What does $0 and $1 mean in Swift Closures?

Question

let sortedNumbers = numbers.sort { $0 > $1 }
print(sortedNumbers)

Can anyone explain, what $0 and $1 means in swift?

Answer 1

TL;DR

Swift 5.3

$0 and $1 are Closure’s first and second shorthand arguments (a.k.a. Shorthand Argument Names or SAN for short). The shorthand argument names are automatically provided by Swift. The first argument can be referenced by $0, the second argument can be referenced by $1, the third one by $2, and so on.

As you know, a Closure is a self-contained block of functionality (a function without name) that can be passed around and used in your code. Closure has different names in other programming languages as well as slight differences in meaning – it's Lambda in Python and Kotlin or Block in C and Objective-C.

Shortening a Closure

let coffee: [String] = ["Cappuccino", "Espresso", "Latte", "Ristretto"]

1. Normal Function

func backward(_ n1: String, _ n2: String) -> Bool {
    return n1 > n2
}
var reverseOrder = coffee.sorted(by: backward)


/* RESULT: ["Ristretto", "Latte", "Espresso", "Cappuccino"] */

2. Inline Closure Expression

reverseOrder = coffee.sorted(by: { (n1: String, 
                                    n2: String) -> Bool in return n1 > n2 } )

3. Inferring Type From Context

reverseOrder = coffee.sorted(by: { n1, n2 in return n1 > n2 } )

4. Implicit Returns from Single-Expression Closures

reverseOrder = coffee.sorted(by: { n1, n2 in n1 > n2 } )

5. Shorthand Argument Names

reverseOrder = coffee.sorted(by: { $0 > $1 } )

/* $0 and $1 are closure’s first and second String arguments. */

6. Operator Methods

reverseOrder = coffee.sorted(by: >)

/* RESULT: ["Ristretto", "Latte", "Espresso", "Cappuccino"] */

Higher Order Function map with dot notation

let companies = ["bmw", "kfc", "ibm", "htc"]

let uppercasedCompanies = companies.map { (item) -> String in item.uppercased() }

/* RESULT: ["BMW", "KFC", "IBM", "HTC"] */

SAN in HOF map with dot notation

let uppercasedCompanies = companies.map { $0.uppercased() }

/* RESULT: ["BMW", "KFC", "IBM", "HTC"] */

SAN in HOF filter with remainder operator

let numbers: [Int] = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

let filteredNumbers = numbers.filter { ($0 % 2) == 0 }

print(filteredNumbers)

/* RESULT: [2, 4, 6, 8, 10] */

Repeating $0

let cubedNumber = { $0 * $0 * $0 } (25)

print(cubedNumber)

/* RESULT:  25^3 = 15625 */

Three Shorthand Argument Names – $0, $1, $2

let math: (Int8, Int8, Int8) -> Int8 = { $0 + $1 - $2 }

func feedClosure() -> (Int8, Int8, Int8) -> Int8 {
    return math
}
feedClosure()(10, 20, 100)

/* RESULT:  (10 + 20 - 100) = -70 */

Five SANs – $0, $1, $2, $3 and $4

let factorial = { $0 * $1 * $2 * $3 * $4 } (1, 2, 3, 4, 5)

print(factorial)

/* RESULT:  5! = 120 */

Key Path Expression

In Swift 5.2 you can access parameters of every instance via key path expression:

struct Lighter {
    let manufacturer: String
    let refillable: Bool
}

let zippo = Lighter(manufacturer: "Zippo", refillable: true)
let cricket = Lighter(manufacturer: "Cricket", refillable: false)

let lighters: [Lighter] = [zippo, cricket]

let refillableOnes = lighters.map(\.refillable)

print(refillableOnes)

/* RESULT:  [true, false] */

Of course, you can alternatively use a familiar syntax:

Regular syntax – $0.property:

let refillableOnes = lighters.map { $0.refillable }

print(refillableOnes)

/* RESULT:  [true, false] */

Shorthand Argument Name with a subscript

let arrays: [[String]] = [["Hello", "Hola"], ["world", "mundo"]]

let helloWorld = arrays.compactMap { $0[0] }

print(helloWorld)

/* RESULT:  ["Hello", "world"] */

Shorthand Argument Name in Completion Handler

let completionHandler: (Bool) -> Void = {
    if $0 {
        print("It is true, sister...")
    }
}

Regular syntax, however, is as following:

let completionHandler: (Bool) -> Void = { sayTheTruth in
    if sayTheTruth {
        print("It is true, sister...")
    }
}

Swift vs Kotlin vs Python

Also, let's see how Kotlin's lambda is similar to Swift's closure:

Swift

let element: [String] = ["Argentum","Aurum","Platinum"]

let characterCount = element.map { $0.count }

print(characterCount)

/* RESULT:  [8, 5, 8] */ 

Kotlin

Often Kotlin's lambda expression has only one parameter with implicit name: it.

val element = listOf("Argentum","Aurum","Platinum")

val characterCount = element.map { it.length }

println(characterCount)

/* RESULT:  [8, 5, 8] */ 

But in Python there's no equivalent of Shorthand Argument Name.

Python

element = ["Argentum","Aurum","Platinum"]

characterCount = list(map(lambda x: len(x), element))

print(characterCount)

/* RESULT:  [8, 5, 8] */