HowToDoErrors.swift

import Foundation

public struct SandwichError: Error, Equatable, CustomStringConvertible {
    private enum SandwichErrorCode: Int {
        case tooLittleSalami = 1
        case tooLittleMustard
        case noTomatoes
        case noBread
    }

    private var code: SandwichErrorCode

    private init(code: SandwichErrorCode) {
        self.code = code
    }

    public var description: String {
        return "SandwichError.\(String(describing: self.code))"
    }

    public static let tooLittleSalami: SandwichError = .init(code: .tooLittleSalami)
    public static let tooLittleMustard: SandwichError = .init(code: .tooLittleMustard)
    public static let noTomatoes: SandwichError = .init(code: .noTomatoes)
    public static let noBread: SandwichError = .init(code: .noBread)
}

func inspectErrorWithSwitch(_ error: Error) {
    if let error = error as? SandwichError {
        switch error {
        case .tooLittleSalami:
            print("NEED MORE SALAMI!")
        case .tooLittleMustard:
            print("NEED MORE MUSTARD!")
        case .noTomatoes:
            print("Can I please get a tomato?")
        default:
            print("ooh, some error I didn't know existed: \(error)")
        }
    } else {
        print("totally unexpected error")
    }
}

func inspectErrorWithCatch(_ e: Error) {
    do {
        throw e
    } catch let error as SandwichError where error == .tooLittleSalami {
        print("NEED MORE SALAMI!")
    } catch let error as SandwichError where error == .tooLittleMustard {
        print("NEED MORE MUSTARD!")
    } catch let error as SandwichError where error == .noTomatoes {
        print("Can I please get a tomato?")
    } catch let error as SandwichError {
        print("ooh, some error I didn't know existed: \(error)")
    } catch {
        print("totally unexpected error")
    }
}

inspectErrorWithSwitch(SandwichError.tooLittleSalami)
inspectErrorWithSwitch(SandwichError.tooLittleMustard)
inspectErrorWithSwitch(SandwichError.noTomatoes)
inspectErrorWithSwitch(SandwichError.noBread)
inspectErrorWithSwitch(NSError())

inspectErrorWithCatch(SandwichError.tooLittleSalami)
inspectErrorWithCatch(SandwichError.tooLittleMustard)
inspectErrorWithCatch(SandwichError.noTomatoes)
inspectErrorWithCatch(SandwichError.noBread)
inspectErrorWithCatch(NSError())