Dictionary Types

Dictionaries have the form {K: V}, where K is the type of the key, and V is the type of the value. For example, a dictionary with Int keys and Bool values has type {Int: Bool}.

// Declare a constant that has type `{Int: Bool}`,
// a dictionary mapping integers to booleans.
//
let booleans = {
    1: true,
    0: false
}

// Declare a constant that has type `{Bool: Int}`,
// a dictionary mapping booleans to integers.
//
let integers = {
    true: 1,
    false: 0
}

Dictionary types are covariant in their key and value types. For example, [Int: String] is a subtype of [AnyStruct: String] and also a subtype of [Int: AnyStruct]. This is safe because dictionaries are value types and not reference types.