Changeable

@Changeable is a Swift macro that generates ergonomic copy-updating helpers for structs.

What you get:

• withChanges(...) to produce a modified copy.
• apply(action:) when the type conforms to Applicable.

Requirements

• Swift 6.2 toolchain (macro support)
• Platforms: macOS 13, iOS 13, tvOS 13, watchOS 6, macCatalyst 13

Installation

Add this package to your SwiftPM dependencies and import Changeable in the files where you use the macro.

Basic Usage

import Changeable

@Changeable
struct Profile {
  let name: String
  let age: Int
}

let profile = Profile(name: "Pat", age: 30)
let updated = profile.withChanges(name: "New Name")

Generated:

public func withChanges(
  name: String? = nil,
  age: Int? = nil
) -> Self {
  Self(
    name: name ?? self.name,
    age: age ?? self.age
  )
}

Optional Properties

To allow setting an optional to nil, the macro uses a closure wrapper for optional properties.

@Changeable
struct Settings {
  let nickname: String?
}

let settings = Settings(nickname: "Pat")
let cleared = settings.withChanges(nickname: { nil })

Generated:

public func withChanges(
  nickname: (() -> String?)? = nil
) -> Self {
  Self(
    nickname: nickname != nil ? nickname!() : self.nickname
  )
}

Applicable Integration

If your struct already conforms to Applicable, @Changeable also generates:

public func apply(action: SetValue<Self, some Any>) -> Self

This is intended for codebases that route mutations through a SetValue action and Applicable protocol.

Constraints

• Apply the macro to a struct only.
• Only stored properties are considered (computed properties are ignored).
• Stored properties must have explicit type annotations.