Convert a binary string of 0s and 1s to a bitstring in elixir

Given a string such as "0110", how do I convert this to a bitstring containing the binary values represented by the string?

I checked the Base module, but it's based on RFC 4648 and is not designed for handling Base 2.

Here's one way:

defmodule Convert do
  def to_bitstring(str) when is_binary(str) do
    for <<byte::binary-1 <- str>>, into: <<>> do
      case byte do
        "0" -> <<0::1>>
        "1" -> <<1::1>>
      end
    end
  end
end

Usage:

iex> Convert.to_bitstring("0110")
<<6::size(4)>>

The benefit of doing it exhaustively using case and matching on binaries is two-fold:

1. The function will reject invalid characters
2. The error message in the above case is easy to understand:

iex> Convert.to_bitstring("0140")
** (CaseClauseError) no case clause matching: "4"

If you just want a quick hack, this works, but it will also happily convert nonsense like "0140" too, so I think the first solution is better.

for <<byte <- str>>, into: <<>>, do: <<(byte - ?0)::1>>