Re: Newbie question: bind a variable on the fly

[Jon Wilson]

Hi Vincent,
Don't try this just yet.  I want other people to give it a look-over and 
make sure it isn't foolish and dangerous before anybody lets it out in the 
wild.


This doesn't use the exception, and it doesn't use a c function, and it 
doesn't do database lookups.  However, it can be used anywhere (inside lets 
and lambdas) to set! a variable that hasn't been defined.  It defines the 
variable at top level, and then sets it wherever you are.  Note: it does 
establish a global binding rather than a local binding.  This may or may 
not be what you want.  I think that using this to establish a local binding 
would be an extremely difficult thing to do, as well as probably a bad idea.

(define-macro (dyn-set! var val)
              `(begin (if (not (defined? (quote ,var)))
                        (primitive-eval `(define ,(quote ,var) #f)))
                      (set! ,var ,val)))

(defined? 'undefined-symbol) ; => #f
;(set! undefined-symbol #t)  Gives an error.
(dyn-set! undefined-symbol #t) ; No error.
(defined? 'undefined-symbol) ; => #t

Regards,
Jon

Vincent De Groote wrote:
> Hello,
>
> Is there a way to catch an "unbound-variable" exception, bind the
> variable on the fly, and continue execution as if the exception didn't
> occurs ?
>
> I'd like to catch this exception in a c function:  the exception context
> should be available, to retrieve the variable or function name.  This
> handler will lookup the value in a relational database.
>
> Is this possible ?  
>
>
> Thanks for you replies
>
> Vincent De Groote
>
>
>
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