[Jsonpath] Index selector [-0] non-existent end of an array?

[Joel Bruner <[e-mail]>]

The other day someone was asking me how to insert a value at the end an array (using my jpt tool)… so I said something like: "Well in JSON Pointer it's "/-" and for JSONPath I borrowed the convention and use $[-]"… to which they replied "the spec seems to indicate it should be [-0]" – which had me quite puzzled since when brought this up in 2022 <https://mailarchive.ietf.org/arch/browse/jsonpath/?q=index%20selector%20like%20json%20pointer> it was deemed out of scope by some and nonsensical to others.

While in "2.3.3.2. Semantics" it is somewhat implied since the $[-1] is the end of an array and it does follow that $[-0] is after that, the ABNF for "2.3.3.2. Semantics" seems to explicitly exclude -0 only allowing "-" with digits 1-9

>  An index selector <index> matches at most one array element value.
>  index-selector      = int                        ; decimal integer
>  int                 = "0" /
>                         (["-"] DIGIT1 *DIGIT)      ; - optional
>  DIGIT1              = %x31-39                    ; 1-9 non-zero digit

Only later in "Figure 2" of "Appendix A" and in "2.3.5.1. Syntax" does "-0" make an appearance since this is a "signed zero" that Javascript allows but JSON can't roundtrip <https://mailarchive.ietf.org/arch/msg/json/RRT0jWUruScN1oXlgZlTqETivSU/> 
> number              = (int / "-0") [ frac ] [ exp ] ; decimal number

So it seems there really is no syntax for the non-existent end of an array in JSONPath – and for a while I was kind of excited there had been a change of heart.

When I look back I see Greg Dennis had this well meaning thing to say:

> I think this is the crucial point: JSON Path is intended as only a query language.  It's not intended to specify a location for days to be set/inserted.

Yet, two years later and it seems folks still haven't "gotten the memo" and are using JSONPath syntax not just for querying but for specifying locations to insert data. Why shouldn't they?  It's like approaching this by saying "People are the problem, the spec is just fine". JSON Point4er RFC6901 managed to define this very useful thing with a couple sentences in its 8 pages, yet in the 62 pages of the JSONPath spec somehow this is out of scope? It's such a simple thing to define, why not? I don't think it's a slippery slope to define either [-] or [-0] as the non-existent end of an array… two years ago I should have pushed back harder. The things not defined now will just mutate down the road.

Thanks,
Joel Bruner