Using Swift 5's Custom String Interpolation in Practice @basthomas, Copenhagen Cocoa, 06-02-2020 1 

Who am I • Swift Weekly Brief • Contravariance (stickers) • RayWenderlich tech editor • iOS Platform at XING @basthomas, Copenhagen Cocoa, 06-02-2020 2 

Where do we come from? @basthomas, Copenhagen Cocoa, 06-02-2020 3 

! @basthomas, Copenhagen Cocoa, 06-02-2020 4 

NSString *name = @"Bas"; [NSString stringWithFormat:@"Hello, %@", name]; [NSString stringWithFormat:@"%.2f", 9.0234]; [NSString stringWithFormat: @"Hello %@, how are you doing this %@? %@", @"Bas", @"evening", @"Great!" ]; @basthomas, Copenhagen Cocoa, 06-02-2020 5 

✨ @basthomas, Copenhagen Cocoa, 06-02-2020 6 

let name = "Bas" "Hello, \(name)" String(format: "%.2f", 9.0234) // "Hello \(name), how are you doing this \(timeOfDay)?" @basthomas, Copenhagen Cocoa, 06-02-2020 7 

It is very good. @basthomas, Copenhagen Cocoa, 06-02-2020 8 

Interlude @basthomas, Copenhagen Cocoa, 06-02-2020 9 

Swift Evolution @basthomas, Copenhagen Cocoa, 06-02-2020 10 

Multiline String Literals (SE-0168) """ Hello, how are you doing this \(timeOfDay)? """ @basthomas, Copenhagen Cocoa, 06-02-2020 11 

String Literal Delimiters (SE-0200) // before "\"Did you put your name into the Goblet of Fire, Harry?\" he asked calmly." // after #""Harry! Did you put your name in the Goblet of Fire?!""# @basthomas, Copenhagen Cocoa, 06-02-2020 12 

And then... @basthomas, Copenhagen Cocoa, 06-02-2020 13 

Custom String Interpolation (aka Fix ExpressibleByStringInterpolation) @basthomas, Copenhagen Cocoa, 06-02-2020 14 

Localization @basthomas, Copenhagen Cocoa, 06-02-2020 15 

let message: LocalizableString = #"The document "\(name)" could not be saved."# alert.messageText = String(localized: message) @basthomas, Copenhagen Cocoa, 06-02-2020 16 

Errors @basthomas, Copenhagen Cocoa, 06-02-2020 17 

extension String.StringInterpolation { mutating func appendInterpolation(_ error: Error) { appendLiteral(error.localizedDescription) } } fatalError( "Something went wrong: \(ConnectionError.invalidRequest)" ) @basthomas, Copenhagen Cocoa, 06-02-2020 18 

Number formatting @basthomas, Copenhagen Cocoa, 06-02-2020 19 

extension String.StringInterpolation { mutating func appendInterpolation( number: Double, formatter: NumberFormatter ) { appendInterpolation( ifNotNil: formatter.string(from: NSNumber(value: number)) ) } } let priceFormatter = NumberFormatter() priceFormatter.numberStyle = .currency "\(number: 100.1234, formatter: priceFormatter)" @basthomas, Copenhagen Cocoa, 06-02-2020 20 

Logging and privacy @basthomas, Copenhagen Cocoa, 06-02-2020 21 

extension String.StringInterpolation { mutating func appendInterpolation(private: String) { #if DEBUG appendLiteral(`private`) #else appendLiteral("@@@") #endif } } let name = "Bas" let password = "Broek" print("User \(name) has password \(private: password)") @basthomas, Copenhagen Cocoa, 06-02-2020 22 

Other instances @basthomas, Copenhagen Cocoa, 06-02-2020 23 

"\(properties == nil ? "nil" : String(describing: properties ?? [:]))" @basthomas, Copenhagen Cocoa, 06-02-2020 24 

extension String.StringInterpolation { mutating func appendInterpolation( dictionary: Dictionary?, default: @autoclosure () -> String = "nil" ) { if let dictionary = dictionary { appendLiteral(String(describing: dictionary)) } else { appendLiteral(`default`()) } } } "\(dictionary: properties)" @basthomas, Copenhagen Cocoa, 06-02-2020 25 

123 > maxBadgeCount ? String("\(maxBadgeCount)+") : String(123) @basthomas, Copenhagen Cocoa, 06-02-2020 26 

extension String.StringInterpolation { mutating func appendInterpolation( number: Int, maximum: @autoclosure () -> Int ) { let max = maximum() if number > max { appendLiteral("\(max)+") } else { appendLiteral("\(number)") } } } "\(number: number, maximum: maxBadgeCount)" @basthomas, Copenhagen Cocoa, 06-02-2020 27 

! @basthomas, Copenhagen Cocoa, 06-02-2020 28 

Polluting a namespace @basthomas, Copenhagen Cocoa, 06-02-2020 29 

Code, documentation, tests @basthomas, Copenhagen Cocoa, 06-02-2020 30 

struct BadgeCount: CustomStringConvertible { let value: Int var maximum = 99 var description: String { if value > maximum { return "\(maximum)+" } else { return "\(value)" } } } "\(BadgeCount(value: 123, maximum: 99))" "\(BadgeCount(value: 123))" @basthomas, Copenhagen Cocoa, 06-02-2020 31 

Going all in @basthomas, Copenhagen Cocoa, 06-02-2020 32 

struct HTMLComponent: ExpressibleByStringLiteral, ExpressibleByStringInterpolation, CustomStringConvertible { struct StringInterpolation: StringInterpolationProtocol { var output = "" init(literalCapacity: Int, interpolationCount: Int) { output.reserveCapacity(literalCapacity * 2) } mutating func appendLiteral(_ literal: String) { output.append(literal) } mutating func appendInterpolation(twitter: String) { output.append("@\(twitter)") } mutating func appendInterpolation(email: String) { output.append("\(email)") } } let description: String init(stringLiteral value: String) { description = "

\(value)

" } init(stringInterpolation: StringInterpolation) { self.init(stringLiteral: stringInterpolation.output) } } @basthomas, Copenhagen Cocoa, 06-02-2020 33 

struct StringInterpolation: StringInterpolationProtocol { var output = "" init(literalCapacity: Int, interpolationCount: Int) { output.reserveCapacity(literalCapacity * 2) } mutating func appendLiteral(_ literal: String) { output.append(literal) } mutating func appendInterpolation(twitter: String) { output.append("@\(twitter)") } mutating func appendInterpolation(email: String) { output.append("\(email)") } } let html: HTMLComponent = "Find me on \(twitter: "basthomas") or send an email to \(email: "[email protected]")" //

Find me on @basthomas // or send an email to [email protected]

@basthomas, Copenhagen Cocoa, 06-02-2020 34 

Optimize code for reading, not writing @basthomas, Copenhagen Cocoa, 06-02-2020 35 

Communication is hard @basthomas, Copenhagen Cocoa, 06-02-2020 36 

Thanks! @basthomas @basthomas, Copenhagen Cocoa, 06-02-2020 37