Slightly moving forward. Simple binary comprehension and options (all but :reduce) do work.

iex|1 ▶ Enum.to_list(stream <<x <- "abcabca">>, do: x)
'abcabca'
iex|2 ▶ Enum.to_list(stream <<x <- "abcabca">>, uniq: true, do: x)
'abc'
iex|3 ▶ stream <<x <- "abcabca">>, take: 2, do: x                 
'ab'

Also :into works as a side effect (as described here)

I am not sure I am into implementing full bitstring support, but :reduce will come soon.

Well, the latest hot discussion about semi-mutable not actually mutable syntactic sugar reminded me of my tech debt and I implemented reduce:.

  test "binary for comprehensions with reduce, generators and filters" do
    bin = "abc"

    acc =
      stream <<x <- bin>>, Integer.is_odd(x), <<y <- "hello">>, reduce: %{} do
        inner_acc -> Map.update(inner_acc, x, [y], &[y | &1])
      end

    assert acc == %{97 => ~c"olleh", 99 => ~c"olleh"}
  end

Under the hood, the slightly modified inner clause is applied to terminate the stream. Enjoy.