[plt-scheme] begin-for-syntax in beginner-level

Do you mean?

;; Registers a new thing.
(define-syntax (reg stx)
  (syntax-case stx ()
    [(_ a-name)
     (begin (register! #'a-name)
      (syntax/loc stx 'a-name))]))

Jos

----- Original Message ----- 
From: "Danny Yoo" <dyoo at cs.wpi.edu>
To: "PLT-list" <plt-scheme at list.cs.brown.edu>
Sent: Tuesday, October 21, 2008 5:07 AM
Subject: [plt-scheme] begin-for-syntax in beginner-level


> I'm trying to write a macro that introduces a begin-for-syntax to
> maintain compile-time registration information.  Here's a
> simplification of the situation I'm seeing: imagine a 'reg' form that
> remembers identifiers that we pass to it.
> 
> ;;;
> #lang scheme/base
> (require (for-syntax scheme/base))
> 
> (begin-for-syntax
>  (define names '()))
> 
> (define-for-syntax (register! name-stx)
>  (set! names (cons name-stx names)))
> 
> ;; Registers a new thing.
> (define-syntax (reg stx)
>  (syntax-case stx ()
>    [(_ a-name)
>     (syntax/loc stx
>       (begin
>         (begin-for-syntax
>           (register! #'a-name))
>         a-name))]))
> 
> ;; prints out all registered things
> (define-syntax (print-regs stx)
>  (syntax-case stx ()
>    [(_)
>     (with-syntax ([(names ...) names])
>       (syntax/loc stx
>         (begin (display (quote (names ...)))
>                (newline))))]))
> 
> (provide reg print-regs)
> ;;;
> 
> 
> When I try to use this in a program in Module language, this seems to
> work ok.  Under Beginner Language, the context appears not to allow
> this.  Here's the program in Beginner Language level:
> 
> ;;;
> (require "test.ss")
> (define x 42)
> (reg x)
> (print-regs)
> ;;;
> 
> I see the following error messsage when I try checking this for syntax.
> 
> define-values-for-syntax: illegal use (not at top-level) in:
> (define-values-for-syntax () (begin (#%app register! (syntax x))
> (values)))
> 
> 
> I suspect that Beginner Level is introducing a layer of syntax that
> prevents me from injecting something via begin-for-syntax.  I can work
> around this by using let-syntax:
> 
> (define-syntax (reg stx)
>  (syntax-case stx ()
>    [(_ a-name)
>     (syntax/loc stx
>       (begin
>         (let-syntax ([foo (lambda (stx-2)
>                             (register! #'a-name)
>                             (syntax/loc stx-2 (void)))])
>           (foo)
>           a-name)))]))
> 
> but I was wondering if I'm interpreting the error message correctly,
> and if my workaround is sound.
> 
> 
> Thanks for any help!
> _________________________________________________
>  For list-related administrative tasks:
>  http://list.cs.brown.edu/mailman/listinfo/plt-scheme
>