Union types

May 7, 2020 · View on GitHub

Enumerations

A simple union type allows us to define an enumeration, like this

type Direction = North | East | South | West

In the above example we've defined the type Direction, but we've also created and defined the values North, East, South and West. If we put this code into UnionTypes.elm then we can use them like this in the REPL:

> import UnionTypes exposing (..)
> East
East : Direction
>

You can see that East is a value, and it has type Direction.

Here's an example of using this

describe : Direction -> String
describe dir =
  case dir of
    North -> "Cold"
    East -> "Arid"
    South -> "Hot"
    West -> "Humid"

Note that the code above won't compile if we miss out one of the directions, because the compiler expects an exhaustive match.

More elaborate union types

Union types also allow us to use more complex constructors. The Character type defined next is a simple enumeration, but the Card type is a mix of tagged Value and tagged String types.

type Character = Ace | King | Queen | Jack
type Value = Pips Int | Name Character
type Card =
  Heart Value
  | Diamond Value
  | Club Value
  | Spade Value
  | Joker String

d10 = Diamond (Pips 10)
hking = Heart (Name King)
joker1 = Joker "Laughing Jeremy"

Let's look at the values' types:

> import UnionTypes exposing (..)
> d10
Diamond (Pips 10) : Card
> hking
Heart (Name King) : Card
> joker1
Joker ("Laughing Jeremy") : Card
>

Union types must create new tags

For a union type to be valid, all the types in the union must be creating a new type. So we can't do this, for example:

type Counter = Int | { name : String, c : Int}

But we can do this if AnonCounter and NamedCounter are entirely new and haven't been defined already:

type Counter = AnonCounter Int | NamedCounter { name : String, c : Int }

Both AnonCounter and NamedCounter act as tags for the types that follow.