How to correct put return from the function into in statement

I create function that return few values into 1 varchar row.select listagg(q'<'>' || expr || q'<'>', ',') into v_returnfunction return varchar2(1000).optionally expr can be number or char.When i select function from duali get normal output like:'1','2','3'When i try to run function like select where ... in (function)I get "ORA-01722: invalid number"

Can varchar return work correct for few values?

I tried:select listagg(q'<'>' || expr || q'<'>', ',')select listagg(expr, q'<','>')select listagg(expr, ',').