World: r4wp

Really? Because I was assuming that FOR would have termination conditions 
(similar to yours). The initial conditions model just picks which 
set of termination conditions to apply, at least as far as the start, 
end and bump parameters are concerned. The body parameter, being 
a code block, is assumed to be under developer control. So if the 
developer wants to hack the termination conditions in the code block 
that is their fault.

Ha, are we still discussing 1993 or some other ticket?

FOREVER is assumed to be a solicited-for infinite loop, because it's 
right there in the name. #864 is assumed to be whatever the developer 
says, because "General loop" is right there in the doc string. #1993-1994 
FOR has the *feature* of not *accidentally* being infinite for any 
value of start, end and bump, its constraint is a feature; of course 
it could be *intentionally* infinite by changing the index in the 
code block, but that just means that there is one parameter that 
the developer would have to be careful about, the body block, and 
since that is a general pattern throughout R3 they would be doing 
that anyway.

Assuming that we are discussing #1993 and assuming:


* you insiste to allow the FOR I 1 1 0 and similar to be "infinite 
by default"

* want to support cycle variable changes in a simple and consistent 
way

the best you can do is to cause an error when finding out that

    all [start = end bump = 0]


is TRUE, since in that case there is no reasonable terminating condition 
that could not cause infinite loop by default. Some people may object, 
but otherwise it looks as not a big deal.

err: "you insist to not allow FOR I 1 1 0 and similar to be "infinite 
by default"" is what I meant

{you insiste to allow the FOR I 1 1 0 and similar to be "infinite 
by default"} -> {you insiste to allow the FOR I 1 1 0 and similar 
to never be "infinite by default"}

Right.

The fact that we even have a FOREVER loop at all means that there 
would be a value in having developers use it, just for their own 
documentation.

And that allows developers to be less careful with start, end and 
bump, for values of "careful" that mean "wrapping FOR in expensive 
conditional code".

We've been trying to reduce the amount of expensive conditional code 
throughout R3. That's why a lot more functions allow none now.

ASSERT/type means that typechecks don't have to be considered expensive 
anymore. Value checks still require EITHER and other conditional 
functions though.

For that matter, typesets make the type tests of function argument 
specs less expensive now too, so we can afford to be picky there 
as well.

Am I to understand that instead of causing an error int that case 
you simply prefer to not loop taking it as an exceptional condition?

An out-of-range condition, yes.

The other model was the trigger-an-error model.

BrianH: I'm really glad to hear that there is a concerted effort 
to reduce the amount of expensive conditional code throughout R3. 
 Great job all!

Hmm, it is not out-of-range in the normal sense, because when START 
= END and BUMP = 0 the values both are and remain "in range", but 
it is "exceptional"

Depends on the model. If bump is a velocity, you can say that only 
velocities above (or below of you're going in reverse) 0 are in range.

That is a matter of coming up with a plausible theoretical explanation 
for something that we want to do for practical reasons.

Then start-vs-end sets the direction, and bump sets the velocity. 
It's just a way to explain *why* to newbiees.

Dealing with the consequences of triggering an error is more expensive, 
so we tend to only want to trigger errors when they really *are* 
errors. If there is a plausible way to just do nothing and/or return 
none when it's not potentially damaging, we should come up with a 
rationale that lets do that instead.

You can say that you "support" zero velocity by "not looping", but, 
in fact, you rather don't support it by failing as silently as possible.

It's really a rationale.

No problem to say it is "zero velocity". The problem is that in "normal 
life" something having zero velocity does not "vanish", rather it 
stays where it is.

So you may be caught as using "inappropriate logic", but I do not 
mind, being able to eventually answer that it is an exceptional case 
that you simply did not want to handle to not cause headaches to 
some users, while causing inconveniences to people being able to 
calculate what it is they should have expected.

We can just arbitrarily declare that we want 0 velocity to be considered 
out of range, as a favor to the developer, and the velocity explanation 
gives us a good excuse to not trigger an error. FOREVER existing 
means that they have other options, and index setting means that 
they can do whatever they want if they really want to, so it's not 
actually a constraint if they don't want it to be.

You can always declare something arbitrarily. The problem is that 
if you do declare a + b = none in case a = 0 you are most probably 
causing inconveniences to all people knowing that there might have 
been a more consistent behaviour...

Remember, #864 is a proposal to replace FOR with a more flexible 
power-user function that would be less safe to use. They lose some 
safety as a tradeoff for more power and prettier sytnax. So, they 
lose two features (safety and backwards compatibility) but gain more 
flexibility. The greater flexibility would come at the expense of 
a slower function: negligably in the case of the function itself, 
but more when you add the conditional wrapper code, so it would have 
to be used carefully if you want it to be efficient. Overall, that 
is the R3 motto right there: R2 is for newbies, R3 for power users.

But let's just forget about it in this specific case. I guess that 
FOR is not expected to be used extensively anyway.

So, I would recommend that the #1993 restrictions against accidental 
infinite loops should go into R3/Backwards and rebol-patches, because 
R2 is for newbies. And I would recommend that #864 be the new FOR 
for R3 and R2/Forward, because R3 is for power users.

Man, you guys are typing faster than I'm reading. :-)

LOL

the best you can do is to cause an error when finding out that

 
   all [start = end bump = 0]

is TRUE, since in that case there 
is no reasonable terminating condition that could not cause infinite 
loop by default.


I think this answers what I was asking. Though it seems that Ladislav 
wants [1 1 0] to be infinite for consistency, while Biran and I want 
it not to be, for perceived user friendliness. :-)

Brian, for that case, does your model make it a "no loop" or "once 
only" condition?

Gregg, we just agreed to make it "no-loop"

OK, I skimmed too fast. Thanks.

(which is "the most silent failure")

once only

 would be rather inconsistent with any terminating condition, because 
 there is none that could cause it to loop exactly once

Updting %new-loop.r now.

Posted %mezz/new-loop.r

Gress, for the start-vs-end-sets-direction bump-is-velocity model:

* start=end means no direction so just loop until the =end termination 
condition is met and ignore bump. If the index gets changed in the 
body block, let the =end termination condition handle it.

* start<end means positive direction, for values of "positive" that 
don't include 0, so bump <= 0 is out of range, meaning no loop. The 
termination condition *if we start looping* is >= end.

* start>end means negative direction, for values of "negative" that 
don't include 0, so bump >= 0 is out of range, meaning no loop. The 
termination condition *if we start looping* is >= start.


Positive and negative directions don't include 0 because if the developer 
wanted to do an infinite loop they would have used FOREVER or R3's 
#864 FOR general loop. R2 was aimed at newbies, and they need extra 
coddling.

Gress -> Gregg

For the bump-sets-direction start-and-end-set-the-range model, 0 
doesn't set a direction so it should trigger an error. Otherwise, 
the same.

Brian, can you point out which test case is incorrect, and what it 
should produce? That way we can match against Ladislav's examples.

I like the model that doesn't trigger an error.

And I tested %new-loop.r only under R2, not R3, just in case.

Test cases for the first model, just using literal numbers as metaphors 
for the principles:
; start = end, start is not constrained, termination is x = end
[i: 0 1 = for x 5 5 1 [if i >= 1 [break/return 'fail] i: i + 1]]
[i: 0 1 = for x 5 5 -1 [if i >= 1 [break/return 'fail] i: i + 1]]
[i: 0 1 = for x 5 5 0 [if i >= 1 [break/return 'fail] i: i + 1]]
; start < end, start is x >= end, termination is x >= end
[i: 0 2 = for x 4 5 1 [if i > 2 [break/return 'fail] i: i + 1]]
[none? for x 4 5 -1 [break/return 'fail]]
[none? for x 4 5 0 [break/return 'fail]]
; start > end, start is x <= end, termination is x <= end
[i: 0 2 = for x 5 4 -1 [if i > 2 [break/return 'fail] i: i + 1]]
[none? for x 5 4 1 [break/return 'fail]]
[none? for x 5 4 0 [break/return 'fail]]


In all cases, bump is added to x after the termination condition 
is not met and before looping again.

Let me fix some comments above:

; start < end, start is bump > 0 and x >= end, termination is x >= 
end

; start > end, start is bump < 0 and x <= end, termination is x <= 
end


So, the direction sets the termination condition, and the bump sets 
the velocity that the loop is advanced between iterations, with range 
limits on the velocity as a starting condition in addition to the 
end range limits.

Ugh, let me fix the comments again, AltME is annoying:

; start < end, start is bump > 0 and x <= end, termination is x >= 
end

; start > end, start is bump < 0 and x >= end, termination is x <= 
end

Thanks. I'll have to make time to reconcile those with the tests 
I have in there now. I was hoping you could just say which don't 
match your model.