Cacheable.swift

protocol ValueContainer {
  associatedtype Value
  var value: Value { get }
  init(_ value: Value)
}

extension ValueContainer where Self : Equatable, Value : Equatable {
  static func == (lhs: Self, rhs: Self) -> Bool {
    return lhs.value == rhs.value
  }
}

protocol Cacheable : Equatable {
  associatedtype Container : (
    ValueContainer & Hashable
  ) where Container.Value == Self
}