foo.ex

defmodule Foo do
  def bar(:a, opts \\ []), do: :a
  def bar(:b, opts \\ []), do: :b
end

gistfile1.txt

» iex
Erlang/OTP 22 [erts-10.6.4] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [hipe]

Interactive Elixir (1.10.2) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> c "foo.ex"
warning: variable "opts" is unused (if the variable is not meant to be used, prefix it with an underscore)
  foo.ex:2: Foo.bar/2

warning: variable "opts" is unused (if the variable is not meant to be used, prefix it with an underscore)
  foo.ex:3: Foo.bar/2


== Compilation error in file foo.ex ==
** (CompileError) foo.ex:3: def bar/2 defines defaults multiple times. Elixir allows defaults to be declared once per definition. Instead of:

    def foo(:first_clause, b \\ :default) do ... end
    def foo(:second_clause, b \\ :default) do ... end

one should write:

    def foo(a, b \\ :default)
    def foo(:first_clause, b) do ... end
    def foo(:second_clause, b) do ... end

    foo.ex:3: (module)
** (CompileError)  compile error
    (iex 1.10.2) lib/iex/helpers.ex:200: IEx.Helpers.c/2
iex(1)>