Enum type emulation: problem with opaque type in PL/pgSQL functions

Guys, hello.

Here is a problem.

--
-- Creating 2 new functions and new type
--
BEGIN;

CREATE FUNCTION enum_week_in (opaque)
    RETURNS int2
    AS '
    DECLARE
        invalue ALIAS for $1;
    BEGIN
        IF invalue='''' OR invalue=''0'' THEN RETURN 0; END IF;
        IF invalue=''Monday'' OR invalue=''1'' THEN RETURN 1; END IF;
        IF invalue=''Tuesday'' OR invalue=''2'' THEN RETURN 2; END IF;
        IF invalue=''Wednesday'' OR invalue=''3'' THEN RETURN 3; END IF;
        RAISE EXCEPTION ''incorrect input value: %'',invalue;
    END;'
    LANGUAGE 'plpgsql'
    WITH (ISCACHABLE);

CREATE FUNCTION enum_week_out (opaque)
    RETURNS text
    AS '
    DECLARE
        outvalue ALIAS for $1;
    BEGIN
        IF outvalue=0 THEN RETURN ''''; END IF;
        IF outvalue=1 THEN RETURN ''Monday''; END IF;
        IF outvalue=2 THEN RETURN ''Tuesday''; END IF;
        IF outvalue=3 THEN RETURN ''Wednesday''; END IF;
        RAISE EXCEPTION ''incorrect output value: %'',outvalue;
    END;'
    LANGUAGE 'plpgsql'
    WITH (ISCACHABLE);

CREATE TYPE enum_week (
    internallength = 2,
    input = enum_week_in,
    output = enum_week_out,
    PASSEDBYVALUE
);

COMMIT;

Well, all is ok after it, e.g. functions and type were registered in system catalog.

Now, when I try to do "SELECT enum_week_in('Monday')", I get the following:

NOTICE: plpgsql: ERROR during compile of enum_week_in near line 0

The same will occure if I

CREATE TABLE test (wday enum_week);
insert into test (wday) values ('Monday')

If I redefine the same functions with input argtype 'text'/'int2' they work fine.
I guess the problem is that PL/pgSQL doesn't handle opaque type correctly.

Any ideas ?

I don't care how but I need to emulate ENUM type, just to convert MySQL dumps to PostgreSQL. E.g. ENUM values
stored in MySQL dump should be restorable in Postgres without any conversion.

I running PostgreSQL 7.0.3 on Linux RedHat 6.2, kernel 2.2.15, Intel Celeron CPU; Postgres was
upgraded from 7.0.2 without changing anything in system catalog.

Thanks,
Max Rudensky.