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# 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.