[REBOL] Re: DyBASE test
From: knizhnik:garret:ru at: 28-Dec-2003 11:13
Hello SunandaDH,
Sunday, December 28, 2003, 1:18:52 AM, you wrote:
Sac> Konstantin:
>> And once object
>> is created it is not possible to add fields to it (very strange
>> restriction for such flexible and dynamic language as Rebol).
Sac> In simple cases (if the object is not yet referenced anywhere else), it is
Sac> very simple....
Sac> an-object: make object! [field1: now]
Sac> an-object: make an-object [field2: true] ;; "adds a field"
Sac> Problems may arise if the original an-object is referenced elsewhere:
Certainly it is referenced!
The main idea of creation object before loading its fields is to be
able to handle cyclic references. So when object is created, it is
placed in hash table which maps OID (object identifier) to the object
instance. So if during fetching object fields I find reference to the
same OID, reference to it can be obtained from hash table and there is
no idle recursion.
Also please notice that there are not only direct self references (in
this case I can use "self" word in initialization of correspondent
field). But fetched fields can contain references to other objects
which also has to be loaded and then indirectly reference original
object.
Certainly it is possible to do fetching of object in two steps - at
first jusrt extract all fields data, store it in some temporary block.
Then create object using this block. And after it resolve references
by performing OID lookup. But it significantly complicates fetch
procedure and also make it not possible to load object methods
(because only data fields are stored in database).
Sac> a-block: copy []
Sac> an-object: make object! [field1: now]
Sac> append a-block an-object
Sac> an-object: make an-object [field2: true]
Sac> append a-block an-object
Sac> probe a-block/1 ;; not the same as
Sac> probe a-block/2 ;; this
Sac> See also
Sac>
http://www.rebol.org/cgi-bin/cgiwrap/rebol/view-script.r?script=extend-an-obje
Sac> ct.r
Sac> and its documentation -- it may be doing what you need (though it doesn't
Sac> solve the existing-references problem)
Sac> Sunanda.
--
Best regards,
Konstantin mailto:
[knizhnik--garret--ru]
