Newts

Better newtypes for Scala based on the following two articles:

• The High Cost of AnyVal subclasses...
• ...and the glorious subst to come

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.