[REBOL] Re: bind
From: joel:neely:fedex at: 18-Oct-2001 13:59
Hi, Graham,
Graham Chiu wrote:
> Would someone be able to explain in english what 'bind does.
> I've read the explanation in the Rebol word browser and am
> no clearer than before :(
>
I'm not sure about my American (I don't speak English... ;-) ,
but please let me know if this is of any use to you. I'll be glad
to try to answer any questions it raises.
-jn-
A REBOL word is just a name or symbol. Its value comes from the
context that gives it meaning. Words in different contexts can
have totally unrelated values, even when the words' names are spelled
the same.
Consider this example:
>> glorp: "I'm global!"
== "I'm global!"
>> wordblock: [glorp]
== [glorp]
>> some-fun: func [b [block!] /local glorp] [
[ glorp: "I'm local!"
[ print [glorp]
[ print b
[ ]
>> some-fun wordblock
I'm local!
I'm global!
The word named "glorp" in the block is the externally-defined word.
That fact doesn't change just because the block is handed in to the
function. Therefore the second line of output is still referring
to the value of the global "glorp" and not the local one.
The role of BIND is to change the context of words in its first
argument (if possible) to be the context of its second (sample)
argument. Therefore, we can do this:
>> glorp: "I'm global!"
== "I'm global!"
>> glunk: "I'm global, too!"
== "I'm global, too!"
>> wordblock: [glorp glunk]
== [glorp glunk]
>> some-fun: func [b [block!] /local glorp foo] [
[ glorp: "I'm local!"
[ foo: 42
[ print [glorp]
[ print b
[ bind b 'foo
[ print b
[ ]
Similar exercise, but I added another global word to the block.
In addition, I added an irrelevant local named FOO, just to show
that the second argument to BIND is simply a *sample* word which
indicates which context to use -- you don't need to worry about
making names match.
>> print wordblock
I'm global! I'm global, too!
Just what we expect.
>> some-fun wordblock
I'm local!
I'm global! I'm global, too!
I'm local! I'm global, too!
Notice that after BIND does its thing, the first word in the block
has been changed (bound) to the function's context. That's why the
last output line is different from the second.
>> print wordblock
I'm local! I'm global, too!
Notice too, that the change is permanent. Now the "glorp" in the
first position of WORDBLOCK is the one in the function's context
and not the global one.
If this kind of aliasing is a problem, you could rewrite the demo
function as:
>> some-fun: func [b [block!] /local glorp foo] [
[ glorp: "I'm local!"
[ foo: 42
[ print [glorp]
[ print b
[ b: bind copy b 'foo
[ print b
[ ]
(We also have to re-create WORDBLOCK...)
>> wordblock: [glorp glunk]
== [glorp glunk]
Now we can do this:
>> some-fun wordblock
I'm local!
I'm global! I'm global, too!
I'm local! I'm global, too!
>> print wordblock
I'm global! I'm global, too!
Since SOME-FUN modified a copy of the argument, the "glorp" in the
original WORDBLOCK is not rebound.
Hope this helps!
-jn-
--
This sentence contradicts itself -- no actually it doesn't.
-- Doug Hofstadter
joel<dot>neely<at>fedex<dot>com
