Using Macro to Make Case Class - scala

Question

Given the following macro (thanks #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"$x must be > 0.")
case Left(NotConstant) => reify { dimension.splice }
}
}
def getInt(c: Context)(dimension: c.Expr[Int]): Either[PosIntCheckResult, Int] = {
import c.universe._
dimension.tree match {
case Literal(Constant(x: Int)) => if (x > 0) Right(x) else Left(LteqZero(x))
case _ => Left(NotConstant)
}
}
}
It works from the REPL:
scala> import spire.math.JetDim
import spire.math.JetDim
scala> JetDim.validate(-55)
<console>:9: error: -55 must be > 0.
JetDim.validate(-55)
^
scala> JetDim.validate(100)
res1: Int = 100
But, I'd like to build this compile-time check (via the JetDimMacro) into the case class's apply method.
Attempt 1
case class JetDim(dimension: Int) {
require(dimension > 0)
}
object JetDim {
private def validate(dimension: Int): Int = macro JetDimMacro.apply
def build(dimension: Int): JetDim = JetDim(validate(dimension))
}
But that failed:
scala> import spire.math.JetDim
import spire.math.JetDim
scala> JetDim.build(-55)
java.lang.IllegalArgumentException: requirement failed
at scala.Predef$.require(Predef.scala:207)
at spire.math.JetDim.<init>(Jet.scala:21)
at spire.math.JetDim$.build(Jet.scala:26)
... 43 elided
Attempt 2
class JetDim(dim: Int) {
require(dim > 0)
def dimension: Int = dim
}
object JetDim {
private def validate(dimension: Int): Int = macro JetDimMacro.apply
def apply(dimension: Int): JetDim = {
validate(dimension)
new JetDim(dimension)
}
}
Yet that failed too:
scala> import spire.math.JetDim
import spire.math.JetDim
scala> JetDim(555)
res0: spire.math.JetDim = spire.math.JetDim#4b56f205
scala> JetDim(-555)
java.lang.IllegalArgumentException: requirement failed
at scala.Predef$.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 modify JetDimMacro#apply to return a JetDim rather than an Int. However, JetDim lives in the core project, which, from what I see, depends on the macros project (where JetDimMacro lives).
How can I use this validate method from JetDim's companion object to check for positive int's at compile-time?

Answer 1

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 non-constant Int.
As an alternative, you could try using an implicit witness for positive ints, which JetDim then takes as a constructor. For instance, 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 that does your check.
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)
}
}
You can then use JetDim(12), which will pass, or JetDim(-12), which will fail (the macro expands the Int to a PositiveInt).