I have tried the code example given in http://www.w3.org/TR/xpath-functions-31/#func-random-number-generator, it has the code 

declare %public function r:random-sequence($length as xs:integer) as xs:double* {
  r:random-sequence($length, fn:random-number-generator())
};
declare %private function r:random-sequence($length as xs:integer, $G as map(xs:string, item())) {
  if ($length eq 0)
  then ()
  else ($G?number, r:random-sequence($length - 1, $G?next())
};


The is a closing `)` missing in the `else` clause, should be

declare %public function r:random-sequence($length as xs:integer) as xs:double* {
  r:random-sequence($length, fn:random-number-generator())
};
declare %private function r:random-sequence($length as xs:integer, $G as map(xs:string, item())) {
  if ($length eq 0)
  then ()
  else ($G?number, r:random-sequence($length - 1, $G?next()))
};