Solution to http://puzzles.sheki.in/puzzle/hamming_numbers/

Hamming.scala

 def merge(as: Stream[Int], bs: Stream[Int], cs: Stream[Int]): Stream[Int] = {
    val h = as.head min bs.head min cs.head
    Stream.cons(
      h,
      merge(as dropWhile {_ == h}, bs dropWhile {_ == h}, cs dropWhile {_ == h})
    )
  }

This function produces a stream containing a constant times each value from another:

  val prod = (k: Int) => (s: Stream[Int]) => s.map{k * _}

With those definitions in hand…

  def hamming: Stream[Int] = Stream.cons(
    1,
    merge(prod(2)(hamming), prod(3)(hamming), prod(5)(hamming))
  )