1968 2005-09-03 12:17:30 +0000 typing for fn:subsequence() too strict 2005-09-08 16:46:06 +0000 1 1 1 Unclassified XPath / XQuery / XSLT Formal Semantics 1.0 Last Call drafts All All CLOSED DUPLICATE 1728 P2 normal --- 1 jens.teubner simeon public-qt-comments oldest_to_newest 5691 0 jens.teubner 2005-09-03 12:17:31 +0000 The XQuery Formal Semantics define a number of specific typing rules for the built-in function fn:subsequence() in order to reveal a static type as specific as possible. Among others, the following rule is included in these specifics: statEnv |- Expr : Type quantifier(Type) in {1, +} ------------------------------------------------------------- statEnv |- fn:subsequence(Expr, $fs:last, 1) : prime(Type) Note that in this inference rule, $fs:last is completely unrelated to the expression Expr. The current context is not necessarily related to the argument of fn:subsequence(). In particular, the size of the current context set (and thus $fs:last) may be larger than the size of Expr. fn:subsequence() returns the empty sequence if its start argument is greater than the length of the sequence. The avove typing rule, however, does not include the empty sequence in the (static) return type of fn:subsequence(). If the start argument is smaller than the length of Expr, fn:subsequence() will return a sequence longer than one. Consider the following query (not really a sensible query, but it demonstrates a wrong type inference when applying the above rule): let $a := <a><b/><b/><b/><b/></a> return $a/b[fn:subsequence( (1,2), last() )] In this query, $a/b evaluates to 4 nodes. last() will thus return 4, which is greater than the length of the sequence (1,2). So the result of fn:subsequence( (1,2), last() ) will be the empty sequence, while we get `xs:integer' as the static return type for it. If we replace the sequence (1,2) in the above example by (1,2,3,4,5), fn:subsequence() will return a list of length 2, though the static type is (still) `xs:integer'. 5692 1 jens.teubner 2005-09-03 13:32:17 +0000 (In reply to comment #0) > If we replace the sequence (1,2) in the above example by (1,2,3,4,5), > fn:subsequence() will return a list of length 2, though the static > type is (still) `xs:integer'. Sorry, I just realized that the rule given in the specs actually includes the length argument of fn:subsequence(). So my last statement does not actually hold. The problem stated before, however, remains. Sorry for the confusion. 5724 2 simeon 2005-09-06 15:49:22 +0000 Yes, this is correct. The rule you mention is wrong, that was picked up already following the treatment of a previous bug report (#1728). The WGs have already decided to remove that rule. Thanks for your feedback, - Jerome Simeon *** This bug has been marked as a duplicate of 1728 ***