Gestalt

An experiment and tutorial on how to create a solid and portable macros system based on solid methodology and principles.

Design Goals:

• Portable -- better IDE experience, works both in Scala and Dotty
• Solid -- compiler crashes to a minimum, no hack required in writing macros
• Friendly -- no knowledge about compiler internals, no dependent universes
• Hygienic -- hygiene guaranteed by the type system

Examples

import scala.gestalt.api_

object Test {
  def plus(a: Int, b: Int): Int = meta {
    q"$a + $b"
  }
}

Features

Gestalt advances the state of the art of meta-programming in Scala:

• Macros are portable among different Scala compilers
• Better IDE support due to portability
• Hygienic macros ensured by type system
• Automatic owner chain management
• Type system defends against incorrect mixing of typed and untyped trees
• Friendly semantic APIs
• No inconvenience related to path-dependent universes
• Easy to implement by compiler vendors

Philosophy

The design space of a macro system is large. Gestalt aims to be simple, solid and portable for the 80% common use cases.

a ) Assumptions on compiler implementation

• more assumptions
• fewer assumptions (✓) : portability & easy implementation

b ) Assumptions on macro usage

• more assumptions (✓) : focus on 80% common cases, make them easy
• fewer assumptions

c) Features

• more features
• fewer features (✓) : metaprogramming is hard, restrict power, implementation solid

Test

• sbt macros/test:run

Paper

Gestalt : The Road for Lightweight Portable Macros