RawRepresentable+Codable.swift

enum Foo: String, Codable {
    case a
    case b
    case c
}

extension RawRepresentable where RawValue == String, Self: Codable {
    init(from decoder: Decoder) throws {
        var container = try decoder.unkeyedContainer()
        let rawValue = try container.decode(String.self)
        do {
            self.init(rawValue: rawValue)!
        }
    }

    func encode(to encoder: Encoder) throws {
        var container = encoder.unkeyedContainer()
        try container.encode(rawValue)
    }
}

let encoded = try? JSONEncoder().encode(Foo.a)
let decoded = try? JSONDecoder().decode(Foo.self, from: encoded!)

struct Hoge: Codable {
    let foo: Foo?

    enum CodingKeys: String, CodingKey {
        case foo
    }
}

enum Foo: String, Codable {
    case a
    case b
    case c
}

struct Hoge: Codable {
    let foo: Foo?
}

let response = """
{
"foo": "d"
}
"""

let json = response.data(using: .utf8)!
do {
    let decoded = try JSONDecoder().decode(Hoge.self, from: json)
    print("decoded value: \(decoded)")
} catch {
    print("error: \(error.localizedDescription)")
    // error: The data couldn’t be read because it isn’t in the correct format.
}