Re: documentation on putp/getp

On Friday 04 March 2005 09:14 pm, Ralf Juengling wrote:
> On Fri, 2005-03-04 at 17:59, Ralf Juengling wrote:
> > [...]
> > 
> > * Hash tables can also rely on pointer equality, as implemented by
> >   function == . These hash tables are useful to associate a piece of
> >   information with a particular lisp object. Functions getp and putp ,
> >   for instance, make use of pointer-equality hash tables. The
> >   associations of a pointer-equality hash table are automatically
> >   removed when the key object is deallocated.
> > 
> > I'm confused.
> > You and the hash table documentation are talking about the key 
> > object that gets dereferenced, whereas in the example above it's 
> > value object that gets dereferenced. 
> > You then just exhibited a documentation bug? (Note also that only
> > symbols are allowed as keys for putp/getp).

> What is the object for putp is the key for the hidden hash table 
> that encodes the association.

It is all implemented in sys/sysenv.lsh


(defvar |*PROPERTIES| (htable))

(de getp(obj key)
  (let ((data (|*PROPERTIES| key)))
    (when data (data obj)) ) )

(de putp(obj key val)
  (let ((data (|*PROPERTIES| key)))
    (when (not data)
      (when (symbolp key)
        (|*PROPERTIES| key (setq data (htable 17 t))) )
      (when (not data)
        (error 'putp "not a symbol" key) ) )
    (data obj val)
    key ) )


The hash table |*PROPERTIES| maps each putp/getp 
symbol key to a second htable with pointer equality.
The keys of this second htable are the putp/getp objects
with a propertie for that object. 

- L.


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