Nested PIPELINED function in pl/sql

I have to write a nested pipelined function in pl/sql which I tried implementing in the following manner.

create package body XYZ AS
    function main_xyz return data_type_1 pipelined is
        begin 
        --code
        pipe row(sub_func);
        end;
    function sub_func return data_type_1 pipelined is
        begin 
        --code
        pipe row(sub_func_1);
        end;
     function sub_func_1 return data_type_1 pipelined is
        begin 
        --code
        pipe row(main_abc);
        end;
 end;

create package body abc AS
        function main_abc return data_type_2 pipelined is
            var data_type_2;
            begin 
            --code
             return var;
            end;
  end;

However, I get the following error

[Error] PLS-00653 : PLS-00653: aggregate/table functions are not allowed in PL/SQL scope

Where am I going wrong? Is it syntax or logic?

Pipelined functions provide rows one by one (on demand), so you cannot put all the rows at one time from pipelined function.

Seems to me you need to change main_xyz this way:

function main_xyz return data_type_1 pipelined is
 begin 

   --code

   FOR rec IN (select * from table(XYZ.sub_func)) LOOP
       pipe row(rec);
   END LOOP;
 end;

Consider that sub_func must be in specification of XYZ package since everything you use in SQL queries including PIPELINED functions are to be public (i.e. visible to a user who runs query).

UPDATE: I forget to alert: do not abuse pipelined functions (if you have another choice) - queries using them might have lame performance because DB engine cannot build a good execution plan for "unpredictable piped rows".