Functions and Closures

Argument Labels

Swift argument labels make function calls read like short phrases.

Name the call-site meaning

count
argument_labels.swift
Replay: real traced execution (multi-file project)
func makeLabel(for item: String, count: Int) -> String {
    return "\(item): \(count)"
}

let count = 3
let label = makeLabel(for: "tickets", count: count)

print(label)

func makeLabel(for item: String, count: Int) -> String {
    return "\(item): \(count)"
}

let count = 1
let label = makeLabel(for: "tickets", count: count)

print(label)

func makeLabel(for item: String, count: Int) -> String {
    return "\(item): \(count)"
}

let count = 5
let label = makeLabel(for: "tickets", count: count)

print(label)

  1. count ← 3

    5let count→ 3 = 3  //@count=1, 56let label = makeLabel(for: "tickets", count: count3)

  2. func makeLabel(for item: String, count: Int) -> String

    1func makeLabel(for itemtickets: String, count3: Int) -> String {2    return "\(itemtickets): \(count3)"3}

  3. label ← tickets: 3

    5let count = 3  //@count=1, 56let label→ tickets: 3 = makeLabel(for: "tickets", count: count3)78print(labeltickets: 3)
    outputtickets: 3

  1. count ← 1

    5let count→ 1 = 16let label = makeLabel(for: "tickets", count: count1)

  2. func makeLabel(for item: String, count: Int) -> String

    1func makeLabel(for itemtickets: String, count1: Int) -> String {2    return "\(itemtickets): \(count1)"3}

  3. label ← tickets: 1

    5let count = 16let label→ tickets: 1 = makeLabel(for: "tickets", count: count1)78print(labeltickets: 1)
    outputtickets: 1

  1. count ← 5

    5let count→ 5 = 56let label = makeLabel(for: "tickets", count: count5)

  2. func makeLabel(for item: String, count: Int) -> String

    1func makeLabel(for itemtickets: String, count5: Int) -> String {2    return "\(itemtickets): \(count5)"3}

  3. label ← tickets: 5

    5let count = 56let label→ tickets: 5 = makeLabel(for: "tickets", count: count5)78print(labeltickets: 5)
    outputtickets: 5

Follow the Call

  1. count starts as 3.
  2. The call is makeLabel(for: "tickets", count: count).
  3. The function uses the item text tickets.
  4. It combines the item with the count.
  5. The program prints tickets: 3. | count | label text | | --- | --- | | 3 | tickets: 3 | | 1 | tickets: 1 | | 5 | tickets: 5 |
labels The external label appears at the call site, while the parameter name is used inside the function body.

Exercise: argument_labels.swift

Reproduce tickets: 3, then use the pinned count variants 1 and 5 to predict tickets: 1 and tickets: 5.