CREATE OR REPLACE FUNCTION f1
   RETURN NUMBER
IS
BEGIN
   RETURN 999;
END;
/

Function created

CREATE OR REPLACE FUNCTION f11
   RETURN NUMBER
IS
BEGIN
   insert into t1 values (1, 2, 3);
   RETURN 999;
END;
/

Function created

create or replace view v1 as
select 1 Num, 'hello world' msg, f1 func1
from dual;

View created

create or replace view v11 as
select 1 Num, 'hello world' msg, f11 func1
from dual;

View created

select * from v1;

NUM                                     MSG         FUNC1                                  
--------------------------------------- ----------- ---------------------------------------
1                                       hello world 999                                    
1 row selected

select * from v11;

ORA-14551: cannot perform a DML operation inside a query 
ORA-06512: at "SCOTT.F11", line 5
ORA-06512: at line 1