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Metaprogramming in Scala 3

The Scala 2.13 macros are compiler-dependent by design. A macro compiled by one version of the compiler cannot be used by another version of the compiler.

Scala 3.0 is overcoming this limitation by introducing a new principled approach of metaprogramming. While this is an uncontested improvement, it also means that previous macro implementations have to be rewritten from the ground up.

Before rewriting a macro

Before getting deep into reimplementing a macro your should check if it can be supported using Scala 3.0 new features.

• Can I encode the logic of the macro using the new scala 3 features?
• Can I use match types to reimplement the interface of my macro?
• Can I use inline and the metaprogramming features in scala.compiletime to reimplement my logic?
• Can I use the simpler and safer expression based macros to implement my macro?
• Do I really need to have access to the raw AST trees?

You can find references to these concepts in the macro tutorial.

Macro turorial

• Scala 3 Macro Tutorial
  • Inline
  • Compile-time operations
  • Macros
  • Quoted code
  • AST Reflection

Additional Resources

Documentation:

• Scala 3.0 Documentation
• Macros: The Plan For Scala 3
• Examples - a repository with small, self-contained examples of various tasks done with Scala 3 macros.

Talks:

• Scala Days - Metaprogramming in Dotty

Projects:

• XML Interpolator
• Shapeless 3

Contributors welcome!