Step-by-step walkthrough of part 1

/commits/part1

Summary of part 1

Inspired by the work of Adam Rosenberger in nalloc, we've sketched a macro-powered zero-overhead option-like class. Thanks to macros, both Adam and us have been able to provide high-order combinators getOrElse and map that get compiled down to first-order code.

final class Optional[+A >: Null](val value: A) extends AnyVal {
  def get: A = value
  def isEmpty = value == null
  def getOrElse[B >: A](alt: => B): B = macro OptionalMacros.getOrElse
  def map[B](f: A => B): Optional[B] = macro OptionalMacros.map
  override def toString = if (isEmpty) "<empty>" else s"$value"
}

Basic macrology

• https://github.com/scalamacros/sbt-example provides a nice template to get a macro project started.
• Use macro bundles and quasiquotes - they significantly simplify writing macros.
• Know your debugging tools: showCode, showRaw, -Xprint:typer and other compiler flags. In the meanwhile we'll be working on IDE integration and other improvements to developer experience.
• Careful with control flow (preserve evaluation order, be sure to cache values of macro arguments if they are used multiple times in expansions)
• Mind hygiene (use c.freshName when generating local variables, when possible use _root_-qualified names to refer to external dependencies). This is something that we hope to do automatically in the future, but for now hygiene needs your attention.
• Most of what we've seen here applies to Scala 2.10, except for macro bundles, relaxed signatures for macro impls and showCode, which require Scala 2.11. Quasiquotes aren't shipping with official Scala 2.10, but they are provided by the macro paradise plugin.

Advanced macrology

• Make sure that your macro isn't susceptible to owner chain corruptions. A good way to test that is to pass owner-chain sensitive tree shapes in its prefix and arguments (see provided tests for a simple yet characteristic tree shape).
• If your macro crashes in GenICode or LambdaLift, it's most likely owner chain corruption, and you need to do c.internal.changeOwner.
• If your macro crashes in RefChecks, then it's probably an unsupported kind of mix of typed and untyped trees, and you need to go for c.internal.typingTransform.
• All this applies equally to both 2.10 and 2.11. However, c.internal only exists in 2.11, on 2.10 you'll have to cast your way to scala.tools.nsc.Global - both techniques are demonstrated in the accompanying code.
• We're working on making all these advanced recipes unnecessary, but there's still a long way to go.

Let's keep on moving!

Materialization, reflection and blackbox/whitebox macros: /tree/part2.