swift-private-extensions-example.swift

import CoreGraphics

let vectors: [CGVector] = [CGVector(dx: 2, dy: 1), CGVector(dx: -3, dy: 0)]

// ∞ノルムの平均の計算

// extension なし
vectors.lazy
    .map { max($0.dx.magnitude, $0.dy.magnitude) }
    .reduce(0, +) / CGFloat(vectors.count)

// extension あり
vectors.lazy.map(\.infNorm).mean()

// private extension
private extension CGVector {
    var infNorm: CGFloat { max(dx.magnitude, dy.magnitude) }
}
private extension Collection where Element: FloatingPoint {
    func mean() -> Element {
        reduce(0, +) / Element(count)
    }
}