Re: Writing a Macro to Test if Value Equals to 5

Actually, I tried to use a `safe constructor` approach, i.e. make the case 
class's constructor to be private & add a factory method.

But that approach did not work.

*Equals5.scala*

package net

import scala.language.experimental.macros
import scala.reflect.macros.Context

case class Equals5 private (value: Int) {
require(value == 5)
}

object Equals5 {

  def build(n: Int): Equals5 = macro verifyIntEquals5

  def verifyIntEquals5(c: Context)(n: c.Expr[Int]): c.Expr[Equals5] = {
    import c.universe._

    val tree = n.tree match {
      case Literal(Constant(x: Int)) if x == 5 =>
        q"_root_.net.Equals5($n)"
      case Literal(Constant(x: Int)) =>
        c.abort(c.enclosingPosition, s"$x != 0")
      case _ => 
        q"_root_.net.Equals5($n)"
    }
    c.Expr(tree)
  }
}

*Testing:*

scala> Equals5.build(1000)

<console>:12: error: 1000 != 0

       Equals5.build(1000)

                    ^


scala> Equals5.build(5)

<console>:12: error: constructor Equals5 in class Equals5 cannot be 
accessed in object $iw

       Equals5.build(5)

                    ^



On Saturday, September 12, 2015 at 10:51:25 PM UTC-4, Kevin Meredith wrote:
>
> Given the following macro:
>
> package net
>
> import scala.language.experimental.macros
> import scala.reflect.macros.Context
>
> case class Equals5(value: Int) {
> require(value == 5)
> }
>
> object Equals5 {
> implicit def wrapInt(n: Int): Equals5 = macro verifyIntEquals5
>
> def verifyIntEquals5(c: Context)(n: c.Expr[Int]): c.Expr[Equals5] = {
>     import c.universe._
>
>     val tree = n.tree match {
>       case Literal(Constant(x: Int)) if x == 5 =>
>         q"_root_.net.Equals5($n)"
>       case Literal(Constant(x: Int)) =>
>         c.abort(c.enclosingPosition, s"$x != 0")
>       case _ => 
>         q"_root_.net.Equals5($n)"
>     }
>     c.Expr(tree)
>   }
> }
>
> I wrote the following tests using scalatest:
>
> package net
>
> import org.scalatest.Matchers
>
> import org.scalatest._
> import org.scalatest.prop.Checkers._
>
> class Equals5Test extends FlatSpec with Matchers {
>
> it should "throw an IllegalArgumentException when trying to make an 
> invalid `Equals5` directly" in {
> intercept[IllegalArgumentException] {
> Equals5(-555) //
> }
> }
>
> it should "not compile when using the macro constructor to build an 
> `Equals5` with a constant" in {
> """
> import net.Equals5._; wrapInt(-555)
> """.shouldNot(compile)
> } 
> }
>
> Both tests succeed.
>
> But, I'd like for `Equals5.apply` to use the `Equals5.wrapInt` method - to 
> make use of the macro.
>
> I believe that I could do this by making Equals5#apply to be private, and 
> then require a factory method (that uses the macro) 
> to be used in order to create an `Equals5` class.
>
> Is there a better alternative? Can the `implicit def wrapInt` be used? Or 
> do implicits not get used during the macro compile-time stage?
>
> Thanks,
> Kevin
>

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