The consequences of broken laws. Sensible code reviewer, realises you should not use foldRight in scala, so refactors to foldLeft. Mwhahaha.

BrokenLaws.scala

// Don't try this at home
object BrokenLaws {
  trait Monoid[F] {
    def zero: F
    def append(a: F, b: F): F
  }

  implicit def DoubleMonoid: Monoid[Double] = new Monoid[Double] {
    def zero = 0.0
    def append(a: Double, b: Double) = a + b
  }

  def sum[A](xs: List[A])(implicit M: Monoid[A]) =
    xs.foldRight(M.zero)((a, acc) => M.append(a, acc))
}


object ContractKiller {
  import BrokenLaws._

  def transactions = List(-8.988465674311579E307, 8.988465674311579E307, 1.0)

  def main(args: Array[String]) {
    val owing = sum(transactions)
    if (owing > 0.0)
      println("bang!!!")
    else
      println("have a nice day")
  }
}

BrokenLawsWithoutStackOverflow.scala

// Don't try this at home
object BrokenLaws {
  trait Monoid[F] {
    def zero: F
    def append(a: F, b: F): F
  }

  implicit def DoubleMonoid: Monoid[Double] = new Monoid[Double] {
    def zero = 0.0
    def append(a: Double, b: Double) = a + b
  }

  def sum[A](xs: List[A])(implicit M: Monoid[A]) =
    xs.foldLeft(M.zero)((acc, a) => M.append(a, acc))
}


object ContractKiller {
  import BrokenLaws._

  def transactions = List(-8.988465674311579E307, 8.988465674311579E307, 1.0)

  def main(args: Array[String]) {
    val owing = sum(transactions)
    if (owing > 0.0)
      println("bang!!!")
    else
      println("have a nice day")
  }
}