Effekt

effect Flip(): Boolean
effect Fail(): Nothing

def choice(n: Int): Int / { Flip, Fail } = {
  if (n < 1) { do Fail() }
  else if (do Flip()) { n }
  else { choice(n - 1) }
}

def triple(n: Int): (Int, Int, Int) / { Flip, Fail } = {
  val i = choice(n)
  val j = choice(i - 1)
  val k = choice(j - 1)
  if ((i * i) == (j * j) + (k * k)) {
    (i, j, k)
  } else {
    do Fail()
  }
}

def solutions(n: Int): List[(Int, Int, Int)] = {
  try {
    val t = triple(n)
    [t]
  } with Flip {
    () => resume(true).append(resume(false))
  } with Fail {
    () => []
  }
}

def main() = {
  solutions(20).foreach { t =>
    println(t)
  }
}