Level 27. Structs: Build Your Own Types

Tuples are great for bundling a few values together, but what if you have three, four, or more fields? And what if you can’t remember whether .0 is the name or the age? That’s where structs come in.

A struct is like designing your own custom box with labelled compartments.

Defining a struct

Use the struct keyword to create a new type:

struct Pet { name: string, species: string, age: int }

This creates a new type called Pet with three named fields.

Making a struct value

Fill in the fields by name:

struct Pet { name: string, species: string, age: int }

fun main() {
  let buddy = Pet { name: "Daisy", species: "cat", age: 3 }
  println(buddy)
}

Output: Pet(name: Daisy, species: cat, age: 3)

Reading fields

Use a dot and the field name, this is much clearer than .0 and .1!

struct Pet { name: string, species: string, age: int }

fun main() {
  let buddy = Pet { name: "Daisy", species: "cat", age: 3 }
  println(buddy.name)     // Daisy
  println(buddy.species)  // cat
  println(buddy.age)      // 3
}

Structs as function parameters

You can pass structs to functions just like any other value:

struct Pet { name: string, species: string, age: int }

fun introduce(p: Pet) : string =>
  "{p.name} is a {p.age}-year-old {p.species}"

fun main() {
  let daisy = Pet { name: "Daisy", species: "cat", age: 3 }
  println(introduce(daisy))
}

Output: Daisy is a 3-year-old cat

Tuples vs Structs

Use tuples when it’s obvious what the values mean (like (x, y) coordinates). Use structs when you want names that explain the data.

🎯 Try it: Create a Player struct with name: string and score: int. Write a function level_up(p: Player) : string that prints "{p.name} reached score {p.score}!".

Updating a struct

Structs can’t change (they’re immutable), but you can make a copy with some fields changed using ...:

struct Pet { name: string, species: string, age: int }

fun main() {
  let daisy = Pet { name: "Daisy", species: "cat", age: 3 }
  let older = Pet { ...daisy, age: 4 }   // everything else stays the same!
  println(older)
}

Think of it like photocopying a form and writing over just one field.

Taking structs apart in match

Remember match? You can use it to look inside a struct, like opening a box and checking what’s in each compartment:

struct Point { x: int, y: int }

fun describe(p: Point) : string => match p {
  Point { x: 0, y: 0 } => "origin",
  Point { x, y: 0 }    => "on x-axis",
  Point { x, y }       => "({x}, {y})"
}

• Point { x, y } — opens the box and names each field
• Point { x: 0, y: 0 } — only matches when both fields are zero
• Point { x } — you can skip fields you don’t care about

🎯 Try it: Create a Pet struct with name, species, and age. Write a describe function that uses match to print different messages for kittens (age 0) vs older pets!