World: r3wp

Let's say, my index is way beyond the tail, and I insert a new element 
there. It may then just be appended to the series, which is at an 
index way before my pointer. What if I then e.g. say:
	remove/part my-index -1

what should happen? And why?

Geomol: "I'm not sure, if this is known or desired behaviour" - Brian 
used a long description, but the fact is, that the best part of it 
is "accident". I bet, that it has not been checked, and it is not 
clear, whether the difference is desirable or not.

As far as I am concerned, the difference is not desirable.

Let's say, my index is way beyond the tail, and I insert a new element 
there.
 - you are out of luck, such an operation is not supported

>> s
== [1 2 3]
>> t: tail s
== []
>> remove/part s 2
== [3]
>> insert t 1
== []
>> s
== [3 1]

>> index? t
== 4
>> system/version
== 2.100.111.2.5

Seems to be.

Hmm, did not actually check, thanks

Now, what do you think about the behaviour? Do you find it useful? 
(I do not)

The more I think about it, I tend to not like it. It must be up to 
the user to be sure, the indexes are in bound when used. It's really 
cryptic code required to make this work.

BTW, "Let's say, my index is way beyond the tail, and I insert a 
new element there." - in fact, I was right saying that the operations 
was not supported, since it did not insert a new element there

yeah :)

Only the part with the behavior of SKIP might be an accident. The 
rest was the result of a blog discussion over two years ago, which 
iirc happened while Ladislav was taking a break from REBOL.

In my opinion, this is clearly unsettled (BACK vs. SKIP), and was 
not discussed before

No, SKIP wasn't discussed. The rest was.

So, the above mentioned INSERT behaviour was discussed?

I think so, but it was in that private world where Carl's GUI for 
R3 was being worked on with Henrik and me. It looks like INSERT works 
like SKIP.

In R2, indexes are constrained to the bounds of the series they reference, 
so if you shorten the series you lose your position
 - this is provably false

(and the proof is quite old)

I was simplifying to avoid having to write a bunch of test code when 
my time is limited. Any proofs are welcome, though maybe the R2 proofs 
should go in Core.

The word "constrained" was a simplification of the real process.

The proof exists, welcome or unwelcome, to demonstrate the actual 
properties of the interpreter, and to prove why the behaviour of 
some function is unuseful.

Yup, especially for R2.

http://www.rebol.net/wiki/Identity#Indices_of_series

It would be possible and in keeping with the metaphor to have an 
out-of-bounds INSERT pad blocks with none values, but since strings 
and binaries don't have a way to have inline nones, that would make 
the behavior of blocks inconsistent. That is why INSERT behaves the 
way it does. If you want INSERT to trigger an error in that case, 
like POKE and set-path modification, that would make sense too.

Or we could just say that INSERT, APPEND, SKIP, AT and LENGTH? are 
constrained to bounds, while PICK, POKE, and the ordinals and path 
accesses that are based on them are not.

OK, that is why the "INSERT *there*" operation is, in fact, unsupported, 
and, in such cases, it is preferable to trigger an error, in my opinion

APPEND is clearly a different case than INSERT, since APPEND always 
uses the tail

regarding the incompatibility of SKIP -1 and BACK - I am not sure, 
whether that is good to have.

I think that the incompatibility of NEXT and BACK is more important, 
and definitely an error, and an accident.
>> a: [1 2 3]
== [1 2 3]
>> b: skip a 2
== [3]
>> remove/part a 3
== []
>> index? b
== 3
>> index? next b

== 3  ; constrained to the argument index, not even the series bounds
>> index? back b
== 2 ; not constrained to the series bounds
>> index? back a
== 1 ; constrained to the head

Option 1:
	NEXT will not advance, if at tail or beyond tail.
Option 2:
	NEXT will not advance, if at tail, but will if beyond tail.
Option 3:
	NEXT will always advance.

And if it will not advance in some cases, should it then trigger 
an error, if that is tried to do?

Option 4:

 NEXT and BACK will be constrained to the series tail, even if that 
 makes the (index? a) <= (index? next a) truism false.

I'm not advocating option 4, I was just making another option besides 
3 that made sense. Option 1 is what we have now.

Note that the other operations that are constrained to the tail of 
the series in R3 are PRINT and EMPTY?, which is why R2 gets that 
out-of-bounds error and R3 doesn't.

Also note that REMOVE/part with a negative part acts like REMOVE/part 
from the other point with a positive part. This is why it's impossible 
to create a before-the-head reference.

With Option 3 that would change, since BACK from the head of the 
series would go out of bounds. REMOVE out of bounds would continue 
to be a noop.

This is something, I need to sleep on to have any useful opinion 
on. :)

The off-by-one error for AT and non-positive indexes still has an 
unresolved ticket too. PICK and POKE were fixed, but not yet AT.

Would a programmer expect this to be true always?

	(index? series) + (length? series) = (index? tail series)


That seems to define some basic rules for indexes and the functions 
involved. But it fails sometimes, as it is now:

>> s: [1 2]       
== [1 2]
>> t: tail s
== []
>> clear s
== []
>> (index? s) + (length? s) = (index? tail s)
== true
>> (index? t) + (length? t) = (index? tail t) 
== false


Problem here is, that LENGTH? t returns 0. It should return -2, if 
the result should be true.

In R2, it is true, but in R2, INDEX? t is also 1 in this case, which 
is also not good.

I noticed a funny thing, when inserting a series in the same series 
with the /only refinement.

>> s: [a b c]
== [a b c]
>> length? skip s 2
== 1				; makes sense
>> insert/only s skip s 2
== [a b c]
>> s

== [[...] a b c]		; reference to the same series is shown this way, 
ok
>> length? s/1
== 2				; unexpected

Wouldn't it be more logical, if that last result were 1?
It's the same in R2.

It's possible to create infinitely deep series with this:

>> s: [a b c]
== [a b c]
>> insert/only tail s s
== []
>> s
== [a b c [...]]
>> s/4
== [a b c [...]]
>> s/4/4
== [a b c [...]]
>> s/4/4/4
== [a b c [...]]

and so on.

No I think it is not unexpected. Because when you insert new values 
into a series its internal positions is changing:

b: [1 2 3 4 5] c: next b
insert b 0
index? c
== 2

in you example s/1 actually a pointer to s itself (the second value 
in s). and when you insert something into s, the pointer (which is 
s/1) is also moves forward.
>> same? head s/1 head s
== true

>> s/1

== [b c] ;which mean it still points to second value in the series.

so length is 2

Hold on.

>> b: [1 2 3 4 5]
== [1 2 3 4 5]
>> c: next b
== [2 3 4 5]
>> index? c
== 2
>> insert b 0 
== [1 2 3 4 5]
>> index? c   
== 2
>> insert b 0 
== [0 1 2 3 4 5]
>> index? c   
== 2