[REBOL] Re: The truth about scope
From: gabriele:colellachiara at: 14-Apr-2005 12:29
Hi Bernie,
On Wednesday, April 13, 2005, 5:40:36 PM, you wrote:
BS> Yes, it's much clearer now. You did a great job. Please continue
BS> with contexts and bindings.
Ok, so to summarize:
The is no scope. Everything is data, and the relation between
words and values is data too. You can manipulate this relation
in the same way you can manipulate the rest of the data.
So, the "connection" (binding) between words and their referred
values is not a "rule" in the language, like "scope" is, but it is
a property of words.
Now... what is a context? And what does binding mean?
You can think of a context as a table of words and values. Nothing
special here - you have words and their "meaning" listed in some
kind of table.
But, if words directly refer to values, why do we need contexts?
The reason we need them, is that words actually refer to values
with an additional layer of indirection. A "double pointer" if you
want to call it this way. Why? Because otherwise each word
instance
would have a different reference. Changing one word
would not change all the other words with the same spelling.
So we have something like:
some-word -----------\
()---------> value
some-word -----------/
some-other-word ----->()---------> value
The () in the above picture are contained in the second column of
contexts.
When you write:
some-word: 3
you are not changing the leftmost arrow in the above picture, you
are changing the rightmost arrow. Basically you are not changing
the word, you are changing the context.
Setting the value of a word is an operation on contexts, not an
operation on words. Words stay unchanged.
Now, the scoop: the leftmost arrow is what is called "binding".
The operation of binding is the operation of changing that arrow
to make it point to some other ().
So: everything is data. Words and their relation to other values
are data too.
What BIND does is take every word in a block and change it so that
it refers to a different context's value slot.
What gives the illusion of scope? As Ladislav said, when words are
loaded (LOAD), they are bound to the global context. Each time a
word is created, a value slot for that word is created in the
global context (SYSTEM/WORDS), and this value slot is initialized
to the UNSET! value.
So, every word starts out being bound there. When code is
evaluated, some functions may change the binding of words.
Actually, many functions do that. What you get in the end is the
illusion of scope, because usually those function are applied on
blocks that are nested inside each other. However, this is not a
requirement, and this usually confuses people, because they are
fooled by the illusion of scope.
The "lookup" in the context table is only done at binding time.
Once the word has been bound, it "directly" (actually, with a
double pointer
) refers to the value, so that when you GET the
value of a word no lookup is necessary. This has the nice side
effect of making REBOL faster, too.
This is why we usually call it "static binding". The connection
between words and value is a "static" data structure, not the
result of a dynamic computation at runtime. Of course, you can
change this structure dynamically, like you can change the
contents of a block dynamically.
Regards,
Gabriele.
--
Gabriele Santilli <[g--santilli--tiscalinet--it]> -- REBOL Programmer
Amiga Group Italia sez. L'Aquila --- SOON: http://www.rebol.it/
