SwiftのOptionalとは何で、どうすれば安全にアンラップできますか？

var name: String? = "Ada"     // Optional String, currently .some("Ada")
var missing: String? = nil    // .none

// if let: bind only when non-nil, scoped to the block
if let name = name {
    print(name.count)         // safe: name is a plain String here
}

// guard let: unwrap or exit early — keeps the happy path un-nested
func greet(_ input: String?) {
    guard let input else { return }  // Swift 5.7 shorthand
    print("Hi \(input)")            // input is non-optional below
}

// nil-coalescing: supply a default
let display = name ?? "Anonymous"

let url = URL(string: "https://a.com")!  // OK: literal is always valid
let user = fetchUser()!                  // BAD: crashes on nil