simulacrum

Type classes rock. Alas, their encoding in Scala requires a lot of boilerplate, which doesn't rock. There is inconsistency between projects, where type classes are encoded differently. There is inconsistency within projects, where object-oriented forwarders (aka. ops, syntax) accidentally differ in exact parameter lists or forwarders are missing where they are expected to be. Even in disciplined teams, the bike-shedding opportunities alone are a source of lost productivity.

This project addresses these concerns by introducing first class support for type classes in Scala 2.11. For example:

import simulacrum._

@typeclass trait Semigroup[A] {
  @op("|+|") def append(x: A, y: A): A
}

Given this definition, something similar to the following is generated at compile time:

trait Semigroup[A] {
  def append(x: A, y: A): A
}

object Semigroup {
  implicit def apply[A](implicit tc: Semigroup[A]): Semigroup[A] = tc

  implicit class Adapter[A](val self: A)(implicit val typeClass: Semigroup[A]) {
    def append(y: A): A = typeClass.append(self, y)
    def |+|(y: A): A = typeClass.append(self, y)
  }
}

This isn't exactly what's generated -- for instance, the Adapter class is really generated as a trait to support various subtyping cases. Subtyping of type classes is supported (e.g., you can define a Monoid type class that extends Semigroup and the generated code adapts accordingly). Higher kinds are also supported -- specifically, type classes that are polymorphic over type constructors, like Functor.

This allows usage like:

implicit val semigroupInt: Semigroup[Int] = new Semigroup[Int] {
  def append(x: Int, y: Int) = x + y
}

import Semigroup.Adapter
1 |+| 2 // 3

This project currently only supports Scala 2.11. The project is based on macro paradise. To use the project, add the following to your build.sbt:

resolvers += "Sonatype Public" at "https://oss.sonatype.org/content/groups/public/"

addCompilerPlugin("org.scalamacros" % "paradise" % "2.0.1" cross CrossVersion.full)

libraryDepdencies += "com.github.mpilquist" %% "simulacrum" % "0.1.0-SNAPSHOT"

Feedback is much appreciated. The generated code is a result of working with project leads of a variety of open source projects that use type classes. However, there's certainly room for improvement, so please open issues or PRs containg feedback. Also, see the TODO.md file for near term improvements.

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simulacrum

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