When I tried writing to an read-only parameter(IN) of a function, Oracle complains with an error. But that is not the case when reading from an write-only(OUT) parameter of a function. Oracle silently allows this without any error. What is the reason for this behaviour?. The following code executes without any assignment happening to "so" variable:

create or replace function foo(a OUT number) return number
  is
    so number;
  begin
    so := a; --no assignment happens here
    a := 42;
    dbms_output.put_line('HiYA there');
    dbms_output.put_line('VAlue:' || so);
    return 5;
  end;
/

declare 
  somevar number;
  a number := 6;
begin
  dbms_output.put_line('Before a:'|| a);
  somevar := foo(a);
  dbms_output.put_line('After a:' || a);
end;
/

Here's the output I got:

Before a:6
HiYA there
VAlue:
After a:42