Function Issue!

Can anyone tell me what is wrong with the function below ?
It throws an ERROR: syntax error at or near "FETCH" at character 551

CREATE OR REPLACE FUNCTION "public"."theo_test2" () RETURNS OPAQUE AS'
BEGIN
declare curr_theo cursor for select * from node_names;
fetch next from curr_theo;
close curr_theo;
END;
'LANGUAGE 'plpgsql' VOLATILE CALLED ON NULL INPUT SECURITY DEFINER;

However, this appears to work :
begin;
declare curr_theo cursor for select * from node_names;
fetch next from curr_theo;
close curr_theo;
end;

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