Swift Type Alias

typealias AliasName = ExistingType

typealias Integer = Int

let aliasedValue: Integer = 42
let standardValue: Int = aliasedValue // Compiles without type conversion

import Foundation

typealias CompletionHandler = (Data?, URLResponse?, Error?) -> Void

// The alias can now be used as a type annotation
var onComplete: CompletionHandler?

// Partially applied generic type
typealias StringDictionary<Value> = Dictionary<String, Value>

let metadata: StringDictionary<Int> = ["count": 10]

typealias HashableAndComparable = Hashable & Comparable

struct CustomKey: HashableAndComparable {
    let id: Int
    
    // Swift synthesizes Hashable and Equatable requirements for the 'id' property.
    // Comparable requires an explicit implementation of the '<' operator.
    static func < (lhs: CustomKey, rhs: CustomKey) -> Bool {
        return lhs.id < rhs.id
    }
}

typealias Coordinates = (x: Double, y: Double, z: Double)

let origin: Coordinates = (x: 0.0, y: 0.0, z: 0.0)

internal struct InternalType {}
public struct PublicType {}

// Compiler Error: Type alias cannot be declared public because its underlying type is internal
public typealias PublicAlias = InternalType 

// Valid: Type alias has a lower or equal access level compared to its underlying type
private typealias PrivateAlias = PublicType