Custom Types

Defining Custom Types

Custom types in Gleam are sum types (also called algebraic data types or tagged unions). They let you define a type that can be one of several variants:

pub type Season {
  Spring
  Summer
  Autumn
  Winter
}

Each variant is a constructor. You create values by calling the constructor:

let current = Summer

Variants with Data

Variants can carry data:

pub type Shape {
  Circle(radius: Float)
  Rectangle(width: Float, height: Float)
  Triangle(base: Float, height: Float)
}

You create them by passing arguments:

let s = Circle(radius: 5.0)
let r = Rectangle(width: 10.0, height: 20.0)

Matching on Custom Types

Use case to handle each variant:

fn describe(shape: Shape) -> String {
  case shape {
    Circle(radius: r) -> "circle with radius " <> float.to_string(r)
    Rectangle(width: w, height: h) -> "rectangle " <> float.to_string(w) <> "x" <> float.to_string(h)
    Triangle(..) -> "triangle"
  }
}

The .. pattern ignores all fields of a variant.

The Power of Sum Types

Sum types make impossible states impossible. Consider representing a traffic light:

pub type Light {
  Red
  Yellow
  Green
}

There is no way to create an invalid light color. The type system guarantees correctness.

Your Task

Define a Direction type with four variants: North, South, East, West. Write a function opposite that returns the opposite direction. Print the opposite of each direction.