Filtering by type in lookup expressions

[michaelhkay]

We have dropped the syntax ??type(T) for filtering the results of lookup expressions, because of problems with syntax ambiguity. This issue seeks an alternative.

Although selection by type also makes sense with shallow lookup, it is most relevant with deep lookup. The main need arises with intermediate steps of a path such as ?? X ?? Y which gives a dynamic error if X selects something that is not a map or array. This is consistent at one level with // X // Y, except that // X can never select something that isn't a node.

The main problems with filtering using an [. instance of record(p, q)] predicate is that it's very long-winded. For example, if we want to select only those members of a selected array that are sequences of a particular record type, without flattening everything else, we have to write something like ?? values::* ?[. instance of record(p, q)+] ? *, which is a bit of a nightmare.

Starting from the end goal, I would like to be able to write something close to ??record(first, last) to select all the items of this record type at any depth. We know that syntax doesn't work, because ??NCName is already taken. That's also true for ??items::record(first, last), unless we change the rules for what can appear after ::.

Also, there's another syntax hazard: what we want here is a SequenceType, not an ItemType, and that means that it can contain a trailing ? occurrence indicator, which is easily confused with the next lookup operator in a path.

Looking at it from all angles, I do feel the best solution is to prefix the record(first, last) with a marker character so that we know we've got a type filter here. Characters that might do the job include @, #, $, %, ^, ~. Of these, my preference remains ~, for three reasons:

(a) it's currently unused: overloading a different symbol is more likely to cause visual confusion (b) one of the traditional uses of ~ is to indicate a "matches" or "is kind of like" relationship. (c) there's a mnemonic association between "tilde" and "type" (compare "at" and "attribute")