What would we do without NULL?

Question

I once read that having nullable types is an absolute evil. I believe it was in an article written by the very person who created them(in Ada?) I believe this is the article

Answer 1

What would we do without NULL? Invent it! :-) You don't have to be a rocket scientist to use 0 if you are looking for an inband pointer value to express actually not a pointer.

Answer 2

Haskell is a powerful language that doesn't have the concept of nullity. Basically, every variable must be initialized to a non-null value. If you want to represent an "optional" variable (the variable may have a value but it may not), you can use a special "Maybe" type.

It's easier to implement this system in Haskell than C# because data is immutable in Haskell so it doesn't really make sense to have a null reference that you later populate. However, in C#, the last link in a linked list may have a null pointer to the next link, which is populated when the list expands. I don't know what a procedural language without null types would look like.

Also, note that many people above seem to be suggesting replacing nulls with type-specific logical "nothing" values (999-999-9999, "NULL", etc.). These values don't really solve anything because the problem people have with nulls is that they are a special case but people forget to code for the special case. With the type-specific logical nothing values, people STILL forget to code for the special case, yet they avoid errors that catch this mistake, which is a bad thing.

Answer 3

Instead of outright declaring that nullable types are evil, I would posit: most languages graft nullability onto entire kinds of types, when the two concepts should really be orthogonal.

For example, all non-primitive Java types (and all C# reference types) are nullable. Why? We can go back & forth, but ultimately I'll bet the answer comes down to "it was easy". There's nothing intrinsic to the Java language that demands widespread nullability. C++ references offered a fine example of how to exorcise nulls at the compiler level. Of course, C++ has a lot more ugly syntax that Java was explicitly trying to curtail, so some good features ended up on the cutting floor alongside the bad.

Nullable value types in C# 2.0 offered a step in the right direction -- decoupling nullability from unrelated type semantics, or worse, CLR implementation details -- but it's still missing a way to do the opposite with reference types. (Code contracts are great & all, but they're not embedded in the type system the way we're discussing here.)

Plenty of functional or otherwise obscure languages got these concepts "straight" from the beginning...but if they were in widespread use, we wouldn't be having this discussion...

To answer your question: banning nulls from a modern language, wholesale, would be just as foolish as the so-called "billion dollar mistake." There are valid programming constructs where nulls are nice to have: optional parameters, any sort of default/fallback calculation where the coalesce operator leads to concise code, interaction with relational databases, etc. Forcing yourself to use sentinel values, NaN, etc would be a "cure" far worse than the disease.

That said, I'll tentatively agree with the sentiment expressed in the quote, so long as I may elaborate to fit my own experience:

1. the # of situations where nulls are desirable is smaller than most people think
2. once you introduce nulls into a library or codepath, it's much harder to get rid of them than it was to add them. (so don't let junior programmers do it on a whim!)
3. nullable bugs scale with variable lifetime
4. correlary to #3: crash early

Answer 4

We use either

1. Discriminators. An extra attribute or flag or indicator that says that a value is "null" and must be ignored.

2. Domain-Specific Nulls. A specific value -- within the allowed domain -- that is interpreted as "ignore this value". For example, a social security number of 999-99-9999 could be a domain-specific null value that says the SSN is either unknown or not applicable.

Answer 5

We'd use option types for the (very) few places where allowing a null value is actually desirable, and we'd have a lot less obscure bugs since any object reference would be guaranteed to point to a valid instance of the appropriate type.

Answer 6

I think you are referring to this talk: "Null References: The billion dollar mistake"