Allow to omit indices in index range expansions

[krobelus]

Description

So omitted range limits in, say $PATH[..] default to the first/last element, just like Python/Go/Rust slices.
This is something I have had expected to work several times (coming from Python). It essentially saves typing the 1/-1 in a .. range and I don't see any harm in defaulting to those values.
However, it's not super well thought out, and a language change so I'm asking if there are any concerns.

TODOs:

• Changes to fish usage are reflected in user documentation/manpages.
• Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.md

[ridiculousfish]

I like the language change, I added some comments on the implementation. I'd like @faho to approve too as he did a lot of work on ranges in the past.

One question is whether empty variables are treated the same as omitted indexes. In this implementation it looks like they are; if that's deliberate we should test and document it.

[ridiculousfish]

Please add a test that makes the behavior clear for an empty variable, e.g.:

echo $test[..$empty]

I do not have a strong opinion about what that should do, but it should be something we decide and test.

[krobelus]

Ah right, I added a test for it.
Turns out this expands to no indices because of cartesian product expansion. It is at least consistent with the same code with 1 on the left side.

[faho]

So currently having an empty variable in a range is allowed, because it triggers the cartesian product (and hence removes the entire token). I.e. this doesn't error:

set -l empty
set -l list 1 2 3
echo $list[1..$empty]

# even this is allowed:
echo $list[..$empty]

Which means if we handled empty variables any differently here it would be a breaking change. I think the cartesian product here is consistent with it elsewhere and therefore the right thing, but then we've never been in love with it in general. However that would be a larger discussion, and I think it's one we could have later.

So I would keep this as-is, except for one test I'd like to add:

echo $list[.."$empty"]
echo $list["$empty"..]

This prints the entire list, which seems logical however you slice it.

Otherwise:

LGTM!

[krobelus]

This prints the entire list, which seems logical however you slice it.

A+ pun right there 😂 😂

Let's ship it!