29428 2016-02-07 13:34:14 +0000 map:fold and map:filter 2016-05-22 06:40:15 +0000 1 1 1 Unclassified XPath / XQuery / XSLT XQuery 3.2 Use Cases and Requirements Candidate Recommendation PC Linux NEW P2 normal --- 1 benito jonathan.robie vladimir public-qt-comments oldest_to_newest 124887 0 benito 2016-02-07 13:34:14 +0000 There is a surprising lack of higher order functions on maps. map:filter Summary Returns an map containing those entries of the $map for which $function returns true. Signature map:filter( $map as map(*), $filter as function($key as xs:anyAtomicType, $value as item()*) as xs:boolean ) as map(*) Rules The effect of the function is equivalent to the following definition: map:merge($map, map:keys() [$filter(., $map(.))] ! map:entry(., $map(.)) ) Error Conditions As a consequence of the function signature and the function calling rules, a type error occurs if the supplied function $function returns anything other than a single xs:boolean item; there is no conversion to an effective boolean value. Examples The expression map:filter( map { "A": 1, "B": 2, "C": "Hello world" }, function($x,$y) {$y instance of xs:integer}) returns map { "A": 1, "B": 2 }. 17.3.13 map:fold Summary Evaluates the supplied function cumulatively on successive values of the supplied map. Signature map:fold( $map as map(*), $zero as item()*, $function as function(item()*, xs:anyAtomicType, item()*) as item()*) as item()* Rules The effect of the function is equivalent to the following definition: fold-left(map:keys($map), $zero, function($aggregate as item()*, $key as xs:anyAtomicType) as item()* { $function($aggregate, $key, $map($key)) }) Notes If the supplied map is empty, the function returns $zero. If the supplied map contains a single entry [$key,$value], the function returns $zero => $function($key, $value). If the supplied map contains two entries [$key, $value] and [$key2, $value2], the function returns $zero => $function($key,$value) => $function($key2,$value2); and similarly for an input map with more than two entries. Examples The expression map:fold(map {"a": true(), "b": true(), "c": false()}, true(), function($x, $key, $y){$x and $y}) returns false(). (Returns true if every value of the input map has an effective boolean value of true().). The expression map:fold(map {"a": true(), "b": true(), "c": false()}, true(), function($x, $key, $y){$x or $y}) returns true(). (Returns true if at least one value of the input map has an effective boolean value of true().). 124889 1 mike 2016-02-07 13:55:08 +0000 Classifying this against future requirements. We're not going to be adding anything to 3.1 just because it seems a good idea; the bar for considering changes to a candidate recommendation has to be set very high. 126475 2 vladimir 2016-05-22 06:40:15 +0000 > 17.3.13 map:fold > > Evaluates the supplied function cumulatively on successive values of the > supplied map. > > Signature > map:fold( $map as map(*), > $zero as item()*, > $function as function(item()*, xs:anyAtomicType, item()*) as item()*) > as item()* Please note that map:fold() would be of limited use, as with current definitions it relies on map:keys() functions, which is non-deterministic with respect to ordering. This means that two calls with the same argument are not guaranteed to call $function in the same order.