Records

A record such as %{name: String, age: Integer} needs no declaration. It emits a BEAM map with atom keys. The field set is exact: extra fields are a different record type. A record is accepted as Map<K, V> when its atom keys and field values fit K and V. Build a smaller record by writing the fields you want.

mod records

pub fn birthday(user: %{name: String, age: Integer}) -> %{name: String, age: Integer} {
  %{user | age: user.age + 1}
}

pub fn origin() -> %{x: Integer, y: Integer} {
  %{x: 0, y: 0}
}

pub fn name_of(user: %{name: String, age: Integer}) -> %{name: String} {
  %{name: user.name}
}

There is no implicit width subtyping. A literal with an extra field is a different type, and a missing field is a type error:

pub fn extra() -> %{name: String, age: Integer} {
  %{name: "ada", age: 36, admin: true}
}