Transaction with in function

I looking for solution for commit, rollback with in function.

Below is Example function

CREATE OR REPLACE FUNCTION test()

  RETURNS void AS

$BODY$

BEGIN

--- set    of statements

Commit;

-- set of statement

commit;

-- if things goes wrong

rollback

RETURN;

END; $BODY$

  LANGUAGE 'plpgsql';

Thanks in advance .

Regards

Ravi Katkar