20632 2013-01-10 12:33:59 +0000 [XP 3.0] Transitivity of subtype relationships 2013-01-29 16:28:48 +0000 1 1 1 Unclassified XPath / XQuery / XSLT XPath 3.0 Last Call drafts PC All RESOLVED FIXED https://www.w3.org/Bugs/Public/show_bug.cgi?id=20643 P2 normal --- 1 mike jonathan.robie tim public-qt-comments oldest_to_newest 81182 0 mike 2013-01-10 12:33:59 +0000 I think we would all agree that there is a fundamental principle that subtyping relationships need to be transitive: A<:B and B<:C implies A<:C. Sadly this is not currently true of the subtyping rules for union types. According to the current rules, subtype(xs:decimal, union(xs:decimal, xs:string)) is true, and subtype(xs:integer, xs:decimal) is true, but subtype(xs:integer, union(xs:decimal, xs:string)) is false. This is clearly an oversight and should be fixed. 81226 1 mike 2013-01-11 10:03:49 +0000 The relevant rules are these two: 1. Ai and Bi are AtomicOrUnionTypes, and derives-from(Ai, Bi) returns true. 2. Ai and Bi are both pure union types, and every type t in the transitive membership of Ai is also in the transitive membership of Bi. If we fix bug #20643, then the meaning of rule 1 changes, and subtype(xs:integer, union(xs:decimal, xs:string)) becomes true. This fixes part of the problem. The other part of the problem is that union(a, b) should be a subtype of union(A, B) if a is a subtype of A and b is a subtype of B. We can achieve this by changing rule 2 to say: 2. Ai is a pure union type, and every type t in the transitive membership of Ai satisfies subtype-itemType(t, Bi) However, this has a side-effect which we may or may not consider desirable. The side-effect is that (for example) union(dayTimeDuration, yearMonthDuration) is now a subtype of duration, which was not the case before. Clearly it is both safe and useful that this relation should hold, but it is definitely a change in the spec. If we want to avoid introducing this side-effect, we could instead write rule 2 as: 2. Ai and Bi are both pure union types, and every type t in the transitive membership of Ai satisfies subtype-itemType(t, Bi) 81604 2 tim 2013-01-20 08:54:32 +0000 (In reply to comment #1) > The relevant rules are these two: > > However, this has a side-effect which we may or may not consider desirable. > The side-effect is that (for example) union(dayTimeDuration, > yearMonthDuration) is now a subtype of duration, which was not the case > before. I believe that, according to Formal Semantics, this was always the case, so shouldn't be regarded as a change, since for every value V that matches the union, V also matches duration. So I'd argue it was true in XQ1.0. 81941 3 jonathan.robie 2013-01-22 17:03:45 +0000 The Working Group decided to change the second of these two rules (the first two rules in the list under 2.5.6.2 The judgement subtype-itemtype(Ai, Bi)): <quote> 1. Ai and Bi are AtomicOrUnionTypes, and derives-from(Ai, Bi) returns true. 2. Ai and Bi are both pure union types, and every type t in the transitive membership of Ai is also in the transitive membership of Bi. </quote> The new wording for the second rule is: 2. Ai is a pure union type, and every type t in the transitive membership of Ai satisfies subtype-itemType(t, Bi) 82278 4 jonathan.robie 2013-01-29 16:28:48 +0000 The Working Group affirmed the wording in Comment #6 today.