1_Newtypable.swift

// This protocol should be in the Standard library
protocol Newtypeable {
  associatedtype Original
  init(value: Original)
  var value: Original { get }
}

// The Standard libary can include useful extensions like this one
extension Newtypeable where Original: Hashable {
  func hash(into hasher: inout Hasher) {
    value.hash(into: &hasher)
  }

  var hashValue: Int {
    return value.hashValue
  }
}

// Similarily, third-party libraries can include a Newtypable extension for their own protocols

2_Newtype_usage.swift

// Declare RecordId using a newtype
newtype RecordId = Int

// Behind the scenes, this generates something like this
struct RecordId: Newtypeable {
  let value: Int
}

// Users can opt-in to the a protocol with a single line
extension RecordId: Hashable {}

// Alternatively, a custom implementation can be provided
extension RecordId: Hashable {
  var hashValue: Int { return 42 }
}

// RecordId can be used as a key in a dictionary
let dictionary: [RecordId: String] = [:]