• Like a function definition, macro name(arg) -> result
• Proper type checking, unlike C
• Macros can be freestanding or attached
• Implementation is defined in a compiler plugin
• Macro definition is provided a swift syntax tree, and returns a separate one
  • Tree includes the macro itself
  • Compiler replaces the provided tree with the result tree
• In xcode, right click, expand macro: to see the result of a macro in source code
  • Or, jump to definition
• The macro function is declared as an externalMacro with the module and type that holds the implementation.
• The implementation must conform to ExpressionMacro (for freestanding macros)
• Macro invokes swift parser to convert return string to a swift tree
• Macro template comes with unit tests, assertMacroExpansion
• Roles
  • Freestanding expression, with a hash symbol
  • Freestanding declaration
  • Attached, with an at symbol
    • peer, accessor, member, memberAttribute, conformance
• Print syntax node structure using debugger in test cases
• Can define generic parameters on the macros

Expand on swift macros

• Macros can have multiple roles, and will be expanded based on context
  • All roles that apply will be expanded, which could be more than one for a single macro usage
  • Each one will expand based on the original code, they wont compose and the order won’t matter