create or replace function your_bool
( p_param1 in number
) return boolean
is 
  l_var1 number;
begin
  return p_param1 > l_var1;
end your_bool;
/

create or replace function your_bool_for_sql
( p_param1 in number
) return varchar2
is
begin
  return case when your_bool(x) then 'TRUE'
              when not your_bool(x) then 'FALSE'
              else 'NULL'
         end;
end your_bool_for_sql;
/