Custom types

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

Custom types are defined with the type keyword.

pub type Cat {
  Cat(name: String, cuteness: Int)
}

Here we have defined a custom type called Cat. Its constructor is called Cat and it has two fields: A name field which is a String, and a cuteness field which is an Int.

The pub keyword makes this type usable from other modules.

Once defined the custom type can be used in functions:

fn cats() {
  // Labelled 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]
}

Multiple constructors

Custom types in Gleam can be defined with multiple constructors, making them a way of modeling data that can be one of a few different variants.

We've already seen a custom type with multiple constructors in the Language Tour - Bool.

Gleam's built-in Bool type is defined like this:

// A Bool is a value that is either `True` or `False`
pub type Bool {
  True
  False
}

It's a simple custom type with constructors that takes no arguments at all! Use it to answer yes/no questions and to indicate whether something is True or False.

The records created by different constructors for a custom type can contain different values. For example a User custom type could have a LoggedIn constructor that creates records with a name, and a Guest constructor which creates records without any contained values.

type User {
  LoggedIn(name: String)  // A logged in user with a name
  Guest                   // A guest user with no details
}

let sara = LoggedIn(name: "Sara")
let rick = LoggedIn(name: "Rick")
let visitor = Guest

Destructuring

When given a custom type record we can pattern match on it to determine which record constructor matches, and to assign names to any contained values.

fn get_name(user) {
  case user {
    LoggedIn(name) -> name
    Guest -> "Guest user"
  }
}

Custom types can also be destructured with a let binding.

type Score {
  Points(Int)
}

let score = Points(50)
let Points(p) = score

p // => 50

Named accessors

If a custom type has only one variant and named fields they can be accessed using .field_name.

For example using the Cat type defined earlier.

let cat = Cat(name: "Nubi", cuteness: 2001)
cat.name // This returns "Nubi"
cat.cuteness // This returns 2001

Generics

Custom types can be be parameterised with other types, making their contents variable.

For example, this Box type is a simple record that holds a single value.

pub type Box(inner_type) {
  Box(inner: inner_type)
}

The type of the field inner is inner_type, which is a parameter of the Box type. If it holds an int the box's type is Box(Int), if it holds a string the box's type is Box(String).

pub fn main() {
  let a = Box(123) // type is Box(Int)
  let b = Box("G") // type is Box(String)
}

Opaque types

At times it may be useful to create a type and make the constructors and fields private so that users of this type can only use the type through publically exported functions.

For example we can create a Counter type which holds an int which can be incremented. We don't want the user to alter the int value other than by incrementing it, so we can make the type opaque to prevent them from being able to do this.

// The type is defined with the opaque keyword
pub opaque type Counter {
  Counter(value: Int)
}

pub fn new() {
  Counter(0)
}

pub fn increment(counter: Counter) {
  Counter(counter.value + 1)
}

Because the Counter type has been marked as opaque it is not possible for code in other modules to construct or pattern match on counter values or access the value field. Instead other modules have to manipulate the opaque type using the exported functions from the module, in this case new and increment.

Record updates

Gleam provides a dedicated syntax for updating some of the fields of a custom type record.

import gleam/option.{Option}

pub type Person {
  Person(
    name: String,
    gender: Option(String),
    shoe_size: Int,
    age: Int,
    is_happy: Bool,
  )
}

pub fn have_birthday(person) {
  // It's this person's birthday, so increment their age and
  // make them happy
  Person(..person, age: person.age + 1, is_happy: True)
}

As Gleam records are immutable the update syntax does not alter the fields in place, instead it created a new record with the values of the initial record with the new values added.

Erlang interop

At runtime custom type records with no contained values become atoms. The atoms are written in snake_case rather than CamelCase so LoggedIn becomes logged_in.

Custom type records with contained values are Erlang records. The Gleam compiler generates an Erlang header file with a record definition for each constructor, for use from Erlang.

// Gleam
Guest
LoggedIn("Kim")

# Elixir
:guest
{:logged_in, "Kim"}

% Erlang
guest,
{logged_in, <<"Kim">>}.

The Gleam Book