You can break this down into 3 easy steps using the modules that @benwilson512 recommends.

1. Convert string to integer String.to_integer("3")

2. Get binary value of integer Integer.to_string(3, 2)

3. Pad binary string with zeros String.pad_leading("11", 8, "0")

You can write the code with guard clauses to handle strings or integers.

  def decimal_to_binary(decimal) when is_binary(decimal) do
    decimal
    |> String.to_integer()
    |> decimal_to_binary()
  end

  def decimal_to_binary(decimal) when is_integer(decimal) do
    decimal
    |> Integer.to_string(2)
    |> String.pad_leading(8, "0")
  end

Oh yeah, it should be "~8.2.0B"

Just format it properly - List.to_string(:io_lib.format("~8.2B", [3]))