cons.ex

defmodule Cons do
  def each_cons(list, num) when length(list) < num, do: []
  def each_cons(list, num) do
    [ _ | tail ] = list
    cons = Enum.take(list, num)
    [ cons | each_cons(tail, num) ]
  end
end

iex(1)> Cons.each_cons(String.to_char_list("abacabaca"), 4)
['abac', 'baca', 'acab', 'caba', 'abac', 'baca']
iex(2)> Cons.each_cons(String.to_char_list("abacabaca"), 3)
['aba', 'bac', 'aca', 'cab', 'aba', 'bac', 'aca']
iex(3)> Cons.each_cons(String.to_char_list("abacabaca"), 2)
['ab', 'ba', 'ac', 'ca', 'ab', 'ba', 'ac', 'ca']

Doug, yes that version of Dave's you posted in the other gist is what I have pretty much verbatim. The only syntax issue I see is that the double slash (//) used to mean a default parameter. That syntax was later changed to be double backslash(\).

There's also been another addition to the standard library since then, so I wouldn't use that today. Instead use:

Enum.chunk('abacabaca', 4, 1)

That third parameter is an offset. When not supplied explicitly it's the same as the 2nd parameter -- the chunk size.

That said, there's still value in writing something like this yourself as a learning exercise. As I mentioned on the mailing list, the most obvious difference is that Dave used an explicit accumulator result. Upon looking at it more closely, I do see a deeper issue. Dave's solution is properly tail recursive -- the very last step is a function call. The VM is optimized at at a low level and treats this as a jump and will not create a new stack frame. In your solution the last step is to construct a list. Therefore each of your intermediate values are stored on the stack and could overflow. Dave's explicit recursive use of result and Enum.reverse at the end is a common pattern in tail recursion. Enum.reverse looks inefficient, but the VM has heavily optimized that operation because it's a common pattern with tail recursion.

Any chance you're going to make it to ElixirConf in a couple of weeks? I'd love to hear more about your thoughts and plans with Elixir.

Great feedback - thanks! I should have caught the syntax issue with default parameters. Just getting started with Elixir but really liking it.

I would love to go to ElixirConf sometime, but unfortunately won't be making it this year. Hope to catch up with you soon, though.