scala user-defined-types scala-macros newtype

Do I need to define functions when using newtype in Scala?

I'm trying to learn about types more and use them to help avoid silly errors.

I asked a similar question here, but ran into problems that are similar to what I show here (I think), and ultimately abandoned the answer. A comment to that question suggested I look into newtype.

It seems simpler on the surface, but I'm still left scratching my head.

I have this code:

  package com.craigtreptow.scrayz
  import io.estatico.newtype.macros.newtype

    package object Colors {

    def multiply(c: Color, n: Double): Color = {
      Color(
        c.red   * n,
        c.green * n,
        c.blue  * n
       )
    }

    @newtype case class Red(toDouble: Double)
    case class Color(red: Red, green: Double, blue: Double)
}

The above produces these errors:

[info] Compiling 1 Scala source to /Users/Ctreptow/code/scrayz/target/scala-2.13/classes ...
[error] /Users/Ctreptow/code/scrayz/src/main/scala/com/craigtreptow/scrayz/Colors/package.scala:11:15: value * is not a member of com.craigtreptow.scrayz.Colors.package.Red
[error]       c.red   * n,
[error]               ^
[error] one error found
[error] (Compile / compileIncremental) Compilation failed
[error] Total time: 4 s, completed Apr 30, 2020 3:19:36 PM

I think I should be able to automatically derive the * since this new type is ultimately a Double.

Can I? If not, how do I define my own function with a param? e.g. *

Solution

You can define * manually

@newtype case class Red(toDouble: Double) {
  def *(n: Double): Red = Red(toDouble * n)
}

   // scalacOptions += "-Ymacro-debug-lite"
//Warning:scalac: {
//  type Red = Red.Type;
//  object Red extends scala.AnyRef {
//    def <init>() = {
//      super.<init>();
//      ()
//    };
//    def apply(toDouble: Double): Red = toDouble.asInstanceOf[Red];
//    final implicit class Ops$newtype extends AnyVal {
//      <paramaccessor> val $this$: Type = _;
//      def <init>($this$: Type) = {
//        super.<init>();
//        ()
//      };
//      def toDouble: Double = $this$.asInstanceOf[Double];
//      def $times(n: Double): Red = Red(toDouble.$times(n))
//    };
//    implicit def opsThis(x: Ops$newtype): Type = x.$this$;
//    @new _root_.scala.inline() implicit def unsafeWrap: Coercible[Repr, Type] = Coercible.instance;
//    @new _root_.scala.inline() implicit def unsafeUnwrap: Coercible[Type, Repr] = Coercible.instance;
//    @new _root_.scala.inline() implicit def unsafeWrapM[M[_]]: Coercible[M[Repr], M[Type]] = Coercible.instance;
//    @new _root_.scala.inline() implicit def unsafeUnwrapM[M[_]]: Coercible[M[Type], M[Repr]] = Coercible.instance;
//    @new _root_.scala.inline() implicit def cannotWrapArrayAmbiguous1: Coercible[_root_.scala.Array[Repr], _root_.scala.Array[Type]] = Coercible.instance;
//    @new _root_.scala.inline() implicit def cannotWrapArrayAmbiguous2: Coercible[_root_.scala.Array[Repr], _root_.scala.Array[Type]] = Coercible.instance;
//    @new _root_.scala.inline() implicit def cannotUnwrapArrayAmbiguous1: Coercible[_root_.scala.Array[Type], _root_.scala.Array[Repr]] = Coercible.instance;
//    @new _root_.scala.inline() implicit def cannotUnwrapArrayAmbiguous2: Coercible[_root_.scala.Array[Type], _root_.scala.Array[Repr]] = Coercible.instance;
//    def deriving[TC[_]](implicit ev: TC[Repr]): TC[Type] = ev.asInstanceOf[TC[Type]];
//    type Repr = Double;
//    type Base = _root_.scala.Any {
//      type __Red__newtype
//    };
//    abstract trait Tag extends _root_.scala.Any;
//    type Type <: Base with Tag
//  };
//  ()
//}

If you want to derive all methods automatically (using corresponding methods of Double), normally this can be done with scala.Dynamic + macro

import scala.language.dynamics
import scala.language.experimental.macros
import scala.reflect.macros.whitebox

case class Red(toDouble: Double) extends Dynamic {
  def applyDynamic(method: String)(args: Any*): Any = macro Macro.impl
}

object Macro {
  def impl(c: whitebox.Context)(method: c.Tree)(args: c.Tree*): c.Tree = {
    import c.universe._
    val q"${methodName: String}" = method
    q"${c.prefix}.toDouble.${TermName(methodName).encodedName.toTermName}(..$args)"
  }
}

object Colors {
  val c: Color = ???
  val n: Double = ???
  c.red * n
}

//Warning:scalac: performing macro expansion Colors.this.c.red.applyDynamic("*")(Colors.this.n) ...
//Warning:scalac: Colors.this.c.red.toDouble.$times(Colors.this.n)

but unfortunately this will not work with @newtype

@newtype case class Red(toDouble: Double) extends Dynamic {
  def applyDynamic(method: String)(args: Any*): Any = macro Macro.impl
}
//Error: newtypes do not support inheritance; illegal supertypes: Dynamic

So you can define one more macro annotation @exportMethods

import scala.annotation.{StaticAnnotation, compileTimeOnly}
import scala.language.experimental.macros
import scala.reflect.macros.blackbox

@compileTimeOnly("enable macro paradise")
class exportMethods extends StaticAnnotation {
  def macroTransform(annottees: Any*): Any = macro ExportMethodsMacro.impl
}

object ExportMethodsMacro {
  def impl(c: blackbox.Context)(annottees: c.Tree*): c.Tree = {
    import c.universe._
    annottees match {
      case q"$mods class $tpname[..$tparams] $ctorMods(...$paramss) extends { ..$earlydefns } with ..$parents { $self => ..$stats }" :: tail =>
        val exportedMethods = paramss match {
          case (q"$_ val $paramName: $paramType = $_" :: Nil) :: Nil =>
            val paramTyp = c.typecheck(tq"$paramType", mode = c.TYPEmode).tpe
            paramTyp.decls.map(_.asMethod)
              .filterNot(s => Set(paramName, TermName("getClass"), TermName("<init>")).contains(s.name))
              .map(s => {
                val paramss1 = s.paramLists.map(_.map(s => q"val ${s.name.toTermName}: ${s.typeSignature}"))
                val paramss2 = s.paramLists.map(_.map(s => q"${s.name.toTermName}"))
                if (s.returnType =:= paramTyp)
                  q"def ${s.name}(...$paramss1): $tpname = ${tpname.toTermName}.apply($paramName.${s.name}(...$paramss2))"
                else
                  q"def ${s.name}(...$paramss1): ${s.returnType} = $paramName.${s.name}(...$paramss2)"
              })
          case _ => c.abort(c.enclosingPosition, "class must have single parameter")
        }
        q"""
          $mods class $tpname[..$tparams] $ctorMods(...$paramss) extends { ..$earlydefns } with ..$parents { $self =>
            ..$stats
            ..$exportedMethods
          }
          ..$tail
        """
    }
  }
}

and use it with @newtype

import io.estatico.newtype.macros.newtype

@exportMethods @newtype case class Red(toDouble: Double)

//Warning:scalac: {
//  type Red = Red.Type;
//  object Red extends scala.AnyRef {
//    def <init>() = {
//      super.<init>();
//      ()
//    };
//    def apply(toDouble: Double): Red = toDouble.asInstanceOf[Red];
//    final implicit class Ops$newtype extends AnyVal {
//      <paramaccessor> val $this$: Type = _;
//      def <init>($this$: Type) = {
//        super.<init>();
//        ()
//      };
//      def toDouble: Double = $this$.asInstanceOf[Double];
//      def toByte: Byte = toDouble.toByte;
//      def toShort: Short = toDouble.toShort;
//      def toChar: Char = toDouble.toChar;
//      def toInt: Int = toDouble.toInt;
//      def toLong: Long = toDouble.toLong;
//      def toFloat: Float = toDouble.toFloat;
//      def unary_$plus: Red = Red.apply(toDouble.unary_$plus);
//      def unary_$minus: Red = Red.apply(toDouble.unary_$minus);
//      def $plus(x: String): String = toDouble.$plus(x);
//      def $eq$eq(x: Byte): Boolean = toDouble.$eq$eq(x);
//      def $eq$eq(x: Short): Boolean = toDouble.$eq$eq(x);
//      def $eq$eq(x: Char): Boolean = toDouble.$eq$eq(x);
//      def $eq$eq(x: Int): Boolean = toDouble.$eq$eq(x);
//      def $eq$eq(x: Long): Boolean = toDouble.$eq$eq(x);
//      def $eq$eq(x: Float): Boolean = toDouble.$eq$eq(x);
//      def $eq$eq(x: Double): Boolean = toDouble.$eq$eq(x);
//      def $bang$eq(x: Byte): Boolean = toDouble.$bang$eq(x);
//      def $bang$eq(x: Short): Boolean = toDouble.$bang$eq(x);
//      def $bang$eq(x: Char): Boolean = toDouble.$bang$eq(x);
//      def $bang$eq(x: Int): Boolean = toDouble.$bang$eq(x);
//      def $bang$eq(x: Long): Boolean = toDouble.$bang$eq(x);
//      def $bang$eq(x: Float): Boolean = toDouble.$bang$eq(x);
//      def $bang$eq(x: Double): Boolean = toDouble.$bang$eq(x);
//      def $less(x: Byte): Boolean = toDouble.$less(x);
//      def $less(x: Short): Boolean = toDouble.$less(x);
//      def $less(x: Char): Boolean = toDouble.$less(x);
//      def $less(x: Int): Boolean = toDouble.$less(x);
//      def $less(x: Long): Boolean = toDouble.$less(x);
//      def $less(x: Float): Boolean = toDouble.$less(x);
//      def $less(x: Double): Boolean = toDouble.$less(x);
//      def $less$eq(x: Byte): Boolean = toDouble.$less$eq(x);
//      def $less$eq(x: Short): Boolean = toDouble.$less$eq(x);
//      def $less$eq(x: Char): Boolean = toDouble.$less$eq(x);
//      def $less$eq(x: Int): Boolean = toDouble.$less$eq(x);
//      def $less$eq(x: Long): Boolean = toDouble.$less$eq(x);
//      def $less$eq(x: Float): Boolean = toDouble.$less$eq(x);
//      def $less$eq(x: Double): Boolean = toDouble.$less$eq(x);
//      def $greater(x: Byte): Boolean = toDouble.$greater(x);
//      def $greater(x: Short): Boolean = toDouble.$greater(x);
//      def $greater(x: Char): Boolean = toDouble.$greater(x);
//      def $greater(x: Int): Boolean = toDouble.$greater(x);
//      def $greater(x: Long): Boolean = toDouble.$greater(x);
//      def $greater(x: Float): Boolean = toDouble.$greater(x);
//      def $greater(x: Double): Boolean = toDouble.$greater(x);
//      def $greater$eq(x: Byte): Boolean = toDouble.$greater$eq(x);
//      def $greater$eq(x: Short): Boolean = toDouble.$greater$eq(x);
//      def $greater$eq(x: Char): Boolean = toDouble.$greater$eq(x);
//      def $greater$eq(x: Int): Boolean = toDouble.$greater$eq(x);
//      def $greater$eq(x: Long): Boolean = toDouble.$greater$eq(x);
//      def $greater$eq(x: Float): Boolean = toDouble.$greater$eq(x);
//      def $greater$eq(x: Double): Boolean = toDouble.$greater$eq(x);
//      def $plus(x: Byte): Red = Red.apply(toDouble.$plus(x));
//      def $plus(x: Short): Red = Red.apply(toDouble.$plus(x));
//      def $plus(x: Char): Red = Red.apply(toDouble.$plus(x));
//      def $plus(x: Int): Red = Red.apply(toDouble.$plus(x));
//      def $plus(x: Long): Red = Red.apply(toDouble.$plus(x));
//      def $plus(x: Float): Red = Red.apply(toDouble.$plus(x));
//      def $plus(x: Double): Red = Red.apply(toDouble.$plus(x));
//      def $minus(x: Byte): Red = Red.apply(toDouble.$minus(x));
//      def $minus(x: Short): Red = Red.apply(toDouble.$minus(x));
//      def $minus(x: Char): Red = Red.apply(toDouble.$minus(x));
//      def $minus(x: Int): Red = Red.apply(toDouble.$minus(x));
//      def $minus(x: Long): Red = Red.apply(toDouble.$minus(x));
//      def $minus(x: Float): Red = Red.apply(toDouble.$minus(x));
//      def $minus(x: Double): Red = Red.apply(toDouble.$minus(x));
//      def $times(x: Byte): Red = Red.apply(toDouble.$times(x));
//      def $times(x: Short): Red = Red.apply(toDouble.$times(x));
//      def $times(x: Char): Red = Red.apply(toDouble.$times(x));
//      def $times(x: Int): Red = Red.apply(toDouble.$times(x));
//      def $times(x: Long): Red = Red.apply(toDouble.$times(x));
//      def $times(x: Float): Red = Red.apply(toDouble.$times(x));
//      def $times(x: Double): Red = Red.apply(toDouble.$times(x));
//      def $div(x: Byte): Red = Red.apply(toDouble.$div(x));
//      def $div(x: Short): Red = Red.apply(toDouble.$div(x));
//      def $div(x: Char): Red = Red.apply(toDouble.$div(x));
//      def $div(x: Int): Red = Red.apply(toDouble.$div(x));
//      def $div(x: Long): Red = Red.apply(toDouble.$div(x));
//      def $div(x: Float): Red = Red.apply(toDouble.$div(x));
//      def $div(x: Double): Red = Red.apply(toDouble.$div(x));
//      def $percent(x: Byte): Red = Red.apply(toDouble.$percent(x));
//      def $percent(x: Short): Red = Red.apply(toDouble.$percent(x));
//      def $percent(x: Char): Red = Red.apply(toDouble.$percent(x));
//      def $percent(x: Int): Red = Red.apply(toDouble.$percent(x));
//      def $percent(x: Long): Red = Red.apply(toDouble.$percent(x));
//      def $percent(x: Float): Red = Red.apply(toDouble.$percent(x));
//      def $percent(x: Double): Red = Red.apply(toDouble.$percent(x))
//    };
//    implicit def opsThis(x: Ops$newtype): Type = x.$this$;
//    @new _root_.scala.inline() implicit def unsafeWrap: Coercible[Repr, Type] = Coercible.instance;
//    @new _root_.scala.inline() implicit def unsafeUnwrap: Coercible[Type, Repr] = Coercible.instance;
//    @new _root_.scala.inline() implicit def unsafeWrapM[M[_]]: Coercible[M[Repr], M[Type]] = Coercible.instance;
//    @new _root_.scala.inline() implicit def unsafeUnwrapM[M[_]]: Coercible[M[Type], M[Repr]] = Coercible.instance;
//    @new _root_.scala.inline() implicit def cannotWrapArrayAmbiguous1: Coercible[_root_.scala.Array[Repr], _root_.scala.Array[Type]] = Coercible.instance;
//    @new _root_.scala.inline() implicit def cannotWrapArrayAmbiguous2: Coercible[_root_.scala.Array[Repr], _root_.scala.Array[Type]] = Coercible.instance;
//    @new _root_.scala.inline() implicit def cannotUnwrapArrayAmbiguous1: Coercible[_root_.scala.Array[Type], _root_.scala.Array[Repr]] = Coercible.instance;
//    @new _root_.scala.inline() implicit def cannotUnwrapArrayAmbiguous2: Coercible[_root_.scala.Array[Type], _root_.scala.Array[Repr]] = Coercible.instance;
//    def deriving[TC[_]](implicit ev: TC[Repr]): TC[Type] = ev.asInstanceOf[TC[Type]];
//    type Repr = Double;
//    type Base = _root_.scala.Any {
//      type __Red__newtype
//    };
//    abstract trait Tag extends _root_.scala.Any;
//    type Type <: Base with Tag
//  };
//  ()
//}

Testing:

multiply(Color(Red(1.0), 2.0, 3.0), 4.0) //Color(4.0,8.0,12.0)

Order of annotations is significant (firstly @exportMethods is expanded, secondly @newtype is expanded).