One sentiment I see from people coming from dynamically typed languages (python, ruby) or even Java and C is a general dismissiveness about static typing and compilers. At best, the sentiment is “Oh, well it makes sure my input is an Int, or a Float, or a Bool… That’s cool I guess, but I can do that with TDD”. At worst, static typing is seen as something to fight against – shackles that limit our creativity and bar us from what we really want to do and the beautiful programs that we could write if only the type-checker got out of our way.

Personally, I think of the type-checker (and by extension the compiler, really) not only as a free suite of rigorous tests built from first principles, but as a friend that is nice enough to correct me, the silly human, when I think I’m making sense but I’m really not. Sure, sometimes that friend is a bit dense (ahem Java, ahem) and can’t quite understand what I’m trying to say, but in the case of a language like Scala I find that the compiler is right more often than not. In fact, I had a whole giant wall of text geared up to talk about the value in using Option over null, in using Either instead of exceptions, in capturing values without using Stringly-Typed data… But then Li Haoyi beat me to it with another addition to his wonderful Strategic Scala Style series: Practical Type Safety. I highly recommend reading it (and the rest of the Strategic Scala Style series) before coming back.

Still here? That post covers a lot of what I wanted to say, but I wanted to put some extra emphasis on one particular topic…

ADTs (Algebraic Data Types) are so, so good. You can take a lot of features away in Scala and I could probably get by, but ADTs – Or at least, the closest Scala approximation – are on the short list that you’d have to pry out of my cold dead hands… Along with higher order functions and pattern matching, probably.

So, a quick review… What are Algebraic Data Types? ADTs are so named because their structure can be described in two operations: Product, and Sum.

#### Product Types

Product types are present in one form or another in the vast majority of mainstream programming languages. People may know it as a struct in C, or a record, or a tuple, or a case class in Scala. It’s essentially a way of mashing multiple types together into one type. The reason it is called a product type is because the cardinality of the type (i.e., the set of all possible values for it), is the product of the cardinality of the type’s constituents. Some quick examples:

// Has 2 possible values: A(true), A(false)
case class A(b: Boolean)

// has (2 * 2 = 4) possible values: AA(true, true), AA(true, false), AA(false, true), AA(false, false)
case class AA(b1: Boolean, b2: Boolean)


#### Sum Types

Where product types express “this and that and the other thing”, sum types (also commonly known as union types) express “this or that or the other thing”. Sum types are so named because the cardinality of a sum type is the sum of the cardinality of the type’s consituents. Unfortunately in Scala 2.x, there isn’t direct support for sum types, but they can be roughly approximated with sealed trait. Some examples:

// Probably not how any of this is implemented in the actual standard lib but you get the idea  :)
sealed trait Boolean
final case object True extends Boolean
final case object False extends Boolean

sealed trait Option[T]
final case class Some(x: T) extends Option[T]
final case class None extends Option[_]

sealed trait Either[A, B]
final case class Left[A, B](a: A) extends Either[A, B]
final case class Right[A, B](b: B) extends Either[A, B]


One cheerful note is that proper union type support was announced for Dotty at ScalaDays 2016, so hurrah! *confetti*

### So What?

At first glance, ADTs might seem simplistic – kinda like fancied up named tuples. But they are deceptively powerful, because combining them effectively allows you to encode invariants about your program’s logic and state in the type system, and therefore leverage the compiler to keep you from violating those invariants and writing buggy software.

Which sounds like a bunch of abstract nonsense, so it’s time for a concrete example!

### Modeling Real Life: Wrangling User Lists

Working in the marketing space, it is very common to deal with lists of users. It can be a list of users from a mailing list, a list of website visitors, or a list of people who have downloaded your white paper. In any case, it is a list of users that you have in your possession, and you want to follow them around the internet and serve them ads. Such a list of users might have attributes like an id and a human-readable name. Since most folks aren’t in the business of building exchanges, it might also have a downstream platform to target (e.g., AdWords, or Facebook). It might also have some different states that keep track of whether the list’s meta-data has been sent to the downstream platform, or whether it’s been archived or soft-deleted and when that state transition happened.

So one reasonable implementation at a data structure for such a user list might look something like:

sealed trait ListStatus
final case object Pending extends ListStatus
final case object Active extends ListStatus
final case object Archived extends ListStatus

sealed trait Platform
final case object Facebook extends Platform // And so on and so forth

case class UserList(
id: Long,
name: String,
platform: Platform,
status: ListStatus,
createdTime: Long,
archivedTime: Option[Long],
downstreamId: Option[String] // i.e., a "foreign key" to the list on the downstream platform
)


At first glance, this seems like pretty reasonable code. The various states and platforms are enumerated, and it seems to carry all the information we want with decent naming and so on. At the very least it’s better than carting around a tab-separated string, or something like val list: (Long, String, ListStatus, Long, Option[Long], Option[String]).

But there’s still something a bit off here. In particular, look around the optional fields. With this current model, it is possible to construct an internally inconsistent list:

val list = UserList(
id = 1,
name = "website visitors",
status = Active,
createdTime = System.currentTimeMillis,
archivedTime = None,
downstreamId = None // Wait, what?
)


The example above is internally inconsistent because the list’s active status indicates that it should be onboarded to the downstream platform, but for some reason we do not have a downstream id. There are other “weird” cases like this that are possible but do not make sense semantically – for example, archivedTime = Some(1) with status = Pending.

This data structure is almost trivially simple, but there area already a good number of things that can go wrong, especially once we take outside input and possibly complex business logic into account. And here there’s nothing stopping us from constructing these degenerate cases other than our ability to keep everything in our heads (spotty even at the best of times) and read the code really really carefully every time we work with this particular data structure (good luck).

Another point to consider is what this does to the code that works with the data:

// Send users from our list to downstream platform so we can serve them ads
def onboardUsersToList(list: UserList, users: Seq[User]) {
list.status match {
case Active => {
val platformId = list.downstreamId.getOrElse {
// this should never happen
}

platformClient(list.platform).sendUsers(platformId, users)
}
case _ => throw new IllegalArgumentException("Cannot onboard users for non-active list")
}
}


The above code is a conceptually simple method that takes users from our list and sends them to a downstream platform, e.g. Facebook. It technically does what we want, but there is a lot of clutter introduced by the management of the status. The block under list.downstreamId.getOrElse is particularly bad, since it basically amounts to saying “well we don’t think we’re wrong, but we technically could be wrong, so we have to sprinkle this boilerplate-ish error handling into our business logic”. Apparently, this sort of thing is not that uncommon.

One possible remedy is to refactor UserList to encode some of the constraints into the type:

final case class UserListMetadata(id: Long, name: String, platform: Platform, createdTime: Long)



This lets us rewrite the above method as:

def onboardUsersToList(list: ActiveList, users: Seq[User]) {

At this point, one might protest, “That just makes you push the status validation somewhere else! The first implementation was just bad practice, you could easily have written this simple version with the old type by refactoring to def onboardUsersToList(downstreamId: String, users: Seq[User]).” Yes, this is perfectly true. The validation still has to take place somewhere since the code will presumably be interacting with the outside world. The difference here is that the former implementation can only enforce cleanliness and correctness with malleable things like documentation and best practice guidelines, whereas the latter implementation enforces it by simply refusing to compile until you fix it. The latter implementation also reduces the amount of “unforced” errors since it is impossible to construct an ActiveList object without a downstreamId, whereas it was possible to accidentally create a UserList(status = Active, downstreamId = None) previously.