macro_playground.ex

defmodule MacroPlayground do

  defmacro para(ast) do
    # 0. Store the pid of the current process
    me = self

    pids = Macro.prewalk(ast, fn
      # 1. Pattern matching can be done with function arguments!
      {:for, meta, args} ->
        # 2. Extract the do block of the comprehension
        [do: do_block] = args |> List.last

      # 3. Wrap the do block around a spawn. Send the result to the
      #    current process.
      spawn_do_block = quote do
        spawn(fn -> send(unquote(me), {self, unquote(do_block)}) end)
      end

      # 4. Swap out the `do_block` (the last element of `args`)
      #    with the `spawn_do_block`
      {:for, meta, List.replace_at(args, -1, [do: spawn_do_block])}

      # 5. We just output the same node for any other node
      node ->
        node
    end)

    quote do
      unquote(pids)
      |> Enum.map(fn pid ->
        receive do
          {^pid, result} ->
            result
        end
      end)
    end
  end
end

defmodule Foo do
  import MacroPlayground

  def foo do

    x = para(for a <- 1..10,
      b <- 1..10,
      c <- 1..10,
      a + b + c <= 10000,
   do: {a, b, c})

    IO.inspect x
  end
end

Foo.foo