Re: limits of the generic
Hi Jeff, > It seems there's something wrong with the pinning algorithm. > Probably, there should be an attempt to do pinning only at the end > of the computation (by analogy with rounding) -- the idea of pinning > to a valid date after adding Y/M components, then later dealing with > the day and time parts seems of dubious merit. Agreed that it's something wrong with the pinning algorithm, but I don't think that leaving the pinning until the end works either. We're trying to find an algorithm where following is true: 2004-01-29T12:00:00 + P1M < 2004-01-30T00:00:00 + P1M < 2004-01-30T12:00:00 + P1M < 2004-01-31T00:00:00 + P1M < 2004-01-31T12:00:00 + P1M < 2004-02-01T00:00:00 + P1M It seems obvious that 2004-01-29T12:00:00 + P1M => 2004-02-29T12:00:00 and that 2004-02-01T00:00 => 2004-03-01T00:00:00. Which means that we need to find something where the following is true: 2004-02-29T12:00:00 < 2004-01-30T00:00:00 + P1M < 2004-01-30T12:00:00 + P1M < 2004-01-31T00:00:00 + P1M < 2004-01-31T12:00:00 + P1M < 2004-03-01T00:00:00 And now things are difficult, to say the least. If the days are pinned back then 2004-01-29T12:00:00 + P1M < 2004-01-30T00:00:00 + P1M won't be true. If they're rounded up, then 2004-01-31T12:00:00 + P1M < 2004-02-01T00:00:00 + P1M won't be true. In either case, keeping the relationship 2004-01-30T12:00:00 + P1M < 2004-01-31T00:00:00 + P1M is going to be hard. It's not just a matter of the times causing problems, or that it's a leap year causing the difficulty. There are similar issues with: 2002-01-28 + P1M < 2002-01-29 + P1M < 2002-01-30 + P1M < 2002-01-31 + P1M < 2002-02-01 + P1M 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')) Cheers, Jeni --- Jeni Tennison http://www.jenitennison.com/
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