Error Protocol and Custom Errors

import Foundation

// Custom error with LocalizedError
enum ValidationError: LocalizedError {
    case emptyField(name: String)
    case tooShort(field: String, minimum: Int)
    case invalidFormat(field: String, expected: String)

    var errorDescription: String? {
        switch self {
        case .emptyField(let name):
            return "\(name) cannot be empty"
        case .tooShort(let field, let min):
            return "\(field) must be at least \(min) characters"
        case .invalidFormat(let field, let fmt):
            return "\(field) must match format: \(fmt)"
        }
    }

    var recoverySuggestion: String? {
        "Please correct the input and try again."
    }
}

struct RegistrationForm {
    var username: String
    var email: String
    var password: String

    func validate() throws {
        guard !username.isEmpty else { throw ValidationError.emptyField(name: "Username") }
        guard username.count >= 3 else { throw ValidationError.tooShort(field: "Username", minimum: 3) }
        guard email.contains("@") else { throw ValidationError.invalidFormat(field: "Email", expected: "user@domain.com") }
        guard password.count >= 8 else { throw ValidationError.tooShort(field: "Password", minimum: 8) }
    }
}

do {
    let form = RegistrationForm(username: "ab", email: "not-an-email", password: "short")
    try form.validate()
} catch let error as ValidationError {
    print(error.errorDescription ?? "Unknown error")
    print(error.recoverySuggestion ?? "")
}