[REBOL] objects without overhead Re:(3)
From: joel:neely:fedex at: 18-Oct-2000 7:22
Hi, Rishi,
[rishi--picostar--com] wrote:
> state: make object! [
> new: func [
> puzzle [block!]
> ][
>
> return make object! [
> puz: copy reduce puzzle
> get-puzzle: :get_puzz
> ]
> ]
> get_puzz: does [return copy puz]
> test-obj: new [1 2 3 4 0 5 6 7 8]
> print test-obj/get-puzzle
> ]
>
[snip]
> I don't understand why get-puzzle cannot access word 'puz.
>
Look at the following (awful ASCII art) diagram, which shows the
contexts and references of the various names involved:
(global)
|
+--------------...--+
| |
state==>(object!) (etc)
|
+--------+-----------+
| | |
new | test-obj==>(object!)
| | |
puzzle | +----------+
| | |
get_puzz<==get-puzzle puz
As the above code is being evaluated, look at the contexts that
are established, and the words they contain.
Context for... Contains words....
---------------- ---------------------------------------------
(REBOL global) 'state
(along with whatever else was there)
STATE 'new 'get_puzz 'test-obj
(the object's "members")
NEW 'puzzle
(the function's argument)
Now, the next-to-last line inside STATE makes 'test-obj (in
the context of STATE) refer to a newly-created object which
has the following context:
TEST-OBJ 'get-puzzle 'puz
Here's the critical issue: TEST-OBJ refers to an object whose
component GET-PUZZLE refers to the *value* of GET_PUZZ, which
contains words defined *in the context of its own definition*
and in that context, *there is no 'puz*. Remember that 'puz
is defined in the context of the object to which TEST-OBJ now
refers, but *not* in the context of STATE.
If you'll pardon the anthropomorphism, TEST-OBJ knows both
GET-PUZZLE and PUZ, and introduced GET-PUZZLE to GET_PUZZ,
but nobody ever introduced GET_PUZZ to PUZ.
What you were trying to do would only have succeeded if REBOL
used "dynamic scoping" such as Lisp or xBase use, in which new
variables are added to the global "environment" and are visible
to everyone after (chronologically!!!) the point of definition.
REBOL contexts don't behave that way.
Hope this helps!
-jn-
