WillBeTrue (Improved)

WillBeTrue.scala

// Intended to be run in shapeless sbt console; use :paste

//--- paste this first to define the macro

import scala.language.experimental.macros

import scala.reflect.macros.whitebox
import scala.util.control.NonFatal

object WillBeTrue {
  def apply(code: String): Boolean = macro WillBeTrueMacros.applyImpl
}

@macrocompat.bundle
class WillBeTrueMacros(val c: whitebox.Context) {
  import c.universe._

  def applyImpl(code: Tree): Tree = {
    // unpack the string
    val Literal(Constant(codeStr: String)) = code

    // try to compile it and evaluate it as a Boolean
    try {
      val dummy0 = TermName(c.freshName)
      val dummy1 = TermName(c.freshName)
      val tree = c.parse(s"{ object $dummy0 { val $dummy1: Boolean = { $codeStr } }; $dummy0.$dummy1 }")
      c.typecheck(tree)
      tree
    } catch {
      case NonFatal(e) => 
        println(s"Exception: $e")
        q"false"
    }
  }
}

//--- paste this second

val wontCompile: Boolean = WillBeTrue(""" ("LambdaJam!": Int) == 42 """)
val willCompile: Boolean = WillBeTrue(""" (42: Int) == 42 """)
val compilesButWrong: Boolean = WillBeTrue(""" (42: Int) == 43 """)

// also captures the environment
val x = true; WillBeTrue(""" x """)

Works for me! I think this'd make a great addition to shapeless, prolly in the getOrElse form ..