condi.exs

defmodule M do
  def test do
    cond do
      length([1,2,3]) == 0 -> false
      true
        "oops, I forgot the arrow"
    end
  end
end

gistfile1.txt

λ elixirc condi.exs
condi.exs:4: warning: this check/guard will always yield the same result

λ iex
Erlang/OTP 17 [erts-6.1] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:10] [hipe] [kernel-poll:false] [dtrace]

Interactive Elixir (0.15.1) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> M.test
** (CondClauseError) no cond clause evaluated to a true value
    M.test/0