Struct

Gleam's struct types are named collections of keys and values. They are similar to objects in object oriented languages, though they don't have methods.

Structs are defined with the struct keyword.

pub struct Cat {
  name: String
  cuteness: Int
}

Here we have defined a struct called Cat which has two fields: A name field which is a String, and a cuteness field which is an Int.

The pub keyword has been used to make this struct usable from other modules.

Once defined the struct type can be used in functions:

fn cats() {
  // Struct fields can be given in any order
  let cat1 = Cat(name: "Nubi", cuteness: 2001)
  let cat2 = Cat(cuteness: 1805, name: "Biffy")

  // Alternatively fields can be given without labels
  let cat3 = Cat("Ginny", 1950)

  [cat1, cat2, cat3]
}

Destructuring structs

To extract values from structs we can pattern match on them with let or case.

let Cat(name: name1, cuteness: cuteness1) = cat1

name1     // => "Nubi"
cuteness1 // => 2001

We can also pattern match using positional arguments:

let Cat(name2, cuteness2) = cat2

name2     // => "Biffy"
cuteness2 // => 1805

Generic structs

Structs types can be parameterised so the same struct can be constructed with different contained types.

pub struct Box(a) {
  tag: String,
  contents: a, // The type of this field is injected when constructed
}

fn run() {
  Box(tag: "one", contents: 1.0) // the type is Box(Float)
  Box(tag: "two", contents: "2") // the type is Box(String)
}

Commonly used structs

Pair(a, b)

There may be times when you want to move two values together, or return them both from a function. To save you from having to define a new struct for this the Gleam standard library implements a Pair type in the gleam/pair module which can contain any two values.

import gleam/pair

fn run() {
  pair.Pair("ok", 100) // type is Pair(String, Int)
  pair.Pair(1.01, [1]) // type is Pair(Float, List(Int))
}

Erlang interop

At runtime Gleam structs are Erlang tuples. All information about the names of the keys is erased at runtime.