SQL> CREATE OR REPLACE FUNCTION f(p_in IN NUMBER) RETURN number
  2  BEGIN
  3  return p_in*2;
  4  END;
  5  /

Warning: Function created with compilation errors.

SQL> sho err
Errors for FUNCTION F:
LINE/COL ERROR
-------- -------------------------------------------------------------------------------------
2/1      PLS-00103: Encountered the symbol "BEGIN" when expecting one of the following:
         . @ % ; is authid as cluster order using external character
         deterministic parallel_enable pipelined aggregate
         result_cache
         The symbol "is" was substituted for "BEGIN" to continue.