Revisit negative array indexing

[ricardbejarano]

So while this was discussed in #113 back in 2016, I wonder if we should revisit this discussion given the recent changes to array slices in #1093.

With array slicing supporting negative indexes, I find it counterintuitive not to have negative indexing in regular array lookups. Especially as we seem to approximate Python's slicing, and Python does have negative indexing.

@sparkprime you made a good argument against it here, I'm curious if you've changed your mind as hinted here.

Note: I've had this related PR in go-jsonnet open for a while.

[mstandley-tempus]

appears to be fixed in v0.21.0, see: https://github.com/google/jsonnet/releases/tag/v0.21.0

Array slices can now have negative start and/or end indexes; these are now wrapped-around (similar behaviour to Python).

[He-Pin]

In jsonpath, it can have negative index too

[ricardbejarano]

@mstandley-tempus no, v0.21.0 includes negative array slicing, I'm asking about negative array indexing.

$ jsonnet -version
Jsonnet commandline interpreter (Go implementation) v0.21.0

$ jsonnet -e 'std.slice([0,1,2,3], -1, null, null)'  # slicing, this is what was added in v0.21.0
[
   3
]

$ jsonnet -e '[0,1,2,3][-1]'  # indexing, this is what this issue is about
RUNTIME ERROR: Index -1 out of bounds, not within [0, 4)
	<cmdline>:1:1-14	$
	During evaluation	

$ 

[mstandley-tempus]

oh word -- i didn't read the release notes carefully enough, because i was expecting [-1] to work in 0.21.0