使用Elixir语言具有递归功能的流

Question

In scala the code below works (I know it's weird, but I want to test stream concat with recursive functions)

def ones(s: Stream[Int]): Stream[Int] = 1 #:: ones(s)
ones(Stream.from(1)).take(10).toList

I wrote some elixir code that should be equivalent to the scala code and it hangs. How can I get the same effect as the above scala code?

defmodule Ones do
  def ones(s) do
    head = Stream.take(s, 1)
    Stream.concat(head, ones(s))
  end
end

Ones.ones(Stream.iterate(1, &(&1 + 1))) |> Enum.take(10) # hang

Answer 1

It seems the #:: operator in scala evaluates its right argument lazily. That's why you can construct "infinite" recursions like that and it still works out. In Elixir there is no notion of lazily evaluated arguments, so ones(s) always calls ones(s) at the end resulting in an infinite loop. In order to avoid that you'd need to use a Stream function that explicitly deals with lazy values to construct your result, for example Stream.resource .