Implementing types that conform to LocalizedError in Swift

errors.swift

//Proper way to implement error types. Implement `errorDescription`, not `localizedDescription` and the global handler `getErrorMessage(error: Error)` is easy to implement, not need a stupid switch case
//Key is implement `errorDescription` instead of `localizedDescription` and must not implement `localizedDescription` at all

import Foundation

struct E1: LocalizedError {
    var errorDescription: String? {
      return "E1's errorDescription"
    }
}
struct E2: LocalizedError {
    var errorDescription: String? {
      return "E2's errorDescription"
    }
}

let e = E1()
let f: LocalizedError = e
let g: Error = e

print("errorDescription:")
print(e.errorDescription)
print(f.errorDescription)
//print(g.errorDescription)

print("\nlocalizedDescription:")
print(e.localizedDescription)
print(f.localizedDescription)
print(g.localizedDescription)

func getErrorMessage(error: Error) -> String {
    return error.localizedDescription //making `getErrorMessage(error:)` unnecessary
}

print("\ngetErrorMessage:")
print(getErrorMessage(error: E1()))
print(getErrorMessage(error: E2()))

And if the error type is defined in a module (eg. Cocoapods) and only the main app provide the localized strings, then:

instead of this in the module:

struct E1: LocalizedError {
    var errorDescription: String? {
      return "E1's errorDescription"
    }
}

do this in the module (i.e do not make it conform to LocalizedError in the module) :

struct E1: Error {
}

and this in the main app (conform to LocalizedError in the app:

extension E1: LocalizedError {
    var errorDescription: String? {
      return "E1's localized errorDescription" //using R.string.localizable.xxx()
    }
}