Top 7 Subtle Swift Features | https://alexdremov.me/top-7-subtle-swift-features/

autoclosure_example.swift

class Foo {
    let bonkProvider: () -> DBConnection
    lazy var bonk: DBConnection = bonkProvider()
    init(_ expression: @escaping @autoclosure () -> DBConnection) {
        self.bonkProvider = expression
    }
    func send() {
    	// Here bonkProvider() is called
        // only for the first call of send()
        bonk.sendMessage()
    }
}

calculate_definition.swift

func calculate(_ expression: @autoclosure () -> Int,
               zero: Bool) -> Int {
    guard !zero else {
        return 0
    }
    return expression()
}

calculate_example.swift

calculate(1 + 2, zero: false) // 3
calculate([Int](repeating: 5, count: 10000000).reduce(0, +),
                zero: false) // 50000000
calculate([Int](repeating: 5, count: 1000).reduce(0, +),
                zero: true) // 0

dynamicCallable.swift

@dynamicCallable
struct RangeGenerator {
    var range: Range<Int>
    func dynamicallyCall(withKeywordArguments args: KeyValuePairs<String, Int>) -> [Int] {
        if args.count > 1 || args.first?.key != "count" {
            fatalError("Unknown arguments \(args)")
        }
        let count = args.first!.value
        return (0..<count).map{ _ in Int.random(in: range) }
    }
}
let gen = RangeGenerator(range: 0..<100)
print(gen(count: 13))
// [2, 89, 4, 17, 65, 26, 73, 86, 93, 13, 25, 96, 96]

dynamicMemberLookup.swift

@dynamicMemberLookup
class Foo {
    subscript(dynamicMember string: String) -> String {
        return string
    }
}
let a = Foo()
print(a.helloWorld)

dynamicMemberLookup_second.swift

class Bob {
    let age = 22
    let name = "Bob"
}
@dynamicMemberLookup
class Foo {
    let himself = Bob()
    subscript<T>(dynamicMember keyPath: KeyPath<Bob, T>) -> T {
        return himself[keyPath: keyPath]
    }
}
let a = Foo()
print(a.age)

enum_as_namespace.swift

enum API {}
extension API {
    static let token = "…"
    struct CatsCounter {
        …
    }
}
let a = API.CatsCounter()
print(API.token)

inlining.swift

struct Foo {
    @inlinable
    @inline(__always)
    func simpleComputation(_ a: Int, _ b: Int) -> Int {
        duplicate(a) + duplicate(b)
    }
    @usableFromInline
    func duplicate(_ c: Int) -> Int {
        c * 2
    }
    func general() {
        print("Hello world")
    }
}

simple_lazy.swift

class Foo {
    lazy var bonk = DBConnection()
    func send() {
        bonk.sendMessage()
    }
}