[REBOL] Re: WYSIWYG programming (was: Block Creation)
From: holger:rebol at: 30-Oct-2000 9:32
On Wed, Oct 25, 2000 at 03:05:21AM +0200, Ladislav Mecir wrote:
> Another example showing what is wrong with the Self Modifying Code:
>
> a: [append a [+ 1] 1]
The above expression modifies a block while it is being evaluated. AFAICT at
this time the result of such an operation should be considered "undefined" and
implementation-dependent
. In other words: don't do that ! I don't think the
behavior can be called a bug as long as no specs have been defined for how
the language should behave in this case.
Most languages have expressions which are well-formed within the language, yet
have unspecified semantics. An example is something like
a: (*p++)+(*p++)
in C. Here it is undefined whether the second access to "p" occurs before or
after incrementing it for the first time (assuming I remember the C specs
correctly...)
A "legal" way to rewrite the above REBOL expression would, e.g., be:
a: [append last a [+ 1] do [1]]
This way the main block does not get changed during evaluation. Only the
block referenced from the main block (the one initialized to be [1]) gets
changed. That is completely legal.
I hope you can see now why I dislike the term "self-modifying code". There is
conceptually a huge difference between the first and second example, and
the term "self-modifying code" tries to be a catch-all phrase that encompasses
(and discourages) within the same category both legal (and potentially useful)
and illegal practices.
--
Holger Kruse
[holger--rebol--com]
