Elixir Cartesian Product Macro

cp.ex

  @doc """
  cp produces cartesian product from a list of lists, calling handler on each
  result. Handler is expected to receive as many arguments as there are groups.

  ## Examples

      iex> HelloWorld.cp([["10", "20"], ["a", "b", "c"], ["xx", "yy"]], fn(x, y,z) -> {x,y,z} end)
      [{"10", "a", "xx"}, {"10", "a", "yy"}, {"10", "b", "xx"}, {"10", "b", "yy"},
       {"10", "c", "xx"}, {"10", "c", "yy"}, {"20", "a", "xx"}, {"20", "a", "yy"},
       {"20", "b", "xx"}, {"20", "b", "yy"}, {"20", "c", "xx"}, {"20", "c", "yy"}]

  """
  defmacro cp(groups, handler) do    
    args = (for idx <- 0..length(groups)-1, do: String.to_atom("a#{idx}"))
           |> Enum.map(fn(i) -> Macro.var(i, __MODULE__) end)

    ast = quote do
      for i <- [] do
        apply(unquote(handler), unquote(args))
      end
    end

    Macro.prewalk(ast, fn(x) ->
      case x do
        {:for, [], iters} ->
          [_ | t] = iters
          {
            :for,
            [],
            (
              groups |>
              Stream.with_index |>
              Enum.map(
                fn({val, idx}) -> {
                  :<-, [], [{String.to_atom("a#{idx}"), [], unquote(__MODULE__)}, val]
                } end)
            ) ++ t
          }

        _ ->
          x 
      end
    end)
  end