NULL values and string

Hi, List!

I'm using PG8.1.

Are there any abilities to represent NULL values as string?

I'm doing something like this:

create function func(int4) returns varchar as'
declare
num_value alias for $1;
string_value varchar;
begin

string_value := \'input value = \' || num_value;
return string_value;

end
'language 'plpgsql';

If I einvoke my function with NULL argument, it return NULL. But I want
'input value = NULL'.
Of course, I can check input value like this:

if(num_value isnull) then
string_value := \'input value = NULL\';
else
string_value := \'input_value = \' || num_value;
end if;

But it is not laconic...

Sergey Karin