Structs

A struct groups several named fields into one value. Use them when you have a few related pieces of data that belong together — coordinates, user records, configuration, anything where naming the fields makes the code clearer.

type Person = {
    first_name: string
    last_name: string
    age: int
}

Construct an instance by calling the type name like a function. Arguments go in field order:

let frank = Person("Frank", "Smith", 34)

Read fields with .:

let fullname = frank.first_name .. " " .. frank.last_name
// "Frank Smith"

And update them the same way:

frank.age = frank.age + 1   // 35

Default field values

A field can have a default value. When constructing the struct, callers can leave any trailing field off:

type Greeter = {
    name: string
    greeting: string = "Hello"
    excited: bool = false
}

extend Greeter {
    fn greet(self) {
        let punct = if self.excited { "!" } else { "." }
        println(self.greeting .. ", " .. self.name .. punct)
    }
}

Greeter("Alice").greet()                         // "Hello, Alice."
Greeter("Bob", "Howdy").greet()                  // "Howdy, Bob."

Named field arguments

You can also pass a field by name, which lets you skip past fields that have defaults:

Greeter("Carol", excited = true).greet()                       // "Hello, Carol!"
Greeter("Dave", greeting = "Hi", excited = true).greet()       // "Hi, Dave!"

Named arguments must come after all positional arguments.

Generic structs

A struct can take a type parameter, so it works with any element type:

type Ref<T> = {
    value: T
}

let int_ref = Ref(42)         // Ref<int>
let str_ref = Ref("hello")    // Ref<string>

See Generics for more.

Adding behavior

To attach methods to a struct, use extend. To make it work with ==, sorting, printing, or other operators, implement an interface for it.