WillBeTrue

WillBeTrue.scala

// (This is an older version of the Gist here: https://gist.github.com/lancelet/0d0713af7eb1401c9b1a)
// (Only hanging around because I posted it to a Gitter group.)
// Intended to be run in shapeless current master 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 tree = c.parse(s"{ { $codeStr }: Boolean }")
      val boolExpr = c.Expr[Boolean](tree)
      val result: Boolean = c.eval(boolExpr)
      q"$result"
    } 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 """)

// expected result:
// wontCompile: Boolean = false
// willCompile: Boolean = true
// compilesButWrong: Boolean = false

// limitation: this doesn't work; my macro-fu fails me
val x = true; WillBeTrue(""" x """) // false (arrgh! - why, Scala, why?)
// ok - I figured out the answer. c.eval() is attempting to evaluate x at compile time,
// and so it doesn't know the value. Instead, you have to splice a tree back in which
// refers to x, like in this (updated) Gist:
// https://gist.github.com/lancelet/0d0713af7eb1401c9b1a