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src/main/scala/newts Improved implicit resolution, new implementation. Sep 26, 2017
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README.md

Newts

Better newtypes for Scala based on the following two articles:

Quick Start

addCompilerPlugin("org.scalameta" % "paradise" % "3.0.0-M10" cross CrossVersion.full)
resolvers += Resolver.bintrayRepo("alexknvl", "maven")
libraryDependencies += "com.alexknvl"  %%  "newtypes" % "0.3.0"

Use this fork of scalameta/paradise until https://github.com/scalameta/paradise/pull/207 is merged in if you need companion object support and can't use paradise-3.0.0-M10 (it supports only 2.12.3+). Clone it and publishM2 in sbt, then change the paradise plugin to:

addCompilerPlugin("org.scalameta" % "paradise" % "3.0.0-alex" cross CrossVersion.full)

Why newts?

Features AnyVal @newtypes.opaque @newtypes.translucent
isInstanceOf Yes No No
Generics box AnyRef subtypes Yes No No
Generics box primitives Yes Yes Yes
Primitives box No Yes No
Up-cast works No No Yes
Down-cast works No No No
Any methods Yes Yes Yes
Overrides Yes No No
Virtual dispatch Yes No No
Typeclass-based dispatch Yes Yes Yes
Wrap List elements O(N) O(1) (using subst) O(1) (using subst)
Unwrap List elements O(N) O(1) (using subst and Is) O(1) (automatic widening)
Supports HK parameters Yes Yes Yes
Supports existential parameters Yes Yes Yes

What does it do?

@opaque type ArrayWrapper[A] = Array[A]

@translucent type Flags = Int

becomes

type ArrayWrapper[A] = ArrayWrapper.Type[A]
object ArrayWrapper {
  type Base$$1
  trait Tag$$1 extends Any
  type Type[A] <: Base$$1 with Tag$$1
  
  object Impl {
    def apply[A](value: Array[A]): Type[A] = value.asInstanceOf[Type[A]]
    def unwrap[A](value: Type[A]): Array[A] = value.asInstanceOf[Array[A]]
    def subst[F[_], A](value: F[Array[A]]): F[Type[A]] = value.asInstanceOf[F[Type[A]]]
  }
}

type Flags = Flags.Type
object Flags {
  trait Tag$$1 extends Any
  type Type <: Int with Tag$$1
  object Impl {
    def apply(value: Int): Type = value.asInstanceOf[Type]
    def unwrap(value: Type): Int = value.asInstanceOf[Int]
    def subst[F[_]](value: F[Int]): F[Type] = value.asInstanceOf[F[Type]]
  }
}

License

Code is provided under the MIT license available at https://opensource.org/licenses/MIT, as well as in the LICENSE file.