a syntax object representing
a quotation of the expressions. That is:

(define-syntax (show-local-context stx)
   (syntax-case stx ()
     ((show-local-context x)
      (let ((x (syntax x)))
        (printf "~s ~s.~n" (syntax-object->datum x)

Jens Axel Søgaard

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