using 'ALTER ROLE' in a function

[Lutz Broedel <lb(at)fggm(dot)uni-hannover(dot)de>]

Dear list,

I'm trying to write a PL/pgSQL function that executes some code every 
time a user changes his/her password. The function is supposed to work 
like this:

CREATE OR REPLACE FUNCTION changePwd(varchar(255)) RETURNS boolean AS $$
DECLARE
	pwd varchar(255);
	curr_user name;
BEGIN
	curr_user = (SELECT session_user);
	pwd = $2;
	
	ALTER ROLE curr_user WITH ENCRYPTED PASSWORD pwd;
	-- [... do some other stuff ...]
	RETURN true;
END;
$$ LANGUAGE 'PLPGSQL';

but PostgreSQL returns:
ERROR:  Error »syntax error« at »$1« at character 13
QUERY:  ALTER ROLE  $1  WITH ENCRYPTED PASSWORD  $2

I have tried this in several ways and it seems, ALTER ROLE just does not 
accept a parameter instead of name. Does anybody have a solution for this?

Thanks in advance and best regards,
Lutz