Simple Encoder

BoolEncoder.swift

import Foundation


class BoolEncoder: SingleValueEncodingContainer {
  var codingPath: [CodingKey]
  private(set) var value: Bool?
  
  init() {
    codingPath = []
  }
  
  func encodeNil() throws {
    value = nil
  }
  
  func encode(_ value: Bool) throws {
    self.value = value
  }
  
  func encode(_ value: String) throws {
    fatalError("\(type(of: self)) only supports bools.")
  }
  
  func encode(_ value: Double) throws {
    fatalError("\(type(of: self)) only supports bools.")
  }
  
  func encode(_ value: Float) throws {
    fatalError("\(type(of: self)) only supports bools.")
  }
  
  func encode(_ value: Int) throws {
    fatalError("\(type(of: self)) only supports bools.")
  }
  
  func encode(_ value: Int8) throws {
    fatalError("\(type(of: self)) only supports bools.")
  }
  
  func encode(_ value: Int16) throws {
    fatalError("\(type(of: self)) only supports bools.")
  }
  
  func encode(_ value: Int32) throws {
    fatalError("\(type(of: self)) only supports bools.")
  }
  
  func encode(_ value: Int64) throws {
    fatalError("\(type(of: self)) only supports bools.")
  }
  
  func encode(_ value: UInt) throws {
    fatalError("\(type(of: self)) only supports bools.")
  }
  
  func encode(_ value: UInt8) throws {
    fatalError("\(type(of: self)) only supports bools.")
  }
  
  func encode(_ value: UInt16) throws {
    fatalError("\(type(of: self)) only supports bools.")
  }
  
  func encode(_ value: UInt32) throws {
    fatalError("\(type(of: self)) only supports bools.")
  }
  
  func encode(_ value: UInt64) throws {
    fatalError("\(type(of: self)) only supports bools.")
  }
  
  func encode<T>(_ value: T) throws where T : Encodable {
    fatalError("\(type(of: self)) only supports bools.")
  }
}

SpanishEncoder.swift

import Foundation

class SpanishEncoder: Encoder, CustomStringConvertible {
  private var container: BoolEncoder
  var description: String {
    switch container.value {
    case true?:
      return "sí"
    case false?:
      return "no"
    case nil:
      return "no sé"
    }
  }
  
  init() {
    container = BoolEncoder()
    codingPath = []
    userInfo = [:]
  }
  
  var codingPath: [CodingKey]
  var userInfo: [CodingUserInfoKey : Any]
  
  func container<Key>(keyedBy type: Key.Type) -> KeyedEncodingContainer<Key> where Key : CodingKey {
    fatalError("This only supports single values.")
  }
  
  func unkeyedContainer() -> UnkeyedEncodingContainer {
    fatalError("This only supports single values.")
  }
  
  func singleValueContainer() -> SingleValueEncodingContainer {
    return container
  }
}

Usage.swift

import Foundation

let encoder = SpanishEncoder()
try! true.encode(to: encoder)
print(encoder) //> sí