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,yFROM 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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