[racket] Macro Problem

That's an hygiene issue: syntax-rules is hygienic, meaning (in that case) that a binding instance introduced by the macro does not capture instances in the original code. 

You need to circumvent hygiene to do what you're after; for this, you need to use syntax-case.

You can look at chapter 36 of PLAI:
http://www.cs.brown.edu/~sk/Publications/Books/ProgLangs

-- Éric


On Jul 8, 2010, at 6:42 PM, Manu wrote:

> Hello
> 
> I am having a problem with the following macro:
> 
> (define-syntax lambda-r
>   (syntax-rules ()
>     ((_ args exp ...)
>      (lambda args
>        (call-with-current-continuation
>         (lambda (return)
>             exp ... ))))))
> 
> it's meant to define a lambda-r form, which wraps a lambda in call/cc, so I can use a "return-like" operator.
> 
> The following is an example of using lambda-r:
> 
> 
> (define test-return
>   (lambda-r ()
>             (let ((input (read)))
>               (if (number? input)
>                   (if (> input 0)
>                       (begin 
>                         (display "positive")
>                         (return input))                        ;; if input is > 0 this should escape out of the procedure
>                       (display "negative or zero"))
>                   (display "not a number"))
>               (display "no return")                         ;; if input is > 0 this shouldn't get displayed
>               )))
> 
> I get the following error when I run the previous example (and enter a positive integer):
> 
>  => reference to undefined identifier: return
> 
> The puzzling thing is that if I run the macro expansion (as shown by the Macro Stepper), it works !
> Here is the macro expansion:
> 
> (define test-return
>   (lambda ()
>     (call-with-current-continuation
>      (lambda (return)
>        (let ((input (read)))
>          (if (number? input)
>            (if (> input 0)
>              (begin (display "positive") (return input))
>              (display "negative or zero"))
>            (display "not a number"))
>          (display "no return"))))))
> 
> 
> What I am missing ?
> 
> Thanks
> 
> M
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