World: r3wp

I also think that Carl hesitates to give us lower-level constructs. 
 This has been the historic case, BUT, now that a lot of people are 
actively contributing and actually producing working concepts, ports, 
prototypes and stuff, Carl is slowly realizing how usefull it is 
for *him* to open up on the lower levels of REBOL.


the exception/return/throw discusission going on is a good example 
of this active participation of the community.  Obviously, not everyone 
is willing or able to participate, but in such deep discussions, 
there is rarelly a huge mass of people anyways.

I really hope that brian, Ladislav and Carl will get together in 
a possibly heated discussion about all of this.  the difference in 
mindsets is a perfect setup to get the best overall solution.

my only desire in all of this discussion is that trhow/catch is kept 
dynamic and that /name be implemented with context matching and that 
catch doesn't handle throw/name.

(note that they need not be *implemented* as dynamic returns, but 
they should function as such)

catch doesn't handle throw/name

 - you could always use catch/name [...] 'none versus throw/name 'none, 
 if you did not want to catch other throw/name calls, so this is still 
 just "cosmetic", and surely not serious in any sense I can imagine.

For an even more interesting thing - a definitional catch/throw pair 
(which can be ported to R2) see the Exception_proposals article. 
Advantage: you do not have to use any name, yet, only the right throw 
is caught.

yes that is interesting, but /name is usefull for some code patterns 
like named exceptions.


thing is that currently, catch  withouth /name does make that impossible 
afaik.  if we built a mezz which re-threw them, afaik, that would 
change the context and thus make context sensing impossible... though 
your skill in this type of low-level manipulations might (will? ;) 
prove me wrong.

:-'

Named exceptions are child's play for definitional catch/throw

ok, what would be the advantage of using a dynamic catch/throw, if 
any?

it is already available, that is about all

but, sure, you may get a different answer if you ask some fans of 
dynamic constructs

ok, so its just less complicated to build mezz code with dynamic 
returns, but that means dropping a few possibilities with definitional 
ones?

its just less complicated to build mezz code with dynamic returns 
- who told you that?

the opposite is true

its just my bumbling appraisal so far, because it seems to me that 
you don't need to redefine and contextualize everything within your 
mezz code... but I do say I'm novice at this subject.  (I'm trying 
hard to completely "get" it/

ok, so its easier cause its managed by the core, but that means you 
can't play around with it within the mezz... is that a better appraisal?

hence, "harder" to adapt, but "easier" out-of-the-box?

Compare this:


dynamic: func [block i] [append block 'i if i > 0 [dynamic block 
i - 1] block]


definitional: closure [block i] [append block 'i if i > 0 [definitional 
block i - 1] block]

probe dynamic [] 3

probe definitional [] 3

Which one is harder?

At the first sight, none is, I bet, but at the second one:

probe reduce dynamic [] 3

probe reduce definitional [] 3

The answer is different

So, which one is harder?

Btw, for DYNAMIC you will get different answers depending on whether 
you are running it in R2 or in R3

R3 
>> probe reduce dynamic [] 3
** Script error: i word is not bound to a context
** Where: reduce
** Near: reduce dynamic [i i i i] 3

>> probe reduce definitional [] 3
[3 2 1 0]
== [3 2 1 0]

So, any favourite?

I'm trying to understand what's going on in R3...

(in the dynamic version)

I could show you a similar example with CATCH/THROW, but it would 
require more code

Dynamic variables in R3: they are "not bound to a context" when the 
function is not running.

(that formulation is a bit unfortunate, meaning, that I would not 
have used it, but, in essence, it means: "don't ask what the value 
of dynamic variable is, when the function is not running")

Nevertheless, you can insert something like


if i = 0 [print mold block] to see the result, and compare that to 
the result in R2

wow, I think you've just illustrated a big part of the issue and 
you should add the above to your exceptions page.

it does a better job at explaining the underlying issue than the 
R3 error page IMHO.

The thing just illustrated were "dynamic variables versus definitional 
variables"

yes, but the error document tries to illustrate it too, and I didn''t 
see it as plainly as I do now.

need to check that

I find that the above is a good intruction which makes the  "Exception 
Marker Scoping"  more understandable in detail.


its not exactly the same, since the error document talks about unwinds, 
but I feel that the above helps me understand *why* the unwind does 
what it does.

I really need to fill in the Exceptions page with the real differences 
and advantages of dynamic and definitional scoping. There are advantages 
that either has that the other does not. It would be a real loss 
to not have both in different circumstances. The different circumstances 
being definitional for returns and dynamic for breaks and throws.

And anyone who thinks definitional is always better needs to look 
at #1521.

with the above example, I'm thinking about the past discussions and 
they are starting to make sense in a new light  :-)

I have discussed a new page organization strategy for the Exceptions 
page with Ladislav. As time permits, I will try to reorganize. It 
will make these issues much more clear.

Max, the errors page you mentioned tried to illustrate the difference 
between dynamic and definitional return, which looks more subtle, 
than the difference between dynamic and definitional variables...

its just less complicated to build mezz code with dynamic returns

 - Depends on the code. Some obscure but important things are impossible 
 if you have dynamic returns. Some less-obscure things aren't even 
 defined as a concept if your returns are definitional..

The reason we can get rid of dynamic returns in R3 is because the 
critical stuff that we used to do with dynamic returns in R2 is *necessarily* 
handled by dynamic breaks in R3, so dynamic returns are no big loss.

A description of the difference between the definitional and dynamic 
return:


*the dynamic return returns to the dynamically closest function (i.e. 
to the function that was called as the last one)

*the definitional return returns to the definitionally closest function 
(i.e. to the function that was defined as the last one and contained 
the return in question)

That's not all. Definitional also requires the affected functions 
to have (and in some cases fake) lexical scope. For R3 this means 
nested blocks.

lad,  what I mean is that the illustration of the variable issues 
above helps us realize that there even *is* a dynamic vs definitional 
distinction. 
it then allows us to more fully understand the return issues. 


you might be surprised that I never even realized there where dynamic 
variables, in the way you present it above.  Even if I've grown to 
understand binding pretty deeply.

aha, OK

in a sense... you can say that I might even begin to understand your 
guru code now  ;-)