Union Types

Many languages have trouble expressing data with weird shapes. They give you a small set of built-in types, and you have to represent everything with them. So you often find yourself using null or booleans or strings to encode details in a way that is quite error prone.

Elm's union types let you represent complex data much more naturally. We will go through a couple concrete examples to build some intuition about how and when to use union types.

Note: Union types are sometimes called tagged unions. Some communities call them ADTs.

Filtering a Todo List

Problem: We are creating a todo list full of tasks. We want to have three views: show all tasks, show only active tasks, and show only completed tasks. How do we represent which of these three states we are in?

Whenever you have weird shaped data in Elm, you want to reach for a union type. In this case, we would create a type Visibility that has three possible values:

> type Visibility = All | Active | Completed

> All
All : Visibility

> Active
Active : Visibility

> Completed
Completed : Visibility

Now that we have these three cases defined, we want to create a function keep that will properly filter our tasks. It should work like this:

type alias Task = { task : String, complete : Bool }

buy : Task
buy =
  { task = "Buy milk", complete = True }

drink : Task
drink =
  { task = "Drink milk", complete = False }

tasks : List Task
tasks =
  [ buy, drink ]

-- keep : Visibility -> List Task -> List Task

-- keep All tasks == [buy,drink]
-- keep Active tasks == [drink]
-- keep Complete tasks == [buy]

So the keep function needs to look at its first argument, and depending on what it is, filter the list in various ways. We use a case expression to do this. It is like an if on steroids:

keep : Visibility -> List Task -> List Task
keep visibility tasks =
  case visibility of
    All ->

    Active ->
      List.filter (\task -> not task.complete) tasks

    Completed ->
      List.filter (\task -> task.complete) tasks

The case is saying, look at the structure of visibility. If it is All, just give back all the tasks. If it is Active, keep only the tasks that are not complete. If it is Completed, keep only the tasks that are complete.

The cool thing about case expressions is that all the branches are checked by the compiler. This has some nice benefits:

  1. If you mistype Compleet by accident, you get a hint about the typo.
  2. If you forget to handle a case, the compiler will figure it out and tell you.

So say you want to add Recent as a fourth possible Visibility value. The compiler will find all the case expressions in your code that work with Visibility values and remind you to handle the new possibility! This means you can change and extend Visibility without the risk of silently creating bugs in existing code.

Exercise: Imagine how you would solve this same problem in JavaScript. Three strings? A boolean that can be null? What would the definition of keep look like? What sort of tests would you want to write to make sure adding new code later was safe.

Anonymous Users

Problem: We have a chat room where people can post whatever they want. Some users are logged in and some are anonymous. How should we represent a user?

Again, whenever there is weird shaped data, you want to reach for a union type. For this case, we want one where users are either anonymous or named:

> type User = Anonymous | Named String

> Anonymous
Anonymous : User

> Named
<function> : String -> User

> Named "AzureDiamond"
Named "AzureDiamond" : User

> Named "abraham-lincoln"
Named "abraham-lincoln" : User

So creating the type User also created constructors named Anonymous and Named. If you want to create a User you must use one of these two constructors. This guarantees that all the possible User values are things like:

  Named "AzureDiamond"
  Named "abraham-lincoln"
  Named "catface420"
  Named "Tom"

Now that we have a representation of a user, lets say we want to get a photo of them to show next to their posts. Again, we need to use a case expression to work with our User type:

userPhoto : User -> String
userPhoto user =
  case user of
    Anonymous ->

    Named name ->
      "users/" ++ name ++ ".png"

There are two possible cases when we have a User. If they are Anonymous we show a dummy picture. If they are Named we construct the URL of their photo. This case is slightly fancier than the one we saw before. Notice that the second branch has a lower case variable name. This means that when we see a value like Named "AzureDiamond", the name variable will be bound to "AzureDiamond" so we can do other things with it. This is called pattern matching.

Now imagine we have a bunch of users in a chat room and we want to show their pictures.

activeUsers : List User
activeUsers =
  [ Anonymous, Named "catface420", Named "AzureDiamond", Anonymous ]

photos : List String
photos =
  List.map userPhoto activeUsers

-- [ "anon.png", "users/catface420.png", "users/AzureDiamond.png", "anon.png" ]

The nice thing about creating a type like User is that no one in your whole codebase can ever "forget" that some users may be anonymous. Anyone who can get a hold of a User needs to use a case to get any information out of it, and the compiler guarantees every case and handles all possible scenarios!

Exercise: Think about how you would solve this problem in some other language. A string where empty string means they are anonymous? A string that can be null? How much testing would you want to do to make sure that everyone handles these special cases correctly?

Widget Dashboard

Problem: You are creating a dashboard with three different kinds of widgets. One shows recent log data, one shows time plots, and one shows scatter plots. How do you represent a widget?

Alright, we are getting a bit fancier now. In Elm, you want to start by solving each case individually. (As you get more experience, you will see that Elm wants you to build programs out of small, reusable parts. It is weird.) So I would create representations for each of our three scenarios, along with view functions to actually turn them into HTML or SVG or whatever:

type alias LogsInfo =
  { logs : List String

type alias TimeInfo =
  { events : List (Time, Float)
  , yAxis : String

type alias ScatterInfo =
  { points : List (Float, Float)
  , xAxis : String
  , yAxis : String

-- viewLogs : LogsInfo -> Html msg
-- viewTime : TimeInfo -> Html msg
-- viewScatter : ScatterInfo -> Html msg

At this point, you have created all the helper functions needed to work with these three cases totally independent from each other. Someone can come along later and say, "I need a nice way to show scatter plots" and use just that part of the code.

So the question is really: how do I put these three standalone things together for my particular scenario?

Again, union types are there to put together a bunch of different types!

> type Widget = Logs LogsInfo | TimePlot TimeInfo | ScatterPlot ScatterInfo

> Logs
<function> : LogsInfo -> Widget

> TimePlot
<function> : TimeInfo -> Widget

> ScatterPlot
<function> : ScatterInfo -> Widget

So we created a Widget type that can only be created with these constructor functions. You can think of these constructors as tagging the data so we can tell it apart at runtime. Now we can write something to render a widget like this:

view : Widget -> Html msg
view widget =
  case widget of
    Logs info ->
      viewLogs info

    TimePlot info ->
      viewTime info

    ScatterPlot info ->
      viewScatter info

One nice thing about this approach is that there is no mystery about what kind of widgets are supported. There are exactly three. If someone wants to add a fourth, they modify the Widget type. This means you can never be surprised by the data you get, even if someone on a different team is messing with your code.


  • Solve each subproblem first.
  • Use union types to put together all the solutions.
  • Creating a union type generates a bunch of constructors.
  • These constuctors tag data so that we can differentiate it at runtime.
  • A case expression lets us tear data apart based on these tags.

The same strategies can be used if you are making a game and have a bunch of different bad guys. Goombas should update one way, but Koopa Troopas do something totally different. Solve each problem independently, and then use a union type to put them all together.

Linked Lists

Problem: You are stuck on a bus speeding down the highway. If the bus slows down, it will blow up. The only way to save yourself and everyone on the bus is to reimplement linked lists in Elm. HURRY, WE ARE RUNNING OUT OF GAS!

Yeah, yeah, the problem is contrived this time, but it is important to see some of the more advanced things you can do with union types!

A linked list is a sequence of values. If you are looking at a linked list, it is either empty or it is a value and more list. That list is either empty or is a value and more list. etc. This intuitive definition works pretty directly in Elm. Let's see it for lists of integers:

> type IntList = Empty | Node Int IntList

> Empty
Empty : IntList

> Node
<function> : Int -> IntList -> IntList

> Node 42 Empty
Node 42 Empty : IntList

> Node 64 (Node 128 Empty)
Node 64 (Node 128 Empty) : IntList

Now we did two new things here:

  1. The Node constructor takes two arguments instead of one. This is fine. In fact, you can have them take as many arguments as you want.
  2. Our union type is recursive. An IntList may hold another IntList. Again, this is fine if you are using union types.

The nice thing about our IntList type is that now we can only build valid linked lists. Every linked list needs to start with Empty and the only way to add a new value is with Node.

It is equally nice to work with. Let's say we want to compute the sum of all of the numbers in a list. Just like with any other union type, we need to use a case and handle all possible scenarios:

sum : IntList -> Int
sum numbers =
  case numbers of
    Empty ->

    Node n remainingNumbers ->
      n + sum remainingNumbers

If we get an Empty value, the sum is 0. If we have a Node we add the first element to the sum of all the remaining ones. So an expression like (sum (Node 1 (Node 2 (Node 3 Empty)))) is evaluated like this:

  sum (Node 1 (Node 2 (Node 3 Empty)))
  1 + sum (Node 2 (Node 3 Empty))
  1 + (2 + sum (Node 3 Empty))
  1 + (2 + (3 + sum Empty))
  1 + (2 + (3 + 0))
  1 + (2 + 3)
  1 + 5

On each line, we see one evaluation step. When we call sum it transforms the list based on whether it is looking at a Node or an Empty value.

Note: This is the first recursive function we have written together! Notice that sum calls itself to get the sum. It can be tricky to get into the mindset of writing recursive functions, so I wanted to share one weird trick. Pretend you are already done.

I always start with a case and all of the branches listed but not filled in. From there, I solve each branch one at a time, pretending that nothing else exists. So with sum I'd look at Empty -> and say, an empty list has to sum to zero. Then I'd look at the Node n remainingNumbers -> branch and think, well, I know I have a number, a list, and a sum function that definitely already exists and totally works. I can just use that and add a number to it!

Generic Data Structures

Problem: The last section showed linked lists that only worked for integers. That is pretty lame. How can we make linked lists that hold any kind of value?

Everything is going to be pretty much the same, except we are going to introduce a type variable in our definition of lists:

> type List a = Empty | Node a (List a)

> Empty
Empty : List a

> Node
<function> : a -> List a -> List a

> Node "hi" Empty
Node "hi" Empty : List String

> Node 1.618 (Node 6.283 Empty)
Node 1.618 (Node 6.283 Empty) : List Float

The fancy part comes in the Node constructor. Instead of pinning the data to Int and IntList, we say that it can hold a and List a. Basically, you can add a value as long as it is the same type of value as everything else in the list.

Everything else is the same. You pattern match on lists with case and you write recursive functions. The only difference is that our lists can hold anything now!

Exercise: This is exactly how the List type in Elm works, so take a look at the List library and see if you can implement some of those functions yourself.

Additional Examples

We have seen a couple scenarios, but the best way to get more comfortable is to use union types more! So here are two examples that are kind of fun.

Binary Trees

Binary trees are almost exactly the same as linked lists:

> type Tree a = Empty | Node a (Tree a) (Tree a)

> Node
<function> : a -> Tree a -> Tree a -> Tree a

> Node "hi" Empty Empty
Node "hi" Empty Empty : Tree String

A tree is either empty or it is a node with a value and two children. Check out this example for more info on this. If you can do all of the exercises at the end of that link, consider yourself a capable user of union types!


We can even model a programming language as data if we want to go really crazy! In this case, it is one that only deals with Boolean algebra:

type Boolean
    = T
    | F
    | Not Boolean
    | And Boolean Boolean
    | Or Boolean Boolean

true = Or T F
false = And T (Not T)

Once we have modeled the possible values we can define functions like eval which evaluates any Boolean to True or False. See this example for more about representing boolean expressions.

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