`is_empty` Guard for Binaries in Elixir

• tricks 51
• elixir 81

I have many times heard the question “How would one implement is_empty guard for binaries in Elixir.”

It is impossible out of the box, basically because guards are compile-time beasts. The documentation on Guards states:

Guards are a way to augment pattern matching with more complex checks; they are allowed in a predefined set of constructs where pattern matching is allowed.

That said, one can not just put String.trim(s) == "" because call to String.trim is not allowed in guard:

iex|1 ▶ defmodule TestEmpty, do: def test(s) when String.trim(s) == "", do: :ok
** (CompileError) iex:1: cannot invoke remote function String.trim/1 inside guard

So far, so bad. But there is a nifty workaround: one might explicitly trim_leading the String with pattern match:

defmodule TestEmpty do
  def trimmer(<<" " :: binary, rest :: binary>>), do: trimmer(rest)

  def trimmer(string) when string == "",
    do: IO.puts "Empty input"
  def trimmer(string) when is_binary(string),
    do: IO.puts "Left trimmed string is: [#{string}]"
end

TestEmpty.trimmer "       "
#⇒ Empty input
TestEmpty.trimmer "    ☆  "
#⇒ Left trimmed string is: [☆  ]

This approach won’t work if one needs to deal with the original input unless it is “empty,” but in most cases it would do the trick. After all, we for some reason already treat spaces as empties.