Converting varchar to bool

Hi,

I have currently trouble working with boolean values and variables in
functions.

As one would expect, a
select '1'::bool, 't'::bool, 'true'::unknown::boolean

works.

As a select '1' tells us this seems as a conversion unknown->bool
or ??maybe?? a boolean literal??

what-o-ever, at least my function gets not accepted:

CREATE OR REPLACE FUNCTION testbool(varchar)
RETURNS boolean AS
$BODY$
SELECT $1::boolean
$BODY$
LANGUAGE 'sql' STABLE STRICT SECURITY DEFINER;

Postgres complains that it can't convert varchar to boolean.
Actually neither $1::unknown::bool works.
It turns out that there exists no conversion varchar/text/unknown ->
boolean.
Actually there's not any conversion -> boolean.

How should this be treaten?
*being confused*

sorry for duplicating (I'm sure) this topic, at least I tried to find it in
the listarcive.