Use is_bitstring or is_binary in Elixir Guard for String?

Question

Let's take this simple function:

  @spec laBionda(String.t()) :: String.t()
  def laBionda(name \\ "you") when is_bitstring(name) do
    "One for #{name}, one for me"
  end

I only want to define the function for String inputs.

Should I use is_bitstring or is_binary on the Guard? Are there any differences? Both seem to be fine in that case.

Answer 1

You should use is_binary/1.

Strings in Elixir are represented as binaries. Elixir binaries are sequences of bytes, whereas bitstrings are sequences of bits. While all binaries are bitstrings, not all bitstrings are binaries.

is_bitstring/1 can return true for some bitstrings that cannot be represented as a binary, for example, the single bit:

iex(1)> is_binary(<<1::1>>)
false
iex(2)> is_bitstring(<<1::1>>)
true

You expect strings only. Bitstrings that are not binaries are never desired, so the more specific is_binary/1 is the better choice.