fn:subsequence-where: equivalent `fn:slice` expression

[ChristianGruen]

fn:subsequence-where: equivalent `fn:slice` expression

[michaelhkay]

I'm trying to get to the bottom of this.

First problem I think is that otherwise binds more tightly than +, so it should be

let $start := index-where($input, $from)[1] otherwise (count($input) + 1)

I'm then getting a problem that in test subsequence-before-002, the expression mock:subsequence-where($in, to:=$pred) returns a different result from mock:subsequence-where($in, from:=true#0, to:=$pred). That must be a bug in Saxon's handling of default function parameters.

Got it: the code wasn't handling keyword arguments correctly in the case of functions imported from a library module. (There aren't many tests for that...)

[michaelhkay]

Most of the subsequence-where tests are now passing using a mock implementation based on the equivalence expression in the spec, with parentheses added as described above.

The exceptions are tests subsequence-after-001, and subsequence-ending-where-101 to -108.

Those turned out to be errors in the way I converted the tests to call the mock version of the function. With that fixed, all the tests are now producing the expected results using the function:

declare function mock:subsequence-where (
  $input	as item()*,	
  $from	as (function(item(), xs:integer) as xs:boolean)?	:= true#0,
  $to	as (function(item(), xs:integer) as xs:boolean)?	:= false#0
) as item()*
{
   let $start := trace(index-where($input, $from)[1] 
              otherwise (count($input) + 1), '$start')
   let $end :=   trace(index-where($input, $to)[. ge $start][1] 
              otherwise (count($input) + 1), '$end')
   return slice($input, $start, $end)
};

As this exercise found a couple of Saxon bugs, I'm inclined to keep the tests for posterity...

[michaelhkay]

Fixed by PR #1025