Filter operator for arrays

[michaelhkay]

I propose to provide ?[...] as a filter operator for arrays.

For example,

let $array := [(1,2,3), (4,5,6,7)]
return $array?[count(.) = 4]

returns

[(4,5,6,7)]

I propose that the operator should work exactly like the familiar [] for sequences in its handling of numeric and boolean predicate values. So for example $array?[2,1] in the above example returns [(4,5,6,7), (1,2,3)]. The result is always an array (which may be a little surprising). This means that $array?[3] has the same effect as [$array?3] or [$array(3)].

[michaelhkay]

We could also use this syntax for filtering maps, with the convention that "." is bound to a KVP representing the map entry. So, for example

let $map := {"x1":1, "y":200, "x3":50}
return $map?[?key => starts-with('x') and ?value > 3]

returns {"x3":50}

[michaelhkay]

Closed by acceptance of PR #1163