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try! Swift macros
Recap of WWDC23 Mercari
https://mercari.connpass.com/event/285179/
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About me
● Nickname
○ kenken
● Company
○ Merpay, Inc. (since May, 2021)
● Occupation
○ Software Engineer, Android (and iOS at some of my previous companies)
● Accounts
○ Twitter: @tkhs0604
○ GitHub: @tkhs0604
○ Bluesky: @kenken.bsky.social
● Other
○ DroidKaigi community staff
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What made me interested in
Swift macros?
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Twitter 🐤
https://twitter.com/ailtonvivaz/status/1666922079294201857
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Comparison between Swift and Kotlin codes
● Swift code with SwiftRequest (https://github.com/ailtonvivaz/swift-request)
● Kotlin code with Retrofit (https://github.com/square/retrofit)
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😳
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Agenda
● What is Swift macros
● Freestanding macros and Attached macros
● How to implement macros
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try! したけど難しかった
Swift macros
Recap of WWDC23 Mercari
https://mercari.connpass.com/event/285179/
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What is Swift macros
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Swift macros
● A feature added from Swift 5.9
○ You can use it after Xcode 15.0 beta
● A tool for generating boilerplate codes repetitively written by hand
● #Preview, @Observable etc. are created with Swift macros
Cited from: https://docs.swift.org/swift-book/documentation/the-swift-programming-language/macros/
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Swift macros
● Transforms the syntax tree of a source code at compile time
○ Input code to a macro and output code of macro expansion are validated syntactically
○ Values passed to a macro and in the code generated by a macro are type-checked
○ If the implementation of a macro has any error, Swift compiler treats it as a compilation error
Cited from: https://developer.apple.com/videos/play/wwdc2023/10167/?time=291
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How to give Swift macros a try
● File > New > Package…
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Package structure
Declaration of a macro
Implementation of a macro
Test of a macro
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Freestanding macros
and
Attached macros
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Role in the context of Swift macros
Cited from: https://developer.apple.com/videos/play/wwdc2023/10167?time=383
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Available macro roles
Cited from: https://developer.apple.com/videos/play/wwdc2023/10167?time=403
● Freestanding macros: Can be used independently
● Attached macros: Can be used with some declaration
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Use case of @freestanding(expression)
Cited from: https://developer.apple.com/videos/play/wwdc2023/10167/?time=483
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● 2 dimensional array
Use case of @freestanding(declaration)
Cited from: https://developer.apple.com/videos/play/wwdc2023/10167/?time=593
● 3 dimensional array
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Use case of @freestanding(declaration)
Cited from: https://developer.apple.com/videos/play/wwdc2023/10167/?time=593
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Use case of @attached(peer)
Cited from: https://developer.apple.com/videos/play/wwdc2023/10167/?time=658
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Use case of @attached(accessor)
Cited from: https://developer.apple.com/videos/play/wwdc2023/10167/?time=735
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Use case of @attached(memberAttribute)
Cited from: https://developer.apple.com/videos/play/wwdc2023/10167/?time=822
Role composition
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Use case of @attached(memberAttribute)
Cited from: https://developer.apple.com/videos/play/wwdc2023/10167/?time=822
Role composition
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Use case of @attached(member)
Cited from: https://developer.apple.com/videos/play/wwdc2023/10167/?time=930
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Use case of @attached(conformance)
Cited from: https://developer.apple.com/videos/play/wwdc2023/10167/?time=1011
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In the end…
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How to implement macros
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Package structure (Repeated)
Declaration of a macro
Implementation of a macro
Test of a macro
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#externalMacro
● Specifies the plug-in (module) which the compiler should launch and the
name of a type inside the plug-in
● When Swift expands the following macro, #stringify(_:) will launch a plug-in
called "MyLibMacros" and ask a type called "StringifyMacro" to expand it
Cited from: https://developer.apple.com/videos/play/wwdc2023/10167/?time=1102
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Macro role protocols
● Used to create a macro implementation
○ https://github.com/apple/swift-syntax/tree/main/Sources/SwiftSyntaxMacros/MacroProtocols
Cited from: https://developer.apple.com/videos/play/wwdc2023/10167/?time=1356
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Abstract syntax tree (AST)
● A tree representation of the abstract syntactic structure of a source code
● SwiftSyntax is a library that helps us parse, inspect, manipulate, and
generate Swift source code
○ https://github.com/apple/swift-syntax
Cited from: https://developer.apple.com/videos/play/wwdc2023/10167/?time=1288
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Swift AST Explorer
● A tool for visualizing AST
○ https://swift-ast-explorer.com/
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Sample of a macro implementation
● Implemented as an extension in this example
○ https://github.com/DougGregor/swift-macro-examples/blob/main/MacroExamplesPlugin/Dictio
naryIndirectionMacro.swift#L63-L84
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😇
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Summary
● Swift macros is a tool for generating boilerplate codes
● Inputs and outputs are syntax-checked and type-checked at compile time
● Some useful macros, like #preview and @Observable have already been
introduced
● SwiftSyntax is an essential tool for implementing macros
○ Quite difficult to understand… 😇
● So… let’s try to create macros by yourself and share knowledges!
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Thank you
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Appendix
● GitHub Gist
○ https://gist.github.com/tkhs0604/d2b6b1a84eeb3bead6faa6a6a1e6af33