Null references redux

[Walter Bright]

Denis Koroskin wrote:
> On Sun, 27 Sep 2009 01:49:45 +0400, Walter Bright 
> <newshound1 at digitalmars.com> wrote:
> 
>> The problem with non-nullable references is what do they default to? 
>> Some "nan" object? When you use a "nan" object, what should it do? 
>> Throw an exception?
>>
> 
> Oh, my! You don't even know what a non-null default is!
> 
> There is a Null Object pattern 
> (http://en.wikipedia.org/wiki/Null_Object_pattern) - I guess that's what 
> you are talking about, when you mean "nan object" - but it has little to 
> do with non-null references.

It's the black hole object. It prevents you from getting a seg fault, 
but I see no rationale for expecting that an unexpected null object 
always returning "I succeeded" means your program will operate correctly.

The white hole object, of course, always throws an exception when it is 
accessed. At least you know something went wrong - but you already have 
that with null.


> With non-null references, you don't have "wrong values", that throw an 
> exception upon use (although it's clearly possible), you get a correct 
> value.

You're not getting a correct value, you're getting another default 
value. If the logic of your program is expecting a prime number > 8, and 
the null object returns 0, now what?

> If an object may or may not have a valid value, you mark it as nullable. 
> All the difference is that it's a non-default behavior, that's it. And a 
> user is now warned, that an object may be not initialized.

He isn't warned, that's just the problem. The null object happily says 
"I succeeded" for all input and returns more default values and null 
objects.

What happens if the output of your program then becomes a null object? 
How are you going to go about tracing that back to its source? That's a 
lot harder than working backwards from where a null exception originated.

I used to work at Boeing designing critical flight systems. Absolutely 
the WRONG failure mode is to pretend nothing went wrong and happily 
return default values and show lovely green lights on the instrument 
panel. The right thing is to immediately inform the pilot that something 
went wrong and INSTANTLY SHUT THE BAD SYSTEM DOWN before it does 
something really, really bad, because now it is in an unknown state. The 
pilot then follows the procedure he's trained to, such as engage the backup.

A null pointer exception fits right in with that philosophy.

You could think of null exceptions like pain - sure it's unpleasant, but 
people who feel no pain constantly injure themselves and don't live very 
long. When I went to the dentist as a kid for the first time, he shot my 
cheek full of novacaine. After the dental work, I went back to school. I 
found to my amusement that if I chewed on my cheek, it didn't hurt.

Boy was I sorry about that later <g>.