World: r4wp

One-based is human friendly, while zero-based is usually more machine 
friendly, so I think REBOL made the right choice

By extension, I would like Red/System to be as close to Red as possible, 
so issues can be explained firmly and just once, and it's easy to 
morph Red code into Red/System code when you decide you need the 
performance

Agreed with last Kaj's remark ....

+1

Being a human myself, I don't find indices-as-ordinals ("one-based") 
particularly human friendly.

For Red/System, a 0-based convention might make more sense, but it 
would push us into the R3 issue I've mentioned above wrt indexes 
<= 0.


With indices-as-offsets ("0-based"), there really is no issue with 
indices <= 0.

As for R3, it did not really introduce "0-based convention implicitly", 
it still is firmly "1-based" in as far as the first element in a 
series can be accessed using index 1.


When you want indices-as-ordinals, you really need to decide: (a) 
is the ordinal "zeroth" meaningful, and if so, what it means; (b) 
are negative indices meaningful, and if so, what they mean.


R3 went with the choices of (a) having meaningful zeroth, defined 
as "the item in a series before the first item", and (b) allowing 
negative indices, having index -1 as the immediate predecessor of 
index 0.


R2 went with the choice of (a) not having a meaningful zeroth, but 
instead of erroring out, functions (pick) & syntax (paths) accepting 
indices are lenient: passing an index of 0 always returns NONE. For 
(b), R2 allows negative indices and defines -1 as the immediate predecessor 
of 1.

Andreas: thanks for the good sum up.


R3: agreed that index 1 is still the first element in a series, but 
index 0 is allowed and there is this ticket #613 that clearly aims 
at introducing 0-based indexing in R3...so my guessing was these 
different changes or wishes were inter-related. http://curecode.org/rebol3/ticket.rsp?id=613


R2: I would have really prefered that index 0 raises an error than 
returning none.

If you wish to allow index computation for series not positioned 
at the head, allowing index 0 is actually quite sensible, unless 
you want to make index computation particularly error prone.

There is also the option proposed by Gabriele to consider: an ordinal! 
datatype (...-2th, -1th, 1st, 2nd, 3rd, 4th,...).


It could solve the whole thing, but I see two cons about this option: 

1) negative ordinals look odd, I don't even know if they can be read 
in engllish?

2) code would be more verbose as it will need conversions (to/from 
ordinals) in many places.


In addition to the pros, making  a difference between an integer 
and an ordinal might help improve code readability.

The problem with no meaningful index 0 is that potentially meaningful 
index values are no longer isomorphic to integers. And as REBOL has 
no actual datatype for indices, all we can compute with are integers 
while relying on a correspondence of those integers to indices.


If you only ever compute indices for series positioned at the head, 
you get a nice correspondence of integers to indices, because meaningful 
indices for this series correspond to the positive integers.


But if you also want to compute indices for series positioned elsewhere, 
this nice integer-to-index correspondence breaks down as you suddenly 
have an undefined "gap" for the integer 0, whereas negative integers 
and positive integers are fine.

Yes, ordinal! would fix that, or the index! I proposed earlier

I also think that an ordinal! (or index!) datatype may be an intriguing 
possiblity to get the best of both worlds.

Andreas: do you have a short code example involving index 0 in computation? 
I don't remember ever having issues with index 0 and I use series 
with offsets a lot! Though, Ladislav claims he and Carl did encounter 
such issue at least once...the use cases for this issues remain a 
mystery well kept by Ladislav. ;-)

I personally avoid computing with non-head positioned series wherever 
possible.

So sorry, I don't have a particular example at hand, but I can easily 
imagine it coming up with e.g. forall or forskip and trying to access 
previous values in an iteration.

Though, Ladislav claims he and Carl did encounter such issue at least 
once

 - I am claiming that I have revealed a bug in Carl's code caused 
 by the fact that indices are not isomorphic to integers, i.e. they 
 "contain a gap". That is a totally different issue than whether indexing 
 should be 1-based or 0-based.

Ladislav: I think it is relevant to this topic as findind out if 
the index 0 gap is a real practical issue or not, could help decide 
about the indexing base.

Then: gap is a practical issue, causing bugs.

(no matter whether indexing is 0-based or 1-based)

That is caused by the fact that there is no gap in the series, the 
gap is only caused by "unreasonable thinking".

Also, if we define 0-gap (possible) then we do not have any right 
to use negative indices.

, i.e. the gap needs to be prolonged to infinity, otherwise we are 
simply inconsistent

I'm inclined to continue on the one-based convention that worked 
pretty well in R2 for many years
 - actually, R2 is "hybrid", since SKIP is zero-based, in fact.

Can you define "unreasonable thinking"?

Have a look at the following illustration:
https://gist.github.com/5af73d4ecf93ac94680a

SKIP works with offsets only, it's not related to indexing.

About the only somewhat reasonable use I can come up with for R2's 
behaviour is that to allow writing literal -1 indices in paths (values/-1) 
to access the value preceding the current position.

Yes, I can in this case: "unreasonable thinking" here is the fact 
that the "mathematical model" - in this case the numbering of positions 
in series differs substantially from the properties of the object 
it is modelling - in this case there is a difference between the 
"no-gap in the series" versus "gap in the mathematical model".

Also, there is one more mathematical inconsistency: if I "hate zero", 
I simply cannot use negative numbers, otherwise I am being inconsistent.

SKIP works with offsets only, it's not related to indexing.

 - that is not true, in fact. It *is* related to indexing, since we 
 may always use PICK SKIP SERIES N M versus PICK SERIES K and these 
 things are realted, like ir or not.

 in this case the numbering of positions in series differs substantially 
 from the properties of the object it is modelling


Is this again the "inbetween position" vs values counting intepretation 
difference?

No.

No

Ok. :-)

It is because the series values actually are a contiguous concept, 
but their respective indices are not a contiguous space of integers.

Which is, frankly, awful when having to compute with indices with 
integers.

... when having to compute indices with integers*

Numbering positions in a series is, in other words, characterized 
as "mathematically modelling 'positions' in a series". Your "inbetween 
positions" are something that does not exist in the series in fact.

Even if you talk about "inbetween", there is _no_ integer in between 
1 and 2, but there is an integer between -1 and 1.

Tail position is inbetween, underlying implementation details cannot 
change that fact.

Tail position is inbetween - actually not. You can write: INSERT 
TAIL SERIES #"a". You do not insert the character "inbetween", in 
fact.

Also, the SKIP function demonstrates the same behaviour quite well 
and consistently.

tail position

 is actually a misnomer, in as far as it corresponds to no proper 
 index of a series. The special behaviour series seen in some functions 
 when operating on series in "tail position" would warrant "tail mode" 
 as a more sensible description of the state the series is in.

as it corresponds to no proper index of a series

 - I reserve the right to disagree. INDEX? TAIL gives some correspondence

Yes, it gives an index that can I'd consider "improper" for the series, 
in as far as the series does not contain a value at that position.

that is arguable, while the INDEX? TAIL is not arguable, giving actual 
correspondence

A rather meaningless correspondence, though.

Subjective interpretations of series tail position are possible, 
as TAIL position exists without pointing to any value.

(And an index that does not correspond to a value in the series, 
at this point in time.)