EnumDictionary

EnumDictionary.swift

struct EnumDictionary<Key: Hashable & CaseIterable, Value> {
    private let dictionary: [Key: Value]
    
    init(_ dictionary: [Key: Value]) {
        for key in Key.allCases {
            assert(dictionary[key] != nil, "All enum cases must be assigned a value.")
        }
        
        self.dictionary = dictionary
    }
    
    subscript(_ key: Key) -> Value {
        return dictionary[key]!
    }
}

extension EnumDictionary: ExpressibleByDictionaryLiteral {
    init(dictionaryLiteral elements: (Key, Value)...) {
        var dictionary = [Key: Value]()
        
        elements.forEach {
            dictionary[$0.0] = $0.1
        }
        
        self.init(dictionary)
    }
}

enum Thing: CaseIterable {
    case this
    case that
    case theOther
}

let x: [Thing: Int] = [.this: 5, .that: 42, .theOther: -8]
let y = EnumDictionary(x)

y[.this] // 5 (Non-optional, since we've already determined every enum key has a value)

let z: EnumDictionary = [Thing.this: "A", .that: "B", .theOther: "C"]
z[.theOther] // C (Non-optional)