Swift Expression Macros: a practical introduction

Swift Expression Macros:
a practical introduction
-実践 Swiftマクロ入門-

Macro proposal timeline
Swift macro overview
How to implement macros
Swift macro essentials
Pros and cons
Various kinds of macros
Declaration macros
Practical Example: Power Assert
Summary

Transcript

1. kishikawa katsumi / @kishikawakatsumi@hachyderm.io
   Swift Expression Macros:
   a practical introduction
   -࣮ફ SwiftϚΫϩೖ໳-
   

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2. Agenda
   • Macro proposal timeline
   
   • Swift macro overview
   
   • How to implement macros
   
   • Swift macro essentials
   
   • Pros and cons
   
   • Various kinds of macros
   
   • Declaration macros
   
   • Practical Example: Power Assert
   
   • Summary
   ໨࣍
   

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3. Macro proposal timeline
   

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4. • A Possible Vision for Macros in Swift

   https://forums.swift.org/t/a-possible-vision-for-macros-in-swift/60900

   https://gist.github.com/DougGregor/4f3ba5f4eadac474ae62eae836328b71
   
   • Swift project in 2023

   https://www.swift.org/blog/focus-areas-2023/
   Macro proposal timeline
   A Possible Vision for Macros in Swift
   

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5. • A Possible Vision for Macros in Swift

   https://forums.swift.org/t/a-possible-vision-for-macros-in-swift/60900
   
   • Swift project in 2023

   https://www.swift.org/blog/focus-areas-2023/
   
   ➡[Pitch] Expression macros

   https://forums.swift.org/t/pitch-expression-macros/61499
   
   • [Pitch #2] Expression macros

   https://forums.swift.org/t/pitch-2-expression-macros/61861
   
   • SE-0382: Expression Macros

   https://forums.swift.org/t/se-0382-expression-macros/62090
   ϚΫϩʢExpression Macros/ࣜϚΫϩʣͷػೳ͕ঝೝ͞ΕΔ·Ͱ
   Macro proposal timeline
   

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6. • A Possible Vision for Macros in Swift

   https://forums.swift.org/t/a-possible-vision-for-macros-in-swift/60900
   
   • Swift project in 2023

   https://www.swift.org/blog/focus-areas-2023/
   
   • [Pitch] Expression macros

   https://forums.swift.org/t/pitch-expression-macros/61499
   
   ➡[Pitch #2] Expression macros

   https://forums.swift.org/t/pitch-2-expression-macros/61861
   
   • SE-0382: Expression Macros

   https://forums.swift.org/t/se-0382-expression-macros/62090
   ϚΫϩʢExpression Macros/ࣜϚΫϩʣͷػೳ͕ঝೝ͞ΕΔ·Ͱ
   Macro proposal timeline
   

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7. • A Possible Vision for Macros in Swift

   https://forums.swift.org/t/a-possible-vision-for-macros-in-swift/60900
   
   • Swift project in 2023

   https://www.swift.org/blog/focus-areas-2023/
   
   • [Pitch] Expression macros

   https://forums.swift.org/t/pitch-expression-macros/61499
   
   • [Pitch #2] Expression macros

   https://forums.swift.org/t/pitch-2-expression-macros/61861
   
   ➡SE-0382: Expression Macros

   https://forums.swift.org/t/se-0382-expression-macros/62090
   ϚΫϩʢExpression Macros/ࣜϚΫϩʣͷػೳ͕ঝೝ͞ΕΔ·Ͱ
   Macro proposal timeline
   

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8. એݴϚΫϩʢDeclaration Macrosʣ
   • [Pitch] Declaration macros

   https://forums.swift.org/t/pitch-declaration-macros/62373/17
   
   • Declaration Macros

   https://github.com/DougGregor/swift-evolution/blob/declaration-macros/proposals/nnnn-declaration-macros.md
   Macro proposal timeline
   

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9. Swift macro overview
   

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10. Swift Macro Overview
    SwiftϚΫϩͷ֓ཁ
    func f(a: Int, _: Double, c: Int) {
    
    
    print(#function)
    
    
    }
    

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11. func f(a: Int, _: Double, c: Int) {
    
    
    print(#function) // => f(a:_:c:)
    
    
    }
    Swift Macro Overview
    SwiftϚΫϩͷ֓ཁ
    

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12. func f(a: Int, _: Double, c: Int) {
    
    
    print(#function) // => f(a:_:c:)
    
    
    }
    Swift Macro Overview
    SwiftϚΫϩͷ֓ཁ
    func f(a: Int, _: Double, c: Int) {
    
    
    print("f(a:_:c:)")
    
    
    }
    

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13. Swift Macro Overview
    SwiftϚΫϩͷ֓ཁ
    let color = #colorLiteral(red: 0.5, green: 0.5, blue: 0.25, alpha: 1.0)
    let color = .init(_colorLiteralRed: 0.5, green: 0.5, blue: 0.25, alpha: 1.0)
    

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14. func doSomething(_ a: Int, b: Int, c d: Int, e _: Int, _: Int, _ _: Int) {
    
    
    #printArguments()
    
    
    ...
    
    
    }
    Swift Macro Overview
    SwiftϚΫϩͷ֓ཁ
    https://forums.swift.org/t/se-0382-expression-macros/62090/9
    

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15. func doSomething(_ a: Int, b: Int, c d: Int, e _: Int, _: Int, _ _: Int) {
    
    
    #printArguments() // => doSomething(42, b: 256, c: 512, e: _, _, _)
    
    
    ...
    
    
    }
    Swift Macro Overview
    SwiftϚΫϩͷ֓ཁ
    https://forums.swift.org/t/se-0382-expression-macros/62090/9
    

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16. Swift Macro Overview
    SwiftϚΫϩͷ֓ཁ
    func doSomething(_ a: Int, b: Int, c d: Int, e _: Int, _: Int, _ _: Int) {
    
    
    print("doSomething(\(a), b: \(b), c: \(d), e: _, _, _)")
    
    
    ...
    
    
    }
    https://forums.swift.org/t/se-0382-expression-macros/62090/9
    func doSomething(_ a: Int, b: Int, c d: Int, e _: Int, _: Int, _ _: Int) {
    
    
    #printArguments() // => doSomething(42, b: 256, c: 512, e: _, _, _)
    
    
    ...
    
    
    }
    

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17. Swift Macro Overview
    SwiftϚΫϩͷ֓ཁ
    #function "f(a:_:c:)"
    #colorLiteral(
    
    
    red: 0.5,
    
    
    green: 0.5,
    
    
    blue: 0.25,
    
    
    alpha: 1.0
    
    
    )
    .init(
    
    
    _colorLiteralRed: 0.5,
    
    
    green: 0.5,
    
    
    blue: 0.25,
    
    
    alpha: 1.0
    
    
    )
    #printArguments() print("doSomething(\(a), b: \(b), c: \(d), e: _, _, _)”)
    

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18. First impressions about macros
    Pros and cons
    • The code might be harder to read
    
    • Macros are hard to imagine what they do
    
    • Incorrect conversions cause unintended behavior
    
    • Macros are hard to debug
    

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19. First impressions about macros
    Pros and cons
    • The code might be harder to read
    
    • Macros are hard to imagine what they do
    
    • Incorrect conversions cause unintended behavior
    
    • Macros are hard to debug
    
    • Macro can eliminate boilerplate
    

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20. How to implement macros
    

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21. How to implement macros
    #colorLiteral()
    let color = #colorLiteral(red: 0.5, green: 0.5, blue: 0.25, alpha: 1.0)
    

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22. How to implement macros
    #colorLiteral()
    @expression public macro
    
    
    colorLiteral(
    
    
    red: Float, green: Float, blue: Float, alpha: Float
    
    
    ) -> _ColorLiteralType
    
    
    = #externalMacro(module: "MacroExamplesPlugin", type: "ColorLiteralMacro")
    

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23. How to implement macros
    #colorLiteral()
    @expression public macro
    
    
    colorLiteral(
    
    
    red: Float, green: Float, blue: Float, alpha: Float
    
    
    ) -> _ColorLiteralType
    
    
    = #externalMacro(module: "MacroExamplesPlugin", type: "ColorLiteralMacro")
    

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24. How to implement macros
    #colorLiteral()
    @expression public macro
    
    
    colorLiteral(
    
    
    red: Float, green: Float, blue: Float, alpha: Float
    
    
    ) -> _ColorLiteralType
    
    
    = #externalMacro(module: "MacroExamplesPlugin", type: "ColorLiteralMacro")
    

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25. How to implement macros
    #colorLiteral()
    @expression public macro
    
    
    colorLiteral(
    
    
    red: Float, green: Float, blue: Float, alpha: Float
    
    
    ) -> _ColorLiteralType
    
    
    = #externalMacro(module: "MacroExamplesPlugin", type: "ColorLiteralMacro")
    

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26. How to implement macros
    #colorLiteral()
    import SwiftSyntax
    
    
    import _SwiftSyntaxMacros
    
    
    public struct ColorLiteralMacro: ExpressionMacro {
    
    
    public static func expansion(
    
    
    of macro: MacroExpansionExprSyntax,
    
    
    in context: inout MacroExpansionContext
    
    
    ) -> ExprSyntax {
    
    
    let argList = replaceFirstLabel(
    
    
    of: macro.argumentList,
    
    
    with: "_colorLiteralRed"
    
    
    )
    
    
    let initSyntax: ExprSyntax = ".init(\(argList))"
    
    
    if let leadingTrivia = macro.leadingTrivia {
    
    
    return initSyntax.withLeadingTrivia(leadingTrivia)
    
    
    }
    
    
    return initSyntax
    
    
    }
    
    
    }
    

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27. How to implement macros
    #colorLiteral()
    import SwiftSyntax
    
    
    import _SwiftSyntaxMacros
    
    
    public struct ColorLiteralMacro: ExpressionMacro {
    
    
    public static func expansion(
    
    
    of macro: MacroExpansionExprSyntax,
    
    
    in context: inout MacroExpansionContext
    
    
    ) -> ExprSyntax {
    
    
    let argList = replaceFirstLabel(
    
    
    of: macro.argumentList,
    
    
    with: "_colorLiteralRed"
    
    
    )
    
    
    let initSyntax: ExprSyntax = ".init(\(argList))"
    
    
    if let leadingTrivia = macro.leadingTrivia {
    
    
    return initSyntax.withLeadingTrivia(leadingTrivia)
    
    
    }
    
    
    return initSyntax
    
    
    }
    
    
    }
    

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28. How to implement macros
    #colorLiteral()
    import SwiftSyntax
    
    
    import _SwiftSyntaxMacros
    
    
    public struct ColorLiteralMacro: ExpressionMacro {
    
    
    public static func expansion(
    
    
    of macro: MacroExpansionExprSyntax,
    
    
    in context: inout MacroExpansionContext
    
    
    ) -> ExprSyntax {
    
    
    let argList = replaceFirstLabel(
    
    
    of: macro.argumentList,
    
    
    with: "_colorLiteralRed"
    
    
    )
    
    
    let initSyntax: ExprSyntax = ".init(\(argList))"
    
    
    if let leadingTrivia = macro.leadingTrivia {
    
    
    return initSyntax.withLeadingTrivia(leadingTrivia)
    
    
    }
    
    
    return initSyntax
    
    
    }
    
    
    }
    

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29. How to implement macros
    #colorLiteral()
    import SwiftSyntax
    
    
    import _SwiftSyntaxMacros
    
    
    public struct ColorLiteralMacro: ExpressionMacro {
    
    
    public static func expansion(
    
    
    of macro: MacroExpansionExprSyntax,
    
    
    in context: inout MacroExpansionContext
    
    
    ) -> ExprSyntax {
    
    
    let argList = replaceFirstLabel(
    
    
    of: macro.argumentList,
    
    
    with: "_colorLiteralRed"
    
    
    )
    
    
    let initSyntax: ExprSyntax = ".init(\(argList))"
    
    
    if let leadingTrivia = macro.leadingTrivia {
    
    
    return initSyntax.withLeadingTrivia(leadingTrivia)
    
    
    }
    
    
    return initSyntax
    
    
    }
    
    
    }
    

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30. How to implement macros
    #colorLiteral()
    import SwiftSyntax
    
    
    import _SwiftSyntaxMacros
    
    
    public struct ColorLiteralMacro: ExpressionMacro {
    
    
    public static func expansion(
    
    
    of macro: MacroExpansionExprSyntax,
    
    
    in context: inout MacroExpansionContext
    
    
    ) -> ExprSyntax {
    
    
    let argList = replaceFirstLabel(
    
    
    of: macro.argumentList,
    
    
    with: "_colorLiteralRed"
    
    
    )
    
    
    let initSyntax: ExprSyntax = ".init(\(argList))"
    
    
    if let leadingTrivia = macro.leadingTrivia {
    
    
    return initSyntax.withLeadingTrivia(leadingTrivia)
    
    
    }
    
    
    return initSyntax
    
    
    }
    
    
    }
    

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31. How to implement macros
    #colorLiteral()
    import SwiftSyntax
    
    
    import _SwiftSyntaxMacros
    
    
    public struct ColorLiteralMacro: ExpressionMacro {
    
    
    public static func expansion(
    
    
    of macro: MacroExpansionExprSyntax,
    
    
    in context: inout MacroExpansionContext
    
    
    ) -> ExprSyntax {
    
    
    let argList = replaceFirstLabel(
    
    
    of: macro.argumentList,
    
    
    with: "_colorLiteralRed"
    
    
    )
    
    
    let initSyntax: ExprSyntax = ".init(\(argList))"
    
    
    if let leadingTrivia = macro.leadingTrivia {
    
    
    return initSyntax.withLeadingTrivia(leadingTrivia)
    
    
    }
    
    
    return initSyntax
    
    
    }
    
    
    }
    

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32. How to implement macros
    #colorLiteral()
    import SwiftSyntax
    
    
    import _SwiftSyntaxMacros
    
    
    public struct ColorLiteralMacro: ExpressionMacro {
    
    
    public static func expansion(
    
    
    of macro: MacroExpansionExprSyntax,
    
    
    in context: inout MacroExpansionContext
    
    
    ) -> ExprSyntax {
    
    
    let argList = replaceFirstLabel(
    
    
    of: macro.argumentList,
    
    
    with: "_colorLiteralRed"
    
    
    )
    
    
    let initSyntax: ExprSyntax = ".init(\(argList))"
    
    
    if let leadingTrivia = macro.leadingTrivia {
    
    
    return initSyntax.withLeadingTrivia(leadingTrivia)
    
    
    }
    
    
    return initSyntax
    
    
    }
    
    
    }
    

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33. Swift Syntax
    

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34. Swift Syntax
    public static func expansion(
    
    
    of macro: MacroExpansionExprSyntax,
    
    
    in context: inout MacroExpansionContext
    
    
    ) -> ExprSyntax {
    
    
    let argList = replaceFirstLabel(
    
    
    of: macro.argumentList,
    
    
    with: "_colorLiteralRed"
    
    
    )
    
    
    let initSyntax: ExprSyntax = ".init(\(argList))"
    
    
    if let leadingTrivia = macro.leadingTrivia {
    
    
    return initSyntax.withLeadingTrivia(leadingTrivia)
    
    
    }
    
    
    return initSyntax
    
    
    }
    

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35. How to implement macros
    #colorLiteral()
    /// Replace the label of the
    fi
    rst element in the tuple with the given
    
    
    /// new label.
    
    
    private func replaceFirstLabel(
    
    
    of tuple: TupleExprElementListSyntax,
    
    
    with newLabel: String
    
    
    ) -> TupleExprElementListSyntax {
    
    
    guard let firstElement = tuple.first else {
    
    
    return tuple
    
    
    }
    
    
    return tuple.replacing(
    
    
    childAt: 0,
    
    
    with: firstElement.withLabel(.identifier(newLabel))
    
    
    )
    
    
    }
    

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36. How to implement macros
    #colorLiteral()
    /// Replace the label of the
    fi
    rst element in the tuple with the given
    
    
    /// new label.
    
    
    private func replaceFirstLabel(
    
    
    of tuple: TupleExprElementListSyntax,
    
    
    with newLabel: String
    
    
    ) -> TupleExprElementListSyntax {
    
    
    guard let firstElement = tuple.first else {
    
    
    return tuple
    
    
    }
    
    
    return tuple.replacing(
    
    
    childAt: 0,
    
    
    with: firstElement.withLabel(.identifier(newLabel))
    
    
    )
    
    
    }
    

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37. How to implement macros
    #colorLiteral()
    /// Replace the label of the
    fi
    rst element in the tuple with the given
    
    
    /// new label.
    
    
    private func replaceFirstLabel(
    
    
    of tuple: TupleExprElementListSyntax,
    
    
    with newLabel: String
    
    
    ) -> TupleExprElementListSyntax {
    
    
    guard let firstElement = tuple.first else {
    
    
    return tuple
    
    
    }
    
    
    return tuple.replacing(
    
    
    childAt: 0,
    
    
    with: firstElement.withLabel(.identifier(newLabel))
    
    
    )
    
    
    }
    

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38. How to implement macros
    #printArguments()
    func doSomething(_ a: Int, b: Int, c d: Int, e _: Int, _: Int, _ _: Int) {
    
    
    #printArguments()
    
    
    ...
    
    
    }
    

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39. How to implement macros
    #printArguments()
    public struct PrintArgumentsMacro: ExpressionMacro {
    
    
    public static func expansion(
    
    
    of node: MacroExpansionExprSyntax, in context: inout MacroExpansionContext
    
    
    ) -> ExprSyntax {
    
    
    var syntax = node.as(Syntax.self)
    
    
    while syntax != nil && syntax!.as(FunctionDeclSyntax.self) == nil {
    
    
    syntax = syntax!.parent
    
    
    }
    
    
    guard let functionSyntax = syntax!.as(FunctionDeclSyntax.self) else { return "" }
    
    
    let signature: FunctionSignatureSyntax = functionSyntax.signature
    
    
    let parameterList = signature.input.parameterList
    
    
    let parameters = parameterList.map { parameter -> String in
    
    
    let potentialLabel = parameter.firstName!.withoutTrivia().description
    
    
    let label = potentialLabel == "_" ? nil : potentialLabel
    
    
    let potentialName = parameter.secondName?.withoutTrivia().description ?? potentialLabel
    
    
    let name = potentialName == "_" ? nil : potentialName
    
    
    var string: String
    
    
    if let label {
    
    
    string = "\(label): "
    
    
    } else {
    
    
    string = ""
    
    
    }
    
    
    if let name {
    
    
    string += "\\(\(name))"
    
    
    } else {
    
    
    string += "_"
    
    
    }
    
    
    return string
    
    
    }
    
    
    let parametersString = parameters.joined(separator: ", ")
    
    
    return "print(\"\(raw: functionSyntax.identifier.description)(\(raw: parametersString))\")"
    
    
    }
    
    
    }
    

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40. How to implement macros
    #printArguments()
    public struct PrintArgumentsMacro: ExpressionMacro {
    
    
    public static func expansion(
    
    
    of node: MacroExpansionExprSyntax, in context: inout MacroExpansionContext
    
    
    ) -> ExprSyntax {
    
    
    var syntax = node.as(Syntax.self)
    
    
    while syntax != nil && syntax!.as(FunctionDeclSyntax.self) == nil {
    
    
    syntax = syntax!.parent
    
    
    }
    
    
    guard let functionSyntax = syntax!.as(FunctionDeclSyntax.self) else { return "" }
    
    
    let signature: FunctionSignatureSyntax = functionSyntax.signature
    
    
    let parameterList = signature.input.parameterList
    
    
    ...
    
    
    }
    
    
    }
    

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41. How to implement macros
    #printArguments()
    public struct PrintArgumentsMacro: ExpressionMacro {
    
    
    public static func expansion(
    
    
    of node: MacroExpansionExprSyntax, in context: inout MacroExpansionContext
    
    
    ) -> ExprSyntax {
    
    
    var syntax = node.as(Syntax.self)
    
    
    while syntax != nil && syntax!.as(FunctionDeclSyntax.self) == nil {
    
    
    syntax = syntax!.parent
    
    
    }
    
    
    guard let functionSyntax = syntax!.as(FunctionDeclSyntax.self) else { return "" }
    
    
    let signature: FunctionSignatureSyntax = functionSyntax.signature
    
    
    let parameterList = signature.input.parameterList
    
    
    ...
    
    
    }
    
    
    }
    

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42. How to implement macros
    #printArguments()
    public struct PrintArgumentsMacro: ExpressionMacro {
    
    
    public static func expansion(
    
    
    of node: MacroExpansionExprSyntax, in context: inout MacroExpansionContext
    
    
    ) -> ExprSyntax {
    
    
    var syntax = node.as(Syntax.self)
    
    
    while syntax != nil && syntax!.as(FunctionDeclSyntax.self) == nil {
    
    
    syntax = syntax!.parent
    
    
    }
    
    
    guard let functionSyntax = syntax!.as(FunctionDeclSyntax.self) else { return "" }
    
    
    let signature: FunctionSignatureSyntax = functionSyntax.signature
    
    
    let parameterList = signature.input.parameterList
    
    
    ...
    
    
    }
    
    
    }
    

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43. public struct PrintArgumentsMacro: ExpressionMacro {
    
    
    public static func expansion(
    
    
    of node: MacroExpansionExprSyntax, in context: inout MacroExpansionContext
    
    
    ) -> ExprSyntax {
    
    
    ...
    
    
    let parameters = parameterList.map { parameter -> String in
    
    
    let potentialLabel = parameter.firstName!.withoutTrivia().description
    
    
    let label = potentialLabel == "_" ? nil : potentialLabel
    
    
    let potentialName = parameter.secondName?.withoutTrivia().description ?? potentialLabel
    
    
    let name = potentialName == "_" ? nil : potentialName
    
    
    var string: String
    
    
    if let label {
    
    
    string = "\(label): "
    
    
    } else {
    
    
    string = ""
    
    
    }
    
    
    if let name {
    
    
    string += "\\(\(name))"
    
    
    } else {
    
    
    string += "_"
    
    
    }
    
    
    return string
    
    
    }
    
    
    let parametersString = parameters.joined(separator: ", ")
    
    
    return "print(\"\(raw: functionSyntax.identifier.description)(\(raw: parametersString))\")"
    
    
    }
    
    
    }
    

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44. Swift macro essentials
    

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45. Swift macro essentials
    • Type safe
    • Syntactic transformations
    
    • Run at build time
    
    • Additional checks at build time
    
    • Sandboxed
    

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46. Swift macro essentials
    • Type safe
    
    • Syntactic transformations
    • Run at build time
    
    • Additional checks at build time
    
    • Sandboxed
    

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47. Swift macro essentials
    if let leadingTrivia = macro.leadingTrivia {
    
    
    return initSyntax.withLeadingTrivia(leadingTrivia)
    
    
    }
    
    
    return initSyntax
    

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48. Swift macro essentials
    • Type safe
    
    • Syntactic transformations
    
    • Run at build time
    • Additional checks at build time
    • Sandboxed
    

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49. Additional checks at build time
    let homepage = #url("https://tryswift.jp/")
    @expression public macro url(_ string: String) -> URL = ...
    @expression public macro sql(_ string: String) -> SQLQuery = ...
    let query = #sql("SELECT * FROM ...")
    More ideas
    

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50. #URL()
    Compile error if argument is an invalid URL
    public static func expansion(
    
    
    of macro: MacroExpansionExprSyntax,
    
    
    in context: inout MacroExpansionContext
    
    
    ) throws -> ExprSyntax {
    
    
    ...
    
    
    if !validate(macro.argumentList.first) {
    
    
    context.diagnose(
    
    
    Diagnostic(
    
    
    node: Syntax(macro),
    
    
    message: SimpleDiagnosticMessage(
    
    
    message: "...",
    
    
    diagnosticID: MessageID(domain: "...", id: "..."),
    
    
    severity: .error
    
    
    )
    
    
    )
    
    
    )
    
    
    }
    
    
    ...
    
    
    }
    

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51. #URL()
    Compile error if argument is an invalid URL
    

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52. Swift macro essentials
    • Type safe
    
    • Syntactic transformations
    
    • Run at build time
    
    • Additional checks at build time
    
    • Sandboxed
    

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53. Sandboxed
    Embed binaries in apps without bundles
    let data = #embed("images/icon.png")
    
    
    let data = Data([0x4d, 0x49, 0x54, ..., 0x65, 0x6e, 0x73])
    

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54. Sandboxed
    Can I access the
    fi
    le system from macros?
    https://forums.swift.org/t/pitch-2-expression-macros/61861/9
    

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55. Pros and cons
    

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56. Pros and cons
    👍 Macros are type safe
    
    👍 Macros can extend Swift
    
    👍 Macros can cooperate with the build process
    
    🤔 Shoot yourself in the foot with any language
    
    • Macros are hard to imagine what they do
    
    • Macros are hard to debug
    • Macros or Functions / Property Wrappers / Result Builders
    
    🤔 Requires knowledge of Swift Syntax
    
    🤔 Things not suited for macro
    

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57. Pros and cons
    Swift Syntax
    

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58. Things not suited for macro
    • Use functions in preference to macros
    
    • Use Result Builders and Property Wrappers
    
    • Macros are code generators
    Pros and cons
    

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59. Various kinds of macros
    

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60. Various kinds of macros
    Declaration macros (Freestanding macros, Attached macros)
    @addCompletionHandler
    
    
    func fetchAvatar(_ username: String) async -> Image? { ... }
    @declaration(peers: [.overloaded]) macro addCompletionHandler: Void
    /// Expansion of the macro produces the following.
    
    
    func fetchAvatar(_ username: String, completionHandler: @escaping (Image?) -> Void) {
    
    
    Task.detached {
    
    
    completionHandler(await fetchAvatar(username))
    
    
    }
    
    
    }
    

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61. Various kinds of macros
    Declaration macros
    @optionSetMembers
    
    
    struct MyOptions: OptionSet {
    
    
    enum Option: Int {
    
    
    case a
    
    
    case b
    
    
    case c
    
    
    }
    
    
    }
    // Expands to...
    
    
    struct MyOptions: OptionSet {
    
    
    enum Option: Int {
    
    
    case a
    
    
    case b
    
    
    case c
    
    
    }
    
    
    
    
    // Synthesized code below...
    
    
    var rawValue: Int = 0
    
    
    
    
    static var a = MyOptions(rawValue: 1 << Option.a.rawValue)
    
    
    static var b = MyOptions(rawValue: 1 << Option.b.rawValue)
    
    
    static var c = MyOptions(rawValue: 1 << Option.c.rawValue)
    
    
    }
    

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62. Various kinds of macros
    Declaration macros
    @traced(logLevel: 2)
    
    
    func myFunction(a: Int, b: Int) { ... }
    @declaration(body) macro traced(logLevel: Int = 0)
    if shouldLog(atLevel: 2) {
    
    
    log("Entering myFunction((a: \(a), b: \(b)))")
    
    
    }
    

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63. Various kinds of macros
    Declaration macros
    • Replace code generators with a macro
    @swiftCodeGen
    
    
    enum Asset {
    
    
    enum Images {
    
    
    static let banana = ImageAsset(value: "Exotic/Banana")
    
    
    static let mango = ImageAsset(value: "Exotic/Mango")
    
    
    }
    
    
    
    
    enum Colors {
    
    
    static let primary = ColorAsset(value: "Vengo/Primary")
    
    
    static let tint = ColorAsset(value: "Vengo/Tint")
    
    
    }
    
    
    enum Symbols {
    
    
    static let exclamationMark = SymbolAsset(name: "Exclamation Mark")
    
    
    static let plus = SymbolAsset(name: "Plus")
    
    
    }
    
    
    }
    @swiftCodeGen
    
    
    enum Asset {
    
    
    }
    

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64. Practical Example:
    

    Power Assert
    

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65. What is Power Assert?
    let john = Person(name: "John", age: 42)
    
    
    let mike = Person(name: "Mike", age: 13)
    
    
    #powerAssert(mike.isTeenager && john.age < mike.age)
    

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66. What is Power Assert?
    #powerAssert(mike.isTeenager && john.age < mike.age)
    
    
    | | | | | | | |
    
    
    | false | | 42 | | 13
    
    
    | | | | Person(name: "Mike", age: 13)
    
    
    | | | false
    
    
    | | Person(name: "John", age: 42)
    
    
    | false
    
    
    Person(name: "Mike", age: 13)
    let john = Person(name: "John", age: 42)
    
    
    let mike = Person(name: "Mike", age: 13)
    
    
    #powerAssert(mike.isTeenager && john.age < mike.age)
    

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67. Why Power Assert?
    Many di
    ff
    erent assertion functions
    public func XCTAssert()
    
    
    public func XCTAssertEqual()
    
    
    public func XCTAssertEqualWithAccuracy()
    
    
    public func XCTAssertFalse()
    
    
    public func XCTAssertGreaterThan()
    
    
    public func XCTAssertGreaterThanOrEqual()
    
    
    public func XCTAssertIdentical()
    
    
    public func XCTAssertLessThan()
    
    
    public func XCTAssertLessThanOrEqual()
    
    
    public func XCTAssertNil()
    
    
    public func XCTAssertNoThrow()
    
    
    public func XCTAssertNotEqual()
    
    
    public func XCTAssertNotEqualWithAccuracy()
    
    
    public func XCTAssertNotIdentical()
    
    
    public func XCTAssertNotNil()
    
    
    public func XCTAssertThrowsError()
    
    
    public func XCTAssertTrue()
    

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68. Why Power Assert?
    Many di
    ff
    erent assertion functions
    

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69. Why Power Assert?
    Many di
    ff
    erent assertion functions
    

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70. Why Power Assert?
    Many di
    ff
    erent assertion functions
    

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71. What is Power Assert?
    • Only simple assert function such as powerAssert(a == b) is su
    ff
    i
    cient
    
    • Display easy-to-understand information on assert failure (when the test does
    not pass)
    
    • Bene
    fi
    t of not having to use many di
    ff
    erent assertion functions
    https://azu.github.io/slide/sakurajs/power-assert.html#/2
    

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72. Why Power Assert?
    That's all you need
    @expression
    
    
    public macro powerAssert(_ expression: @autoclosure () throws -> Bool)
    

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73. Implement Power Assert with
    macros
    

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74. Implement Power Assert with macros
    #powerAssert(a * b == 91)
    

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75. Implement Power Assert with macros
    https://swift-ast-explorer.com/
    Swift AST Explorer
    

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76. Implement Power Assert with macros
    #powerAssert(a * b == 91)
    TupleExprElementList
    Inspect the macro syntax structure
    

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77. Implement Power Assert with macros
    #powerAssert(a * b == 91)
    TupleExprElementList
    Identi
    fi
    erExpr
    Inspect the macro syntax structure
    

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78. Implement Power Assert with macros
    #powerAssert(a * b == 91)
    TupleExprElementList
    Identi
    fi
    erExpr
    BinaryOperatorExpr
    Inspect the macro syntax structure
    

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79. Implement Power Assert with macros
    #powerAssert(a * b == 91)
    TupleExprElementList
    Identi
    fi
    erExpr
    BinaryOperatorExpr
    IntegerLiteralExpr
    Inspect the macro syntax structure
    

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80. Implement Power Assert with macros
    PowerAssert.Assertion(#"#powerAssert(a * b == 91)"#, line: 2)
    
    
    .assert(a * b == 91)
    
    
    .capture(expression: a.self, column: 13)
    
    
    .capture(expression: b.self, column: 17)
    
    
    .capture(expression: a * b == 91, column: 19)
    
    
    .capture(expression: 91, column: 22)
    
    
    .render()
    

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81. Implement Power Assert with macros
    PowerAssert.Assertion(#"#powerAssert(a * b == 91)"#, line: 2)
    
    
    .assert(a * b == 91)
    
    
    .capture(expression: a.self, column: 13)
    
    
    .capture(expression: b.self, column: 17)
    
    
    .capture(expression: a * b == 91, column: 19)
    
    
    .capture(expression: 91, column: 22)
    
    
    .render()
    

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82. Implement Power Assert with macros
    PowerAssert.Assertion(#"#powerAssert(a * b == 91)"#, line: 2)
    
    
    .assert(a * b == 91)
    
    
    .capture(expression: a.self, column: 13)
    
    
    .capture(expression: b.self, column: 17)
    
    
    .capture(expression: a * b == 91, column: 19)
    
    
    .capture(expression: 91, column: 22)
    
    
    .render()
    

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83. Implement Power Assert with macros
    PowerAssert.Assertion(#"#powerAssert(a * b == 91)"#, line: 2)
    
    
    .assert(a * b == 91)
    
    
    .capture(expression: a.self, column: 13)
    
    
    .capture(expression: b.self, column: 17)
    
    
    .capture(expression: a * b == 91, column: 19)
    
    
    .capture(expression: 91, column: 22)
    
    
    .render()
    

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84. Implement Power Assert with macros
    PowerAssert.Assertion(#"#powerAssert(a * b == 91)"#, line: 2)
    
    
    .assert(a * b == 91)
    
    
    .capture(expression: a.self, column: 13)
    
    
    .capture(expression: b.self, column: 17)
    
    
    .capture(expression: a * b == 91, column: 19)
    
    
    .capture(expression: 91, column: 22)
    
    
    .render()
    

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85. Implement Power Assert with macros
    PowerAssert.Assertion(#"#powerAssert(a * b == 91)"#, line: 2)
    
    
    .assert(a * b == 91)
    
    
    .capture(expression: a.self, column: 13)
    
    
    .capture(expression: b.self, column: 17)
    
    
    .capture(expression: a * b == 91, column: 19)
    
    
    .capture(expression: 91, column: 22)
    
    
    .render()
    // => #powerAssert(a * b == 91)
    
    
    | | | |
    
    
    10 9 | 91
    
    
    false
    

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86. Implement Power Assert with macros
    PowerAssert.Assertion(#"#powerAssert(a * b == 91)"#, line: 2)
    
    
    .assert(a * b == 91)
    
    
    .capture(expression: a.self, column: 13)
    
    
    .capture(expression: b.self, column: 17)
    
    
    .capture(expression: a * b == 91, column: 19)
    
    
    .capture(expression: 91, column: 22)
    
    
    .render()
    // => #powerAssert(a * b == 91)
    
    
    | | | |
    
    
    10 9 | 91
    
    
    false
    #powerAssert(a * b == 91)
    

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87. Implement Power Assert with macros
    #powerAssert(String(data: data, encoding: .utf8) == "test")
    FunctionCallExpr
    Identi
    fi
    erExpr MemberAccessExpr
    

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88. Implement Power Assert with macros
    #powerAssert(String(data: data, encoding: .utf8) == "test")
    FunctionCallExpr
    Identi
    fi
    erExpr MemberAccessExpr
    

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89. Implement Power Assert with macros
    PowerAssert.Assertion(#"#powerAssert(String(data: data, encoding: .utf8) == "test")"#, line: 2)
    
    
    .assert(String(data: data, encoding: .utf8) == "test")
    
    
    .capture(expression: String(data: data, encoding: .utf8), column: 13)
    
    
    .capture(expression: data.self, column: 26)
    
    
    .capture(expression: .utf8, column: 42)
    
    
    .capture(expression: String(data: data, encoding: .utf8) == "test", column: 49)
    
    
    .capture(expression: "test", column: 52)
    
    
    .render()
    

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90. Implement Power Assert with macros
    PowerAssert.Assertion(#"#powerAssert(String(data: data, encoding: .utf8) == "test")"#, line: 2)
    
    
    .assert(String(data: data, encoding: .utf8) == "test")
    
    
    .capture(expression: String(data: data, encoding: .utf8), column: 13)
    
    
    .capture(expression: data.self, column: 26)
    
    
    .capture(expression: .utf8, column: 42)
    
    
    .capture(expression: String(data: data, encoding: .utf8) == "test", column: 49)
    
    
    .capture(expression: "test", column: 52)
    
    
    .render()
    

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91. Implement Power Assert with macros
    .capture(expression: data.self, column: 26)
    
    
    .capture(expression: .utf8, column: 42)
    
    
    .capture(expression: String(data: data,
    No type information in syntax tree
    

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92. Implement Power Assert with macros
    https://github.com/apple/swift-evolution/blob/main/proposals/0382-expression-macros.md#macro-argument-type-information
    No type information in syntax tree
    

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93. Implement Power Assert with macros
    https://github.com/apple/swift-evolution/blob/main/proposals/0382-expression-macros.md#macro-argument-type-information
    

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94. Implement Power Assert with macros
    https://github.com/apple/swift-evolution/blob/main/proposals/0382-expression-macros.md#example-expression-macros
    

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95. https://github.com/kishikawakatsumi/swift-power-assert
    Implement Power Assert with macros
    

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96. Summary
    Macros are fun!
    • Macros are a very powerful feature that can extend Swift itself
    
    • Shoot yourself in the foot if used incorrectly
    
    • Swift Syntax becomes more important
    
    • Macros are hard to debug
    
    • Post your ideas!
    

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97. Easy? Difficult?
    https://forums.swift.org/t/pitch-declaration-macros/62373/21
    

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98. Easy? Difficult?
    https://forums.swift.org/t/pitch-declaration-macros/62373/25
    

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99. Summary
    Macros are fun!
    • Macros are a very powerful feature that can extend Swift itself
    
    • Shoot yourself in the foot if used incorrectly
    
    • Swift Syntax becomes more important
    
    • Macros are hard to debug
    
    • Post your ideas!
    

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100. References
     • A Possible Vision for Macros in Swift

     https://forums.swift.org/t/a-possible-vision-for-macros-in-swift/60900
     
     • Swift project in 2023

     https://www.swift.org/blog/focus-areas-2023/
     
     • [Pitch] Expression macros

     https://forums.swift.org/t/pitch-expression-macros/61499
     
     • [Pitch #2] Expression macros

     https://forums.swift.org/t/pitch-2-expression-macros/61861
     
     • SE-0382: Expression Macros

     https://forums.swift.org/t/se-0382-expression-macros/62090
     
     • [Pitch] Declaration macros

     https://forums.swift.org/t/pitch-declaration-macros/62373/36
     
     • 5෼͙Β͍ͰΘ͔Δpower assert

     https://azu.github.io/slide/sakurajs/power-assert.html
     

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