Swift Conditional String Interpolation

Question

I often trap myself writing code like this when I need to construct a string depending on some condition (in this case isFavorite):

let me = Contact(name: "Stefan", isFavorite: true)

var message = "Contact \(me.name)"
if me.isFavorite {
    message.append(" is a favorite contact")
}

These are 4 lines or alternatively a complex ternary operator (if ? then : else) for such a simple task. I always have a bad conscience with this...

Is there a way to do this more elegantly with Swift?

Answer 1

Swift 5

Indeed there is - I found the answer at the UIKonf 2019 where I heard from Erica Sadun that there is a way by using Swift 5 String Interpolation to achieve this in one single line. All you need is this reusable extension:

extension String.StringInterpolation {
    mutating func appendInterpolation(if condition: @autoclosure () -> Bool, _ literal: StringLiteralType) {
        guard condition() else { return }
        appendLiteral(literal)
    }
}

It enables you to transform your 4 lines into just one:

"Contact: \(me.name)\(if: me.isFavorite, " is a favorite contact")"

You can read about the different possibilities of Swift's string interpolation in in Erica's article: https://ericasadun.com/2018/12/12/the-beauty-of-swift-5-string-interpolation/

Playground

Start getting your hands dirty with this:

import Foundation

// *****************************************************************************
// Many thanks to Erica Sadun for describing the new possibilities of string
// interpolation in Swift 5 👍
// *****************************************************************************


// *****************************************************************************
// Conditional Interpolation

struct Contact {
    let name: String
    let isFavorite: Bool
}
let me = Contact(name: "Stefan", isFavorite: true)

// ***************************************************************** Swift 4

var message = "Contact \(me.name)"
if me.isFavorite {
    message.append(" is favorite")
}
message // we need 4 lines to construct this message!!!
"Contact: \(me.name)\(me.isFavorite ? " is favorite" : "")" // or a complex ternary operator

// ***************************************************************** Swift 5

extension String.StringInterpolation {

    mutating func appendInterpolation(if condition: @autoclosure () -> Bool, _ literal: StringLiteralType) {
        guard condition() else { return }
        appendLiteral(literal)
    }
}
"Contact: \(me.name)\(if: me.isFavorite, " is favorite")" // simple and clean - no extras

// *****************************************************************************
// Optional Interpolation

let optionalMe: Contact? = Contact(name: "Stefan", isFavorite: true)

// ***************************************************************** Swift 4

"Contact: \(optionalMe?.name)" // shows warning

// ***************************************************************** Swift 5

// Interpolate nil value
extension String.StringInterpolation {
    /// Provides `Optional` string interpolation without forcing the
    /// use of `String(describing:)`.
    public mutating func appendInterpolation<T>(_ value: T?, default defaultValue: String) {
        if let value = value {
            appendInterpolation(value)
        } else {
            appendLiteral(defaultValue)
        }
    }
}
let nilContact: Contact? = nil
"Contact: \(nilContact?.name, default: "nil")"

// Strip `Optional`
extension String.StringInterpolation {
    /// Interpolates an optional using "stripped" interpolation, omitting
    /// the word "Optional" from both `.some` and `.none` cases
    public mutating func appendInterpolation<T>(describing value: T?) {
        if let value = value {
            appendInterpolation(value)
        } else {
            appendLiteral("nil")
        }
    }

}
"Contact: \(describing: optionalMe?.name)"