Re: Altering functions cast

On Fri, Aug 26, 2005 at 12:22:27PM -0700, Matt A. wrote:
> How do I get this to work?
>
> create function nullif_bool(boolean) returns
> bool as ' select nullif(''$1'','''')::bool;' language
> sql;

If an empty string is a possible input then the argument type can't
be boolean because you'll get a syntax error before the function
is called:

CREATE FUNCTION nullif_bool(boolean) RETURNS boolean AS '
SELECT TRUE; -- for testing
' LANGUAGE sql;

SELECT nullif_bool('');
ERROR:  invalid input syntax for type boolean: ""

The argument type should probably be text.  But then you get a
different error, this time during function creation:

CREATE FUNCTION nullif_bool(text) RETURNS boolean AS '
SELECT nullif($1,'''')::boolean;
' LANGUAGE sql;
ERROR:  cannot cast type text to boolean

One possible solution is to use PL/pgSQL instead of SQL and rely
on PL/pgSQL's automatic type conversion of the return value:

CREATE FUNCTION nullif_bool(text) RETURNS boolean AS '
BEGIN
    RETURN nullif($1, '''');
END;
' LANGUAGE plpgsql IMMUTABLE STRICT;

SELECT nullif_bool('');
 nullif_bool
-------------

(1 row)

SELECT nullif_bool('t');
 nullif_bool
-------------
 t
(1 row)

SELECT nullif_bool('f');
 nullif_bool
-------------
 f
(1 row)

Is that what you're looking for?

--
Michael Fuhr