マクロの役割：フリースタンディングとアタッチド

// Macros are invoked with # (freestanding) or @ (attached)
#warning("Fix this later")
@Observable class Counter { var count = 0 }

// #stringify expands to a tuple of the value and its source string
let (val, src) = #stringify(1 + 2)
print(val, src) // 3, "1 + 2"

// #makePair emits two variables
#makePair("point", x: 1, y: 2)
// expands to: let pointX = 1; let pointY = 2

@AddID
struct User { var name: String }
// Expands to: var id = UUID()

struct Point {
  @Clamped(0...100)
  var x: Int = 0
  // accessor macro generates get/set with clamping
}

class Service {
  @AddAsync
  func fetchSync(completion: @escaping (Data)->Void) { ... }
  // peer macro adds: func fetch() async -> Data
}

@EquatableByID
struct Product { var id: Int; var name: String }
// adds: extension Product: Equatable { ... }

@CustomCodable
struct Config { var host: String; var port: Int }
// adds: extension Config: Codable { custom encode/decode }

// Right-click macro usage in Xcode → "Expand Macro"
// to see the generated source code

@freestanding(expression)
public macro stringify<T>(_ value: T) -> (T, String)
  = #externalMacro(module: "MyMacros", type: "StringifyMacro")

// Freestanding
let result = #stringify(40 + 2)
// Attached
@Observable class Model { var count = 0 }