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

[Geoffrey K. <ge...@kn...>]

Good question.  MIT/GNU Scheme has:

procedure: environment-reference-type environment symbol

Returns a symbol describing the reference type of symbol in  
environment or one of its ancestor environments.   The result is one  
of the following:

normal - means symbol is a variable binding with a normal value.
unassigned - means symbol is a variable binding with no value.
macro - means symbol is a keyword binding.
unbound - means symbol has no associated binding.

http://www.gnu.org/software/mit-scheme/documentation/scheme_14.html

Geoffrey
--
Geoffrey S. Knauth | http://knauth.org/gsk


On Jan 10, 2006, at 16:42, 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---