[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: limits of the generic
Jeni, I think you've hit the nub of this issue. We expect '+' to be commutative, associative, and have additive inverses. At least associativity and additive inverses have been shown in the examples in this thread to have been violated when dates and one or more durations are added. We also expect consequences of these properties, such as a + x = a + y => x = y x < y => x + a < y + a (where a is a duration and x and y are dates) to be true, and they are not (the converse of the first one is true, however). The algebra of DateTime x Duration with operation + is not the algebra of a group or any other familiar structure, so many of our expectations for the '+' operation will be violated. Typically this is occuring because a thing that looks like a fixed duration, such as P1M, is actually a different length in units of days or seconds depending upon the context in which it is used in addition. As you say, however, it is useful to be able to add and subtract dates and durations. If someone with a decent background in algebra can spell it out formally, there may be some way for people to use '+' and not have their intuition violated too often. Otherwise, (or perhaps even if there is a formal algebra, but it's too exotic) treating the addition as a named function with arbitrary but reasonable semantics, without all the assumptions about '+' that we drag into every addition we see, may be the right way to proceed. Jeff ----- Original Message ----- From: "Jeni Tennison" <jeni@j...> To: "Jeff Greif" <jgreif@a...> Cc: <xml-dev@l...> Sent: Monday, September 30, 2002 4:27 AM Subject: Re: limits of the generic > Hi Jeff, ... > > Hmm... The thing is that adding durations to date/times is a useful > thing to do, so I'd be loath to make it impossible in XPath 2.0. If it > were done through a function call instead, that would enable the + > operator to retain its usual semantics. What do you think? Would it > matter if: > > 2002-01-28 < 2002-01-30 > > but not: > > add-yearMonthDuration-to-date( > xs:date('2002-01-28'), xs:yearMonthDuration('P1M')) < > add-yearMonthDuration-to-date( > xs:date('2002-01-30'), xs:yearMonthDuration('P1M')) >
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