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 > _________________________________________________ > For list-related administrative tasks: > http://lists.racket-lang.org/listinfo/users _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
