- From: Jos de Bruijn <debruijn@inf.unibz.it>
- Date: Mon, 04 May 2009 20:20:04 +0200
- To: Sandro Hawke <sandro@w3.org>
- CC: RIF <public-rif-wg@w3.org>
- Message-ID: <49FF31D4.7090709@inf.unibz.it>

Sandro Hawke wrote: >> XPath functions and operators essentially says that each implementation >> can decide for itself which length for decimals it supports. > > As long as it's at least 16 digits > http://www.w3.org/TR/xmlschema11-2/#partial-implementation > >> For >> example, implementation A could support decimals of length 16, while >> implementation B supports decimals of length 20. In addition, each >> implementation can use its own rounding algorithm for representing >> numbers that need larger decimals. For example, implementation A could >> truncate numbers, while implementation B rounds them. >> >> this poses problems for us when defining things like casting functions >> and arithmetic operations. >> for example, according to XPath casting a string >> "0.11111111111111111111111111111111111111111111111111111111111111111111111111 >> 11111111111111111111111111" >> to a decimal results either in an error or a decimal of some length >> (given by the implementation) that is obtained from the number >> corresponding to the string by some arbitrary rounding algorithm. >> >> It is not possible for us to define this kind of behavior in DTB, >> because functions are defined as functions: you have some input values >> that define an output value. >> In addition, it is really bad for interchange, since some >> implementations do something, while other implementations do something >> else, and you get no warning. >> So, for casting, I propose to define the xs:decimal casting function >> such that the result of casting a string to a decimal is simply the >> number with the input string as lexical representation, and so we have >> no exception behavior. >> >> We might define conformance such that implementations only need to >> support decimals of a particular length. > > Am I going to be able to use an XPath number handling library for this? I guess there is no such thing, because XPath does not tell you what to do. > Or would you be making me round numbers differently? Again, XPath does not tell you how to round. > > I guess 0.6666666666666666 (17 digits) might reasonably round to > 0.6666666666666667 (16 digits), but that wouldn't conform to the > definition you're proposing... Indeed. And XPath does not tell you what to do at all. > > This functions-are-functions thing is worrying me, too. This means list > "union", "intersect", and "except" need to be defined with stable > ordering, I think, which I believe means they're stuck with n^2 > algorithms. Is it really impossible to allow for non-determinism / > non-specification on these things? I had been expecting the order of > lists returned from these operations to be undetermined (which would > allow an implementation to really use hash tables to b-trees behind the > scenes.) Hmmm, I guess there are tricks one could still do -- add a > list-index field, and then sort the final result by that field -- but > ouch.... > >> The problem with numeric functions are similar. With addition, >> subtraction, and multiplication we run into the same problem. Again, I >> propose to define the functions such that the output values are simply >> the decimals which are the result of the arithmetic operations and not >> from some implementation-dependent modification. >> With division it gets a bit more complicated. For example, there is no >> decimal that can represent the result of dividing 1 by 3, because there >> are no infinite-length decimals. If we had owl:real we could still >> properly defined the division function, although there is no syntactical >> representation for the result. I have two possible solutions for you: >> (1) We reintroduce owl:real and use it for the definition of >> numeric-divide (I think we need it only there). >> (2) We define the domain of numeric-divide such that only pairs of >> numbers a,b (if they are decimals) are included if a/b can be >> represented using a decimal. This means that if 1,3 are the arguments, >> the value of the function is not specified by DTB and is left up to the >> implementation. >> >> I think these are crucial issues and we need to have at least an idea of >> where we want to go before DTB can go to last call. Otherwise, it will >> not be possible to make any RIF implementations. > > FWIW, I don't think this is all that crucial. *shrug* Well, if things are not defined, people have no idea what to implement. Jos > > - Sandro -- +43 1 58801 18470 debruijn@inf.unibz.it Jos de Bruijn, http://www.debruijn.net/ ---------------------------------------------- Many would be cowards if they had courage enough. - Thomas Fuller

Received on Monday, 4 May 2009 18:20:56 UTC