Hi,
I feel, that Joel tried to warn us, that the "Poor man's module" is not as
reliable as some may need it to be:
Rebol []
make object! [
protected1: 1
protected2: 2
if true [
unprotected1: 3
]
]
and he is right, because 'protected1 and 'protected2 will not destroy their
global context couterparts, but 'unprotected1 can!
> Hi Joel
>
> You have been coming up with some fun examples lately. Maybe I can throw a
> little light on the topic.
>
> First the effect in your example is also obtained with this code:
>
> ob1: make object! [
> a: 1
> init: func [n [integer!]] [a: n]
> set 'ob11 make object! [
> b: 10
> init: func [n [integer!]] [b: n]
> wot: func [] [a + b]
> ]
> ]
>
> All of your tests below give the same result as when using the do []
> construct. The form above is the poor man's module capability (until the
> real thing comes along in REBOL/Core 3). Carl, myself, and many others
often
> use this construction. In fact, much of VID is written using this
mechanism.
>
> For example, if you do source LAYOUT, you will find that layout calls a
> number of functions like TRACK, EXPAND-SPECS, DO-FACETS, GROW-FACETS, etc.
> which are not defined globally. This works because LAYOUT was defined in
the
> VID object, but it was defined using SET as above and thus exported to the
> global context.
>
> This is very useful because it allows you to export a few interface
> functions for the object to the global context, while at the same time
being
> able to reference all of the words in the "parent" object. At the same
time,
> the global context is shielded from the many named data-structures,
layouts,
> sub-objects, styles, helper functions, etc. which are included in the
parent
> object.
>
> So how does all this work? There are others who can surely give a better
> explanation than I can, but I will make a few comments. There are stages
in
> object creation, and in one of the early stages the object spec block is
> scanned for all first-level set-words. A context for the object is created
> with these words bound to it.
>
> After this stage, the code in the object spec is executed, in exactly the
> same way as a block of code in the global context. The only difference is
> that those words which occurred in the spec-block as set-words are bound
to
> the objects context. Any code can appear and will be executed. For
example:
>
> >> x: 21 ob: make object! [ a: 52 print [a x]]
> 52 21
>
> This feature is also very handy. I often wrap a whole script this way:
>
> REBOL []
> ctx-my-script [
>
> .... all of the script code just as it was when its words were in the
global
> context
>
> ] end ctx-my-script
>
> The global context is protected from all of the set-words used in the
> script. There are some extra precautions when using vid which I will skip
> here.
>
> There are a couple of facts about "embedded" objects which are good to
know:
>
> 1) Sub-objects which are defined with a set-word in the parent object are
> treated differently than other datatypes when the object is cloned.
> Sub-objects are not cloned, the pointer (which what a word really
> references) still points to the original sub-object. Other datatypes are
> copied into the cloned object.
> .
> 2) Sub-objects exported with your do [] construct or by using SET are not
> really "embedded", the words which reference them are global, but the
words
> contained in their definition blocks are bound to the context of the
object
> in which they are created. To me it is similar to this example:
>
> >> b: [] repeat j 3 [use [x][x: j append b 'x]]
> == [x x x]
> >> b
> == [x x x]
> >> reduce b
> == [1 2 3]
>
> The word b is in the global context, it's value is a block which contains
> the words x, x, and x each of them created in a different unnamed context.
> You can do much the same thing in LISP, Scheme or other symbolic
programming
> languages.
>
> Your conclusion can be extended to state that it is often misleading to
> apply the concepts of object-oriented programming in trying to understand
> REBOL. REBOL's objects do not support the key concepts of OO. They are
> primarily name-spaces, and have their own distinctive properties relating
to
> the underlying symbolic language. They are one-of-kind objects, sometimes
> called objects by prototype in the books.
>
> Perhaps others will contribute further insights or corrections. (Carl,
Jeff,
> Holger, Gabriele, Ladislav?)
>
> -Larry
>
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