Welcome to Scala 2.11.8 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_92).
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scala> trait RationalTrait {
     |   val numerArg: Int
     |   val denomArg: Int
     | }
defined trait RationalTrait

scala>

scala> new RationalTrait {
     |   val numerArg = 1
     |   val denomArg = 2
     | }
res0: RationalTrait = $anon$1@5a9f4771

scala>

scala> val x = 2
x: Int = 2

scala>

scala> new RationalTrait {
     |   val numerArg = 1 * x
     |   val denomArg = 2 * x
     | }
res1: RationalTrait = $anon$1@15713d56

scala>

scala> trait RationalTrait {
     |   val numerArg: Int
     |   val denomArg: Int
     |   require(denomArg != 0)
     |   private val g = gcd(numerArg, denomArg)
     |   val numer = numerArg / g
     |   val denom = denomArg / g
     |   private def gcd(a: Int, b: Int): Int =
     |     if (b == 0) a else gcd(b, a % b)
     |   override def toString = numer + "/" + denom
     | }
defined trait RationalTrait

scala>

scala> new RationalTrait {
     |   val numerArg = 1
     |   val denomArg = 2
     | }
java.lang.IllegalArgumentException: requirement failed
  at scala.Predef$.require(Predef.scala:212)
  at RationalTrait$class.$init$(<console>:14)
  ... 36 elided

scala>

scala> new RationalTrait {
     |   val numerArg = 1 * x
     |   val denomArg = 2 * x
     | }
java.lang.IllegalArgumentException: requirement failed
  at scala.Predef$.require(Predef.scala:212)
  at RationalTrait$class.$init$(<console>:14)
  ... 36 elided

scala>

scala> new {
     |   val numerArg = 1
     |   val denomArg = 2
     | } with RationalTrait
res4: RationalTrait = 1/2

scala>

scala> new {
     |   val numerArg = 1 * x
     |   val denomArg = 2 * x
     | } with RationalTrait
res5: RationalTrait = 1/2

scala>