Re: Question about how to check a symbol is bound

[Pan Xie]

#+begin_quote
Perhaps if you can explain why you need to know if a symbol is bound or
unbound, we might be able to help you better achieve your goal.
#+end_quote

For example, if I want to do things shown in following codes, it is useful to 
get the
interned symbols from their names and also get their bound procedures:

 (define-record egg-info
   name author desc)
 
 (define (show-egg-info egg)
   (define (symbol-value sym)
     (##sys#slot sym 0))
   
   (define (getter field-name)
     (symbol-value
      (string->symbol
       (format #f "egg-info-~a"
               field-name))))
   
   (let ((fields '(name author desc)))
     (for-each
      (lambda (f)
        (format #t "~a: ~a~%"
                f
                ((getter f) egg)))
      fields)))

 (show-egg-info (make-egg-info
                 "F-operator"
                 "Kon Lovett"
                 "Shift/Reset Control Operators"))

I think it is a very common idiom in languages from Lisp family. So it is 
important to know
how to check symbol is bound and get its value. Every scheme implementation 
means for
business seriously should have the ability.

Thanks
Pan


On 6/23/23 15:40, Peter Bex wrote:
On Fri, Jun 23, 2023 at 03:32:38PM +0800, Pan wrote:
Ah, that make sense. It seems I can just use the '##sys#slot' procedure to
accomplish all that tasks.
Please don't use ##sys#slot unless you know what you're doing - the
procedure is unsafe and will cause segmentation faults when used on
non-block objects.  Anything that starts with ##sys# or ##core# etc
is intentionally undocumented because it's not meant for user code.

Would you please elaborate about the "transformed
name"?  I see there are codes reference symbols like "##sys#slot" or
"scheme#list", but I can't find document describe them. Is there any
document I can look into? More specifically, how can I transfer "list" to
"scheme#list"? Is there procedure can do that?
scheme#list is the fully qualified symbol that means it's the "list"
procedure from the "scheme" module.  If you do "(import scheme)" and then
just use "list" without prefix, it will get rewritten under the hood.

Again, this is an implementation detail and not meant to be used
directly.

Perhaps if you can explain why you need to know if a symbol is bound or
unbound, we might be able to help you better achieve your goal.

Cheers,
Peter