Failable Initializers

Part of the Object Oriented Programming section of Coddy's Swift journey. Lesson 20 of 57.

Some values cannot be built from every input. A failable initializer, written init?, returns nil instead of an instance when the input is invalid, so no invalid instance can ever exist:

struct Percentage {
    let value: Int

    init?(_ value: Int) {
        guard value >= 0, value <= 100 else { return nil }
        self.value = value
    }
}

print(Percentage(42)?.value ?? -1)
print(Percentage(150)?.value ?? -1)

Output:

42
-1

The result of a failable initializer is an optional, so the caller must unwrap it, usually with if let or guard let:

struct Email {
    let address: String

    init?(_ text: String) {
        let parts = text.split(separator: "@")
        guard parts.count == 2, parts[1].contains(".") else { return nil }
        address = text.lowercased()
    }
}

for text in ["Ada@Mail.com", "not-an-email"] {
    if let email = Email(text) {
        print("valid: \(email.address)")
    } else {
        print("invalid: \(text)")
    }
}

Output:

valid: ada@mail.com
invalid: not-an-email

Swift's own types use the same idea: Int("42") and Double("x") are failable initializers, and so is an enum's init(rawValue:). A failable initializer can call these and pass the failure on:

struct Coordinate {
    let lat: Double
    let lon: Double

    init?(text: String) {
        let parts = text.split(separator: ",")
        guard parts.count == 2,
              let lat = Double(parts[0]), let lon = Double(parts[1]),
              abs(lat) <= 90, abs(lon) <= 180 else { return nil }
        self.lat = lat
        self.lon = lon
    }
}

print(Coordinate(text: "51.5,-0.1") != nil)
print(Coordinate(text: "95,10") != nil)

Output:

true
false

Use init? when a missing value is enough information. Use a throwing initializer, init(...) throws, when the caller should learn which rule failed:

struct Username {
    let text: String
    init(_ text: String) throws {
        guard text.count >= 3 else { throw NameError.tooShort }
        guard text.allSatisfy({ $0.isLetter }) else { throw NameError.badCharacters }
        self.text = text
    }
}

Challenge

Easy

Complete the struct Rgb, which stores a color as red, green and blue from 0 to 255. Its failable initializer init?(hex:) reads text like #FF8000: it returns nil unless the text is # followed by exactly six hexadecimal digits. Use Int(text, radix: 16) for each pair. Add a computed label that returns rgb(255, 128, 0).

The supplied code reads one color per line and prints the label or invalid: text.

Your code goes in Rgb.swift. main.swift holds the supplied input/output code and cannot be edited.

Try it yourself

// Supplied input/output code: keep it as it is
var input: [String] = []
while let line = readLine() { input.append(line) }
for line in input {
    if let color = Rgb(hex: line) {
        print(color.label)
    } else {
        print("invalid: \(line)")
    }
}