Nathan Kleyn

The other day, I was asked by somebody why we had Unit in Scala, and what was the difference over just using null. I thought this was a really interesting question — so here’s my response in a slightly longer form!

The Boolean Type

Let’s start from something we know well — the Boolean type! When writing code, we use these all the time. As you know, they can only have two possible values — true or false.

The boolean type is the smallest possible type with which we can represent information. It can show something was true, or false, on or off, yes or no, 1 or 0 — just a single bit is all it has to work with, but it’s enough ¹.

Imagine the following example function:

def doSomething(bool: Boolean): Boolean = ???

We have a function that takes a Boolean and returns a Boolean. We know immediately some things about this function:

• It is going to take either a true or false value.
• It is going to return either a true or false value.
• It cannot take or return any other values.

We know all of these things because the number of possible values for Boolean is limited. This limitation is what allows us to reason about what a function can do just from it’s type — and this is an incredibly useful tool that static-typing gives you.

Parameters and Return Values As Tuples

Remember that function we wrote before?

def doSomething(bool: Boolean): Boolean = ???

We can think about functions in a slightly different way: we can think about their parameters being passed as tuples and them returning tuples:

// Is the same thing as:
val doSomething: (Boolean) => (Boolean) = ???
//               ^ tuple      ^ tuple

This is how a compiler actually sees the functions you wrote — names given to parameters and syntax around showing the return type is just sugar that makes it read better to our human eyes. Really, behind the scenes, it’s more like passing tuples of arguments and receiving tuples of results back.

Keep this tuple thing in mind!

The Unit Type

Sometimes we do things where we don’t care about or can’t get a result — things that may have side-effects, for example. Often we need a way to say “this function returns nothing useful” — how can we do that without wasting space on a boolean if a boolean is the smallest piece of information we have to work with?

Well, it turns out there is a type for saying “I have nothing useful to return” — and that type is called Unit.

Unit is a type that only has a single value (hence the name). This single value is also called “unit” but to differentiate the value from the type, in some languages we write it as ().

Notice something? That looks like a tuple! And indeed, that’s not a mistake — Unit is just a way of saying “an empty tuple”!

Unit is useful because of an important property about the empty tuple — there is only one possible value of it, (). Where, for example, (Boolean) could become (true) or (false), Unit will always be () only. As a result of only having a single value, this means it is entirely useless for actually encoding any information — after all, what information can you impart if you’re only allowed to respond with one thing?

Being able to say “I have no information” is still a useful thing, and so we use Unit and () to do just that:

def doSomething(file: File): Unit = ???

Just from looking at the definition of this function, and as you were able to with the boolean example, you can tell this function must have some side-effect ². We could even use this information to guess what this function does (perhaps it deletes the file, or maybe calls touch?)

Null: The Odd One Out

So now to the question at hand: what makes Unit useful over null?

The issue with null is that it is, basically, a hack! null is a special value that can take the place of any type, but it is not a type itself. This completely breaks our ability to reason using just types. Take for example this function:

def doSomethingpath: Path): File = ???

We might look at this and think “oh cool, maybe it’s opening a File for us?”. Well, if we didn’t have null you might possibly be right³ — but unfortunately it’s completely valid for this function to be implemented thusly:

def doSomethingpath: Path): File = null

What’s wrong about null is that it is a magic value not represented in the type-system, and therefore it limits a lot of our ability to use the types to their full power. This is why most idiomatic Scala eschews the use of null over things like Unit, Option[A], or Either[A, B] — these types tell the full story of what is going on, without hacks, and allow us the full power of deductive reasoning. null in Scala exists purely because of Java compatibility, and you would be wise to avoid it at all costs!