how can I returns a set of integer in a plpgsql function?

hi, all:
there is a function in postgresql contrib "int_arrgreagte":

CREATE OR REPLACE FUNCTION int_array_enum(int4[])
RETURNS setof integer
AS '$libdir/int_aggregate','int_enum'
LANGUAGE C IMMUTABLE STRICT;

I can use this function like this:
chry=# SELECT int_array_enum('{1,2}');
or
chry=# SELECT * from int_array_enum('{1,2}');
result:
int_array_enum
----------------
1
2

also, I can use it like this:
SELECT int_array_enum('{1,2}'), int_array_enum('{1,3}');
result:
int_array_enum | int_array_enum
----------------+----------------
1 | 1
2 | 3

I try to write my own function with the same return type in plpgsql:
create or replace function my_int_array_enum(state integer[]) returns
setof integer as $$
declare
i integer;
begin
for i in array_lower(state,1)..array_upper(state,1) loop
return next state[i];
end loop;
return;
end;
$$ language 'plpgsql' immutable;

but, when I use my function like this: test=# SELECT
my_int_array_enum('{1,2}');
I got the error said as below:
ERROR: set-valued function called in context that cannot accept a set
(I know, SELECT * from my_int_array_enum('{1,2}') is ok)

can anybody tell me the reason, thanks for any help:D

regards,