JSONiq follows the W3C standard for partial application. The following explanations, provided as an informal summary for convenience, are non-normative.

A static or dynamic function call also have placeholder parameters, represented with a question mark in the syntax. When this is the case, the function call returns a function item that is the partial application of the original function, and its arity is the number of remaining placeholders.

       let $f := function($x as integer, $y as integer) as integer { $x + $y }
       let $g := $f(?, 2)
       return $g(2)
     

JSONiq dynamic function calls follow the W3C specification.