How to declare cursor if tablename is a variable?

See this:
“CREATE or replace function geo_polygon(tablename varchar) RETURNS integer
AS $$
DECLARE
objectid varchar;
tab varchar;
x1 float;
y1 float;

obj_num integer;
point_num integer;

polygonstr varchar;

cur_obj CURSOR FOR SELECT mrc_xy_position FROM tablename;
cur_point CURSOR FOR SELECT x,y FROM tablename where mrc_xy_position =
objectid;

BEGIN
…….“

Error if call the function. Neither to use “execute” . Who can solve it?
Thanks!

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