Improvements to syntax for defining multiple locals

[ekimekim]

Currently, if I wish to define a large list of locals (eg. "importing libraries" at the top of a file), I have two reasonable ways to format it:

local foo = import "foo.libsonnet";
local bar = import "bar.libsonnet";
local baz = import "baz.libsonnet";

local
  foo = import "foo.libsonnet",
  bar = import "bar.libsonnet",
  baz = import "baz.libsonnet";

The first is annoying due to the repetition, it would be nice to be able to define many locals without saying "local" every time and the syntax local x = 1, y = 2; solves this problem nicely for single-line statements.

The second is annoying when spaced over many lines. You either need to remember to end the last line with a ; instead of a , as above, or you can do a quasi-"closing brace"-style thing like so:

local
  foo = import "foo.libsonnet",
  bar = import "bar.libsonnet",
  baz = import "baz.libsonnet"
;

However, now you need to remember not to include a trailing comma on the final line.

I see two fixes to this problem that I hope should be relatively easy:

1. Allow a redundant , before the ;, ie. so that an expression like local x = 1, y = 2,; is valid. This allows a "trailing comma" in the quasi-closing-brace-style shown above, which will go a long way to making this not a pain when changing or re-ordering the list. This is the smallest change.

2. Allow a multiple-definition syntax like local (x = 1, y = 2);, with optional trailing commas. This would make my example:

local (
  foo = import "foo.libsonnet",
  bar = import "bar.libsonnet",
  baz = import "baz.libsonnet",
);

This is the best option in my opinion as the parentheses indicate immediately to the user that the definitions form a "block" of things which the local keyword applies to, and it has a very visible end while allowing re-ordering or modification of the list of definitions without needing to worry about treating any one specially.

It is also reminiscent of the python syntax:

from foo import (
  bar,
  baz,
)

[andreabedini]

local foo = import "foo.libsonnet"
   ,  bar = import "bar.libsonnet"
   ,  baz = import "baz.libsonnet"
   ;

?

there's still an asymmetry between the first import and the following ones but it's right at the front of the line and easy to spot.

[sparkprime]

#307