"Connect by" can't be put in Function?

I created a table and used this for querying very well:

       select filename from docindex
       start with refno = 42
       connect by prior parentnode = refno;

But I failed to create this Function object:

    create or replace function GetFullPath(pDocIndexRefNo DocIndex.RefNo%type)

        return varchar
        as
     begin
        select filename from docindex
        start with refno = 42
        connect by prior parentnode = refno;

        return 'Hello';
     end;

Just a simple function, where are my wrong?

--
Best regards,
Diviner.