Using a macro to create a class

Given the following macro (thanks to @TravisBrown for this help ):

JetDim.scala

case class JetDim(dimension: Int) {
  require(dimension > 0)
}

object JetDim {
  def validate(dimension: Int): Int = macro JetDimMacro.apply
  def build(dimension: Int): JetDim = JetDim(validate(dimension))
}

      

JetDimMacro.scala

import reflect.macros.Context

object JetDimMacro {

    sealed trait PosIntCheckResult
    case class LteqZero(x: Int) extends PosIntCheckResult
    case object NotConstant extends PosIntCheckResult

    def apply(c: Context)(dimension: c.Expr[Int]): c.Expr[Int] = {

        import c.universe._

        getInt(c)(dimension) match {
            case Right(_)          => reify { dimension.splice }
            case Left(LteqZero(x)) => c.abort(c.enclosingPosition, s"$x must be > 0.")
            case Left(NotConstant) => reify { dimension.splice }
        }
    }

    def getInt(c: Context)(dimension: c.Expr[Int]): Either[PosIntCheckResult, Int] = {

        import c.universe._

        dimension.tree match {
            case Literal(Constant(x: Int)) => if (x > 0) Right(x) else Left(LteqZero(x))
            case _                         => Left(NotConstant)
        }
    }
}

      

Works with REPL:

scala> import spire.math.JetDim
import spire.math.JetDim

scala> JetDim.validate(-55)
<console>:9: error: -55 must be > 0.
              JetDim.validate(-55)
                             ^

scala> JetDim.validate(100)
res1: Int = 100

      

But I would like to build this compile-time check (via JetDimMacro

) in a case class method apply

.

Attempt 1

case class JetDim(dimension: Int) {
  require(dimension > 0)
}

object JetDim {
  private def validate(dimension: Int): Int = macro JetDimMacro.apply
  def build(dimension: Int): JetDim = JetDim(validate(dimension))
}

      

But it failed:

scala> import spire.math.JetDim
import spire.math.JetDim

scala> JetDim.build(-55)
java.lang.IllegalArgumentException: requirement failed
  at scala.Predef$.require(Predef.scala:207)
  at spire.math.JetDim.<init>(Jet.scala:21)
  at spire.math.JetDim$.build(Jet.scala:26)
  ... 43 elided

      

Attempt 2

class JetDim(dim: Int) {
  require(dim > 0)

  def dimension: Int = dim
}

object JetDim {
  private def validate(dimension: Int): Int = macro JetDimMacro.apply
  def apply(dimension: Int): JetDim = {
    validate(dimension)
    new JetDim(dimension)
  }
}

      

But that also failed:

scala> import spire.math.JetDim
import spire.math.JetDim

scala> JetDim(555)
res0: spire.math.JetDim = spire.math.JetDim@4b56f205

scala> JetDim(-555)
java.lang.IllegalArgumentException: requirement failed
  at scala.Predef$.require(Predef.scala:207)
  at spire.math.JetDim.<init>(Jet.scala:21)
  at spire.math.JetDim$.apply(Jet.scala:30)
  ... 43 elided

      

I thought to change JetDimMacro#apply

to return JetDim

, not Int

. However JetDim

lives in a project core

which I can see depends on the project macros

(where it JetDimMacro

lives).

How can I use this method validate

from JetDim

the companion object to check for a positive int at compile time?

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1 answer


The problem is that by the time we call validate

in apply

, we are no longer dealing with a constant (singleton type). So validate gets a volatile Int.

Alternatively, you can try using an implicit witness for positive int, which JetDim then accepts as a constructor. For example, something like:

package com.example

case class JetDim(n: PositiveInt)

case class PositiveInt(value: Int) {
  require(value > 0)
}

      



Then we add an implicit (macro) conversion from Int => PositiveInt

which does your validation.

import scala.language.experimental.macros

import scala.reflect.macros.blackbox.Context

object PositiveInt {
  implicit def wrapConstantInt(n: Int): PositiveInt = macro verifyPositiveInt

  def verifyPositiveInt(c: Context)(n: c.Expr[Int]): c.Expr[PositiveInt] = {
    import c.universe._

    val tree = n.tree match {
      case Literal(Constant(x: Int)) if x > 0 =>
        q"_root_.com.example.PositiveInt($n)"
      case Literal(Constant(x: Int)) =>
        c.abort(c.enclosingPosition, s"$x <= 0")
      case x =>
        c.abort(c.enclosingPosition, s"cannot verify $x > 0")
    }
    c.Expr(tree)
  }
}

      

Then you can use JetDim(12)

which will pass, or JetDim(-12)

which will fail (the macro expands Int to PositiveInt).

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