[BUG] function supplied to fn:filter must return xs:boolean

[line-o]

Describe the bug

eXistDB does not enforce the return type of the user provided filter function.

Relevant spec:

https://www.w3.org/TR/xpath-functions/#func-filter

especially,

there is no special treatment for numeric predicate values, and no conversion to an effective boolean value

Expected behavior

Return types of function parameter to fn:filter other than xs:boolean must throw XPTY0004.

To Reproduce

xquery version "3.1";

module namespace hofs="http://exist-db.org/xquery/test/higher-order-functions";

declare namespace test="http://exist-db.org/xquery/xqsuite";

declare
    %test:assertError("XPTY0004")
function hofs:function-has-wrong-return-type () {
    filter((0 to 1), function ($a) { $a })
};

declare
    %test:assertError("XPTY0004")
function hofs:return-mixed-types () {
    filter((true(), false(), 1, ""), function ($a) { $a })
};

Context (please always complete the following information):

• OS: macOSX 10.15.4
• eXist-db version: since the feature was released in 2.2 until latest HEAD (5.3.0-SNAPSHOT)
• Java Version 8

[adamretter]

I think you will also find it doesn't enforce the parameter types either (certainly not statically anyway)

[line-o]

If you mean that fn:filter does not check the second parameter to be function(item()) as xs:boolean, then yes. That is a different issue though.

[joewiz]

See #1917.