Encodable decoder.swift

extension Encodable {
  // This lets any encodable encode itself into a container.
  fileprivate func encode(to container: inout SingleValueEncodingContainer) throws {
    try container.encode(self)
  }
}

// This wraps an encodable so it can be encoded. It uses the above extension so that the 
// we don't have to know the encodable's type.
struct AnyEncodable : Encodable {
  var value: Encodable
  init(_ value: Encodable) {
    self.value = value
  }
  func encode(to encoder: Encoder) throws {
    var container = encoder.singleValueContainer()
    try value.encode(to: &container)
  }
}

// Usage example, enum with an Encodable associated value.
enum A {
    case b
    case c(Encodable)
}

let b = A.b
let c = A.c(123)

if case A.c(let encodable) = c {
    let result = try? JSONEncoder().encode(AnyEncodable(encodable))
    String(data: result!, encoding: .utf8) // -> "123"
}