1159 Add filter expressions for maps and arrays

[michaelhkay]

No description provided.

[ChristianGruen]

A nice proposal…

• Maybe $value?[predicate] could be changed to value?[predicate], as the first variant implies that the value must be a variable reference.
• It’s ok that numeric predicates are not supported, but I didn’t understand the reasoning. My assumption would be that $array?[1, 3] and $array?[position() = (1, 3)] are simply equivalent.
• Typo: specificd

[michaelhkay]

Fix #1159

[michaelhkay]

I dropped numeric predicates because I felt people would expect ["a","b","c"]?[1] to return "a" rather than ["a"], but then what would ["a","b","c"]?[1,2] return? And there are plenty of other ways of selecting a single member of an array, we don't need another.

[ChristianGruen]

I dropped numeric predicates because I felt people would expect ["a","b","c"]?[1] to return "a" rather than ["a"], but then what would ["a","b","c"]?[1,2] return?

Would an implementation then be allowed to return the full array for ["a","b","c"]?[1, 2], i.e., would it be one of those cases in which an error happens, but which doesn’t necessarily have to be reported (see also #1014 (comment))?

Ironically, people might still write $array?[1] and be surprised that the input array is returned unchanged. If we want to reduce EBV confusion (see the discussion in #829 (comment) and others), we could require the map/array filter predicate to return a boolean value:

(: valid :)
$array?[position() = 1]
$array?[true()]
[true()]?[?1]
['a']?[boolean(?1)]

(: invalid :)
$array?[1]
$array?['a']
$array?[./a]
['a']?[?1]

[michaelhkay]

Minor detail: should "?[" be one token or two? I can't see any strong arguments either way. Making it one token gives us a bit more flexiibility to change the operator precedence, but I don't see any particular reason to do that.

[ChristianGruen]

Minor detail: should "?[" be one token or two? I can't see any strong arguments either way. Making it one token gives us a bit more flexiibility to change the operator precedence, but I don't see any particular reason to do that.

I have no preference either.

I’m convinced that I would use the new syntax once it existed… but I can’t quite get used to the fact that we treat arrays and maps differently (this runs counter to our efforts to standardize the two data structures).

Next, while the classical filter expression is comparatively clean, we now run the risk of doing a lot of special casing. For expressions like…

$map-or-array?[. = 123]
$map-or-array?[?key = 123]

…there will be no chance to raise static errors.

[ndw]

The CG agreed to merge this issue at meeting 074