"xdmp:eval" throws "XDMP-ARG: -- vars is invalid" exception when "()" [empty sequence] is passed for an optional external parameter

Question

Below is the sample code where $p2 is an optional external parameter (i.e. with ? modifier); the code gives XDMP-ARG: -- vars is invalid exception when empty sequence-() is passed against $p2.

Tried it on Marklogic 8 & 9

let $query := 
"
declare variable $p1 as node()? external;
declare variable $p2 as node()? external;
(
if($p1) then xdmp:log('P1') else ()
,
if($p2) then xdmp:log('P2') else ()
)

"
let $p1 := <p></p>
let $p2 := ()
return 
xdmp:eval(
          $query,
          (xs:QName('p1'), $p1, xs:QName('p2'), $p2)
         )

I expected the code to run and print logs. Can I get some deep insights into how the exception occurred?

Answer 1

If you pass in vars as a Sequence of key,val,key,val it needs to be even (multiple of 2). You can't embed sequences like that, as nested sequences get flattened automatically in XQuery. Pass in your vars using a map:map:

map:new((
  map:entry("p1", $p1),
  map:entry("p2", $p2),
))

HTH!