and output parameters aren't allowed 

SQL> ed
Wrote file afiedt.buf

  1  create or replace function a (x in number, y out number)
  2  return number
  3  is
  4  begin
  5  y:=x*2;
  6  return 10;
  7* end;
SQL> /

Function created.

SQL> var m number;
SQL> var n number;
SQL> exec :m := a(2,:n)

PL/SQL procedure successfully completed.

SQL> print m

         M
----------
        10

SQL> print n

         N
----------
         4

SQL> select a(2,:p) from dual;
select a(2,:p) from dual
       *
ERROR at line 1:
ORA-06572: Function A has out arguments