29870 2016-09-26 06:42:55 +0000 Tuple types 2016-09-26 06:42:55 +0000 1 1 1 Unclassified XPath / XQuery / XSLT Requirements for Future Versions Candidate Recommendation PC All NEW P2 normal --- 1 mike jim.melton public-qt-comments oldest_to_newest 127530 0 mike 2016-09-26 06:42:55 +0000 A common use case for maps is to represent composite structures such as the result of fn:random-number-generator, which is a map containing number: an xs:double next: a function permute: a function The problem about using maps for such structures is that we can't describe or constrain their type very accurately: this example simply becomes map(xs:string, item()). We could introduce tuple types simply as new syntax for constraining the type of a map. declare function fn:random(...) as tuple{number : xs:double, next : function(*), permute : function(item()*) as item()*} The instances of this type would still be maps, and they would still be instances of map(xs:string, item()), and one could use all the same operations (e.g. random($seed)?permute($x)), but the new item type syntax would allow the required type to be specified more descriptively and precisely, and would allow better static type checking. The feature has been prototyped in Saxon: see http://dev.saxonica.com/blog/mike/2016/09/tuple-types-and-type-aliases.html (In this particular example it would be nice to be able to make the type recursive, since the "next" function also returns tuples of this type; but that's a separate requirement).