Union types

In Elm, Union Types are used for many things as they are incredibly flexible. A union type looks like:

type Answer = Yes | No

Answer can be either Yes or No. Union types are useful for making our code more generic. For example a function that is declared like this:

respond : Answer -> String
respond answer =
    ...

Can either take Yes or No as the first argument e.g. respond Yes is a valid call.

Union types are also commonly called tags in Elm.

Payload

Union types can have associated information with them:

type Answer = Yes | No | Other String

In this case, the tag Other will have an associated string. You could call respond like this:

respond (Other "Hello")

You need the parenthesis otherwise Elm will interpret this as passing two arguments to respond.

As constructor functions

Note how we add a payload to Other:

Other "Hello"

This is just like a function call where Other is the function. Union types behave just like functions. For example given a type:

type Answer = Message Int String

You will create a Message tag by:

Message 1 "Hello"

You can do partial application just like any other function. These are commonly called constructors because you can use this to construct complete types, i.e. use Message as a function to construct (Message 1 "Hello").

Nesting

It is very common to 'nest' one union type in another.

type OtherAnswer = DontKnow | Perhaps | Undecided

type Answer = Yes | No | Other OtherAnswer

Then you can pass this to our respond function (which expect Answer) like this:

respond (Other Perhaps)

Type variables

It is also possible to use type variables or stand-ins:

type Answer a = Yes | No | Other a

This is an Answer that can be used with different types, e.g. Int, String.

For example, respond could look like this:

respond : Answer Int -> String
respond answer =
    ...

Here we are saying that the a stand-in should be of type Int by using the Answer Int signature.

So later we will be able to call respond with:

respond (Other 123)

But respond (Other "Hello") would fail because respond expects an integer in place of a.

A common use

One typical use of union types is passing around values in our program where the value can be one of a known set of possible values.

For example, in a typical web application, we can trigger messages to perform actions, e.g. load users, add user, delete user, etc. Some of these messages would have a payload.

It is common to use union types for this:

type Msg
    = LoadUsers
    | AddUser
    | EditUser UserId
    ...

There is a lot more about Union types. If interested read more about this here.