Kernel.to_boolean/1

kernel_to_boolean.ex

defmodule Kernel do
  @doc """
  Converts any expression to boolean.

  Returns `true` if `expr` is truthy (ie. it does not evaluate to `nil`, nor `false`),
  otherwise returns the `false`.

  Allowed in guard tests.
  """
  @spec to_boolean(Macro.t) :: boolean
  defmacro to_boolean(expr) do
    quote do
      not unquote(expr) in [false, nil]
    end
  end
end

defmodule Kernel.ToBooleanTest do
  use ExUnit.Case, async: true

  def has_second_and_third_char?(string) when to_boolean(binary_part(string, 1, 1)) and to_boolean(binary_part(string, 2, 1)),
    do: true
  def has_second_and_third_char?(_),
    do: false

  test "Kernel.to_boolean/1 in guards" do
    assert has_second_and_third_char?("f") == false
    assert has_second_and_third_char?("") == false
    assert has_second_and_third_char?("fo") == false
    assert has_second_and_third_char?("foo") == true
    assert has_second_and_third_char?("foooooo") == true
  end

  test "Kernel.to_boolean/1" do
    assert to_boolean(true) == true
    assert to_boolean(false) == false
    assert to_boolean(nil) == false
    assert to_boolean("") == true
    assert to_boolean([]) == true
    assert to_boolean("yes") == true
  end
end