Support empty list/set/map

[Aiee]

Neo4j:

Nebula:

(root@nebula) [nba]> RETURN []
[ERROR (-1004)]: SyntaxError: syntax error near `]'


(root@nebula) [nba]> RETURN [1]
+-----+
| [1] |
+-----+
| [1] |
+-----+
Got 1 rows (time spent 574/922 us)


(root@nebula) [nba]> RETURN {}
[ERROR (-1004)]: SyntaxError: syntax error near `}'


(root@nebula) [nba]> RETURN {a:123}
+----------+
| {a:123}  |
+----------+
| {a: 123} |
+----------+
Got 1 rows (time spent 645/1025 us)

(root@nebula) [nba]> LOOKUP ON player WHERE player.age in []
[ERROR (-1004)]: SyntaxError: syntax error near `]'

[jievince]

Empty set has conflict with empty map in the parser syntax

[CPWstatic]

This is not supported at the beginning of design.

[jievince]

GQL supports list, set, multiset, map, its syntax rule is like:

<multiset value constructor by enumeration> ::=
MULTISET <left brace> <multiset element list> <right brace>

For example

List{1, 2, 3}
Set{1, 2, 3}
Multiset{1, 2, 2}
Map{a:1, b:2}

And empty list, empty set, empty map is:

List{}
Set{}
MultiSet{}
Map{}

In this way, there will be no grammatical conflict.

[jievince]

Since we decided to make gql compatible in versions after 3.0, I recommend that this also could be done in versions after 3.0