gistfile1.txt

defmodule Specs do
  @type network :: :mainnet | :testnet3
  @spec magic(network) :: binary
  defp magic(:mainnet) do
    <<0xf9,0xbe,0xb4,0xd9>>
  end
  defp magic(:testnet3) do
    <<0x0b, 0x11, 0x09, 0x07>>
  end
end

iex(1)> c "defp.ex"
defp.ex:4: warning: function magic/1 is unused

== Compilation error on file defp.ex ==
** (CompileError) defp.ex:3: spec for undefined function Specs.magic/1
    (stdlib) lists.erl:1336: :lists.foreach/2
    (stdlib) erl_eval.erl:657: :erl_eval.do_apply/6

** (exit) shutdown: 1
    (elixir) lib/kernel/parallel_compiler.ex:199: Kernel.ParallelCompiler.handle_failure/5
    (elixir) lib/kernel/parallel_compiler.ex:182: Kernel.ParallelCompiler.wait_for_messages/8
    (elixir) lib/kernel/parallel_compiler.ex:55: Kernel.ParallelCompiler.spawn_compilers/3
       (iex) lib/iex/helpers.ex:94: IEx.Helpers.c/2