[REBOL] Re: Context - ...etc...
From: joel::neely::fedex::com at: 14-Sep-2001 1:53
Hi, Holger,
THANK YOU!
This is the kind of rationale and operational explanation that
some of us have been eagerly desiring for quite some time! I
truly believe that, given this kind of concise detail, the
REBOL community would gladly rise to the task of writing
tutorials and explanations that would present the workings of
REBOL in a manner suitable both for the beginning programmer
and the newcomer to the language (with other programming
experience). I, for one, have given lots of thought (and made
public presentations) in the general area of "REBOL for Perl
programmers" (or other backgrounds). However, I've been VERY
reluctant to commit too much to writing because of the number
of subtleties in REBOL's implementation for which I had no
authoritative description, and therefore some degree of doubt
that my understanding was complete.
Having said that...
I must add, in defense of everything else that has been
written in this thread, that the prior unavailability of
clear, authoritative information such as you've just provided
has forced many of us (myself included -- recalling the past
discussions of "mutability", "self-referentiality", and "model
of series" discussions) to try to make up our own models as
a basis of explaining and predicting REBOL behavior. It is
inevitable -- absent any authoritative responses to the oft-
raised questions about such matters -- that our home-brewed
models and descriptions will be incomplete, inaccurate, or
redundant in some ways.
Therefore...
Please accept my profound thanks for providing us a glimpse
into the world of "The Inner Game of REBOL" (a working title
I actually played with and discarded in frustration), and
likewise accept my earnest plea for your open and frank
responses to REBOL questions to continue. More of this will
go a VERY long way to helping us use and promote REBOL more
effectively!
-jn-
Holger Kruse wrote:
> - Words do not "store" values, they ARE values...
>
> When evaluating a value the result is, usually, another
> value... Those values are not stored in words, they are
> stored in contexts.
>
> - Contexts are not word tables, but name-value lookup tables...
>
...
> - to-block! does not bind words into a context. make word!
> and to-word bind all words into the global context...
>
> - The global context expands automatically...
>
> - Aliases ... are global, not local per context.
>
> - Scoping: REBOL uses lexical scoping, not dynamic scoping,
> but allows words to be rebound during block evaluation...
>
> - The "bug" you described when combining 'use with functions
> and other contexts is not a bug. It is intentional. There
> are only four ways how a function can (theoretically) access
> its local variables and arguments, in any language:
>
> - Using a hidden context pointer...
>
> - By creating a completely new "environment" for each function...
>
> - By keeping a single function body for all function invocations,..
>
> - By keeping a single function body for all function invocations,..
>
> - ... the context of the outer function remains intact (indefinite
> extent) AND retains its values after the outer function returns
> (for non-recursive function invocations), under certain
> circumstances. This is an implementation detail you should not
> rely on, because it is likely to change in future versions, i.e.
> there is a good chance that in the future after a (non-recursive)
> function invocation the values of that function's context are
> set to unset!. This would break the way your inner function
> references the outer function's arguments after the outer
> function has returned.
>
...
> This also resolves the drawback you mentioned, i.e. use
>
> f: func [x] [func [] reduce [x]]
>
> and the behavior is as expected and compatible with future versions.
>
-jn-
--
------------------------------------------------------------
Programming languages: compact, powerful, simple ...
Pick any two!
joel'dot'neely'at'fedex'dot'com
