Re: [Jscheme-user] Testing if a symbol is bound?

[Michael T. <mt...@bb...>]

Hmmm - thanks.  I think I'm going to stick to my definition:
(define (bound? symbol) (.isDefined symbol))
even though performance isn't much of an issue in the cases I want to 
use it.

I guess the followup question ought to be "How do I unbind a symbol?" 
but not only is this a hairier problem, but I don't really require an 
answer... ;-)

Cheers,
    -mik

Timothy J Hickey wrote:

> Hi mik,
>
> On Jan 10, 2006, at 3:10 PM, Michael Thome wrote:
>
>>  What is the "proper" way to test to see if a symbol is bound?
>
> There is no proper way, as far as I can tell.
>
>>
>>  I was expecting something like (bound? 'foo).
>
> If you have created a scheme environment using the jscheme.JScheme 
> constructor,
> then there is an isDefined method which tests whether a symbol is 
> defined in that interpreter,
> e.g.
>
>> [Timothy-Hickeys-Computer:~/Research/Software/jscheme] tim% java -jar 
>> lib/jscheme.jar
>> JScheme 7.2 (1/10/06 8:55 AM) http://jscheme.sourceforge.net
>> > (define js (jscheme.JScheme.))  ;; create a new scheme interpreter
>> $1 = jscheme.JScheme@5db13f
>>
>> > (.eval js '(define x 1))   ;; evaluate a define in that interpreter
>> $2 = 1
>>
>> > (.isDefined js "x")   ;; check to see if the symbol corresponding 
>> to the string "x" is defined in that interpreter
>> $3 = #t
>>
>> > (.isDefined js "y")  ;; notice that "y" is not defined
>> $4 = #f
>>
>> > (.isDefined 'x)  ;; also notice that 'x is not defined in the 
>> toplevel Scheme environment, only in js
>> $5 = #f
>>
>> > (.eval js 'x)  ;; we can get the value of 'x in js using eval 
>> again....
>> $6 = 1
>>
>
> If you want to check whether a symbol is defined in the toplevel then 
> (.isDefined 'foo) works fine.
> You could also use a macro with tryCatch  to test for defined-ness 
> without having to quote the symbol...
>
>> > (tryCatch (begin x #t) (lambda(e) #f))
>> $7 = #f
>>
>> > (define-macro (isDefined s) `(tryCatch (begin ,s #t) (lambda(e) #f)))
>> $8 = (macro isDefined (s)...)
>>
>> > (isDefined sin)
>> $9 = #t
>>
>> > (isDefined sine)
>> $10 = #f
>
>
>
> I like the try/Catch approach better because the (.isDefined 'x) 
> relies on the implementation of the Symbol.java class
> in the jsint package, which could conceivably change at some point, 
> whereas the tryCatch approach relies only on
> the syntax of macros and tryCatch and the fact that evaluating an 
> undefined symbol generates an exception....
> But, using (.isDefined 'foo) is ten times faster then the tryCatch 
> approach  ...
>
>>
>> > (time (isDefined foo) 100000)
>> $10 = (#f (2929 msec) (367880 bytes))
>>
>> > (time (.isDefined 'foo) 100000)
>> $11 = (#f (211 msec) (135856 bytes))
>
>
> Does that help??
>
> Best,
> ---Tim---
>