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Swift Macros

Disclaimer: This is not definitive, just my understanding.

Macro Types

• Freestanding Expression macro

  • can basically do let a = #macroName(args)
  • Declaration
    • example declaration keyword: @freestanding(declaration, names: named(MyClass))
      • (yanked directly from the examples - unsure if names is required, but it kinda seems like it is)
    • Generates [DeclSyntax]
  • Expression
    • example declaration keyword: @freestanding(expression)
    • Generates ExprSyntax

• Attached Macros

  • Accessor
    • Adds accessors (like get, set, didSet, willSet, and more complex ones too (see docs)
    • example declaration keyword: @attached(accessor)
    • Generates [AccessorDeclSyntax]
  • CodeItem
    • could not find example declaration keyword example
    • Generates [CodeBlockItemSyntax]
  • Extension
    • example declaration keyword: @attached(extension, conformances: Equatable)
      • (again yanked from examples. conformances doesn't look explicitly required, but the one time it's omitted there's an alternative argument provided)
    • Generates [ExtensionDeclSyntax]

  • Member

    • Can add members to the declaration it's attached to

    • example declaration keyword:

      @attached(
        member,
        names: named(Storage),
        named(_storage),
        named(_registrar),
        named(addObserver),
        named(removeObserver),
        named(withTransaction)
      )

      • appears as if names can take one or more arguments
    • Generates [DeclSyntax]

    • I believe this means something like

      @MahMancro
      class Foo {}

      Can expand to something like

      class Foo {
          var bar: Int
          func baz () {}
      }

  • MemberAttribute

    • Can add attributes to members inside the declaration it's attached to
    • example declaration keyword: @attached(memberAttribute)
    • Generates [AttributeSyntax]

    • I believe this means something like

      @MahMancro
      class Foo {
          var bar: Int
          var baz: String
      }

      Can expand to something like

      class Foo {
          @MahSubMancro
          var bar: Int
          @MahSubMancro
          var bax: String
      }

  • Peer

    • Can add "peer" declarations alongside its attached declaration

    • examples of declaration keyword:

      @attached(peer, names: overloaded)
      @attached(peer, names: suffixed(_peer))

    • Generates [DeclSyntax]

    • I believe this means something like

      @MahMancro
      let url = URL(string: "https://the-google.com/")

      Can expand to something like

      let url = URL(string: "https://the-google.com/")
      let url_request = URLRequest(url: url)

references:

• swift ast
• swift-syntax docs
• macro-examples (dated 4/21/24)
  • macro protocol headers (dated 4/21/24)
• Mah Man

this document last modified: April 22 2024 00:23

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