Chris Martin

Null as a sort of bottom

“It uses the faculty of the imagination. But that does not mean making things up. It is a form of seeing.”

“Not real traveling, then,” said Lyra, “Just pretend...”

“No,” said Xaphania, “nothing like pretend. Pretending is easy. This way is hard, but much truer.”

— Philip Pullman, The Amber Spyglass

I struggle to talk about programming languages that aren’t based in some mathematical formalism. I have to choose ways of reasoning about them. I find it’s often helpful to choose to mentally declare null to be a kind of bottom when reasoning about languages that don’t provide their own sufficient system of reason.

I’m aware that this is blasphemy. The official Haskell party line goes something like this:

Novice: “So is bottom sort of like null?”

Master: “No, they couldn’t be more different! Null is a value; bottom is not. There is one null value; there are no bottom values.”

The real bottom

Here are some bona fide exemplary bottoms.

Haskell:

bottom :: forall a. a
bottom =
  let x = x
  in  x

Scala:

def bottom[A]: A = {
  while (true) {}
  ???
}

Non-terminating evaluation. The infinite loop. That possibility lurking in all Turing-complete systems, that exciting consequence of the undecidability of the Entscheidungsproblem. It is a fascinating and irksome reality. If you understand what this is and why it must be, be proud of your knowledge. But keep going.

The real bottom is annoying.

Here are some partial functions, defined using the bottoms I showed above.

Haskell:

fromJust :: forall a. Maybe a -> a
fromJust =
  \case
    Just a  -> a
    Nothing -> let x = x
               in  x

Scala:

def fromSome[A](o: Option[A]): A =
  o match {
    case Some(a) => a
    case None    => while (true) {}
                    ???
  }

This approach works, but it is a harsh way to treat the poor evaluator. Remember there’s no general way to know that you’ve hit nonterminating recursion. So all we can do with fromJust Nothing when executing this sort of program is spin indefinitely trying to evaluate it.

Tagged bottoms

Once you’re in a situation with nonterminating recursion, it’s impossible to either finish evaluating or know that you’ll never finish evaluating. It’s a dreadful state, so it seems unnecessarily cruel to ever intentionally produce it for the evaluator to either puzzle out or chew on forever. If we know that an expression will be bottom, let’s state that explicitly in our code.

fromJust =
  \case
    Just a  -> a
    Nothing -> undefined

fromJust Nothing is still bottom — it’s still an expression of type a that does not evaluate to any value of type a — but we’ve added a little metadata that lets us fail quickly when attempting to evaluate it.

We can go further than that, though, and provide not only a proclamation of bottomness, but an explanation for why the expression is bottom.

fromJust =
  \case
    Just a  -> a
    Nothing -> error "fromJust Nothing"

Don’t be fooled: the expression fromJust Nothing is still bottom. But when that expression is evaluated, the fact that it is bottom is known, and that fact comes with some additional information.

“Tagged bottom” is a term I just made up, for lack of a better phrase. It refers to the result of determining that an expression cannot be evaluated, including some justification if available, like “this expression can’t be evaluated because it includes a division by zero”. In the case of undefined, the tag merely asserts that the expression is bottom.

Bottom is nothing. It has no values. It is the epitome of emptiness. — These are half truths. In Haskell, bottoms can carry information, and we can even inspect them (only in IO):

try   :: Exception e => IO a -> IO (Either e a)
catch :: Exception e => IO a -> (e -> IO a) -> IO a

Non-null reasoning is morally correct

Officially, null is a value that inhabits all Nullable types in Scala (which is most of them).

But we do not operate in this mindset on a regular basis. Like bottom, we ignore it, and allow it to propagate upward — often implicitly converted to another form of bottom:

throw new NullPointerException()

Consider this Scala function that adds one to an integer:

def succ1(x: BigInt): BigInt =
  x + 1

Technically, this function is partial. Because when x is null, it throws NullPointerException.

You could diligently handle nulls throughout your code:

def succ2(x: BigInt): BigInt =
  if (x == null) null
  else           x + 1

Or you can, as we did in the first version, choose to interpret null as a bottom value — not an inhabitant of any type, but merely one of the many forms of tagged bottom. In that case, we do not have to consider succ1 to be a partial function, because null no longer belongs to its domain.

When we choose not to ignore null — when we check if (x == null) or use try/catch — we are writing meta-code that reasons about whether our program has been reasoning correctly.

The dark path

From a software design perspective, bottoms are characterized by their use as a last resort, when writing a total function is not possible or would be particularly inconvenient.

Sometimes the temptation to use bottom tags in unextraordinary circumstances leads us to dark places. We can do this by overusing throw and catch in Haskell, and we can do this by overusing null as idiomatic Java does.

I do not believe null was Java’s billion-dollar mistake. It is no more inherently harmful than ??? in Scala or undefined in Haskell. The billion-dollar mistakes were more serious fundamental flaws — notably, its omission of sum types and pattern matching — that drive Java’s users to abuse tagged bottoms for lack of sufficient means to express themselves properly within the normal confines of the type system. We can see from Scala that, with this limitation removed, programmers typically choose to relegate null-tagged bottom to a footnote along with the rest of the bottoms.