World: r3wp

How then can I get this code, executed from the command line, to 
always quit?
  r3-a97-3-1.exe --do "print true attempt [exit] print true quit"

does it not print an error message?

It's the missing CATCH that is the error, not the THROW.

I guess ATTEMPT is no longer an extension of error? try like in R2.

No, but it works the same, just faster. Almost all functions that 
had a [throw] attribute in R2 (or needed one, like ATTEMPT) were 
converted to natives in R3. We're still waiting for the new equivalent 
of the [throw] attribute to be added to R3 - right now USE, OBJECT 
and CONTEXT are the only built-in functions that still need it, afaik.

Note that the ticket was marked as a problem, not dismissed (yet). 
The problem was explained in the comments, and more thoroughly now 
in my more recent reply there.

I think I can answer my own question about executing arbitary code.

Just tried the ancient error? try technique:

   error? try [break]  ;; I'm happy
   attempt [break]   ;;  still think this is a bug!

Actually, the first is an error too. It turns out that the way BREAK, 
EXIT, RETURN, THROW, HALT and QUIT work is by throwing pseudo-errors 
that are technically instances of the error! type, but not really 
errors. The ERROR? function disables that throwing for all error! 
values, even the fake ones. It's an error in ERROR?.

There's even a CureCode ticket for it, which you referred to in your 
comment.

It's the same error as RETURN BREAK.

From my perspective, attempt [break] is working correctly. ATTEMPT 
has nothing to do with the BREAK, it simply passes it to the outer 
context (if you can say that) and then the BREAK doesn't hold up, 
because it's not inside a function. Seems simple to me.

Loops, but yeah.

yes, sorry, loops, not functions.

loop 2 [attempt [break]] works as expected.

>> a: does [attempt [exit] 1]
>> a
>>

Looks fine and dandy.

What would you expect from:
  loop 2 [print true attempt [break] print true]
true printed 0..6 times?

Or:
  loop 2 [print true attempt [continue] print true]

Once, and it did.

Twice, and it did.

The question (only for Carl) is whether task-local runtime contextual 
information can be made available to BREAK, EXIT, RETURN and THROW 
to let them know that they are going to be handled by the console, 
which would let them throw the error that the console should be throwing, 
on behalf of the console. Basically, letting them be more polite. 
The problem is that it is *really* likely (only Carl would know) 
that this would slow down all function! and closure! calls, all loops, 
and every call to DO (even indirectly, as in IF) by quite a bit. 
It could make R3 a lot slower (maybe several times slower).

I'm not sure what you would expect here. Wrapping break and continue 
in attempt has no effect, because we know there are no errors inside 
the attempt block and the context outside is correct for break and 
continue. The attempt has absolutely no effect here.

Right.

I'm curious now: How do those functions know they are inside loops 
and functions?

They ask Carl :-)

They don't, not in the slightest.

They are thrown like exceptions. The default handler is in the console.

I see. That further proves how little ATTEMPT has to do with these 
errors. :-)

And in REBOL there is no way to statically determine whether an exception 
will be handled without tracing the code. You could in theory determine 
it at runtime (from the internals), but that would have overhead. 
The question is how much overhead.

ATTEMPT and TRY push a handler for exceptions with codes over 100 
on the task-local handler stack. Those exceptions are generated by 
DO MAKE error! (or code called by natives with the same effect).

The interesting part of the default console handler is that it is 
exactly the same handler that displays regular errors. The messages 
come from here:
>> system/catalog/errors/throw
== make object! [
    code: 0
    type: "throw error"
    break: "no loop to break"
    return: "return or exit not in function"
    throw: ["no catch for throw:" :arg1]
    continue: "no loop to continue"
    halt: ["halted by user or script"]
    quit: ["user script quit"]
]

Sorry for the awkward phrasing above. The comments in the ticket 
read better.

Brian, I agree with you.  I think if we make Try or Attempt handle 
these other exceptions then we are going to slow things down.  Again, 
we have  a way to handle this now.  No need to pursue the problem 
at this point.

This `attempt [break]` debate is really interesting.

I think it uncovered not a bug in the implementation, but provokes 
a discussion on the language semantics.

I guess I'll have to write that up in more detail :)

Thanks for the repeated attempts to explain, Brian!

For me, the work around is:
    error? try [....]
That seems to work in all cases I have tried.


[Ironically, 99% of all the REBOL code I have ever written uses ERROR? 
TRY --  ATTEMPT did not exist when I started REBOL, and my fingers 
have learned to type the idiom. It's only with R3 that I'ved tried 
to update my error handling.....Seems I should stick with my older 
ways for a while...:)]

Ok, I've written up some of my thoughts at http://www.rebol.net/wiki/User_talk:Andreas#Notes_on_Bug.231506_--_on_control_transfer_functions

The gist of it: I think most control transfer functions should be 
unbound globally, and only bound in code executing within their respective 
control transfer handler.

For example, BREAK should be unbound globally, and bound only within 
loop bodies.

That means, an erroneous use of e.g. BREAK would simply result in 
a "has no value" error, just as the use of any other unset word. 
And this error would of course be nicely caught by ATTEMPT.

The crux of the issue for me is this:
  attempt [badcode] print true
I (naively perhaps) suggest that this should always either:
-- close the console (if badcode has a QUIT); or
-- halt (if  badcode has a HALT; or
-- print TRUE in all other cases.


Brian argues that there are internal engineering reasons for a fourth 
possibility:
-- ATTEMPT returns an error message

I think that your suggestion is not naive but very reasonable

And I also think there's an easy way to fix things that they work 
as you expect

Easy
 from a language design perspective, that is.

I also think it is reasonably straightforward to implement and has 
no significant impact on performance. But I don't have the time to 
write down why I think this, at the moment.

Sunanda, that error? try [....] code you keep mentioning doesn't 
work in all cases you mention. The problem is this:
>> error? break
== true

This is an error (bug#771 and bug#862): It should not return true, 
it should break and then later on trigger an error for an unhandled 
break. To illustrate why:
>> loop 1 [error? break 'buggy]
== buggy

That should have returned unset, not continued. The BREAK is supposed 
to halt execution and resume it somewhere else, either after the 
enclosing loop or at the top level. The same error is affecting your 
error? try [....] code, which is not behaving correctly. 


Your problem, Sunanda, is that you are thinking that ATTEMPT is not 
catching bad code (bugs) that it is supposed to catch - this is not 
strictly true. ATTEMPT isn't supposed to catch bugs, it is supposed 
to catch triggered errors. And a lot of logic bugs don't trigger 
errors, because the code itself is technically correct (what REBOL 
can check for), but not in accordance with the programmer's intentions 
(which REBOL can't know).


Now in this case the logic bug is that you have forgotten that the 
purpose of the BREAK, CONTINUE, THROW, EXIT and RETURN functions 
is escape from the normal flow of the code ("structured" gotos, aka 
non-local returns, exceptions, ...) - you remembered this about HALT 
and QUIT - and didn't realize that the error you are trying to catch 
with ATTEMPT is not in the code that you have wrapped in the ATTEMPT: 
It is not bad code. The error is triggered by the code on the other 
end, the handler, which is not wrapped in the ATTEMPT at all.

So you need to realize that this code is technically correct:
>> attempt [break]
** Throw error: no loop to break
but doesn't do what you thought it should.

And this code is absolutely incorrect:
>> error? try [break]
== true

but doesn't trigger an error because of a weird design quirk in R3 
that is arguably a bug, but not necessarily fixable (see bug#771 
and bug#862).

Created a ticket (bug#1509) to explain the real problem with these 
functions.

do we really want this behavior:

>> obj: construct [a: b: c: 1]
== make object! [
    a: 1
    b: 1
    c: 1
]

I think we need a way to pass that block so that 'a and 'b can evaluate 
to none

It is clear why it setting a and b to 1 but we need to have something 
a bit safer.