FSM snippet at https://www.meetup.com/Elixir-Shanghai/events/238614338/

fsm.exs

defmodule FSM do
  defmacro __using__(opts) do
    quote do
      import FSM.DSL, only: [trans: 3]
      Module.register_attribute(__MODULE__, :events, accumulate: true)
      @before_compile FSM.DSL
    end
  end
end

defmodule FSM.DSL do
  defmacro trans(name, from, to) do
    quote do
      @events unquote(name)
      def unquote(name)(unquote(from)) do
        unquote(to)
      end
    end
  end

  defmacro __before_compile__(env) do
    fields = Module.get_attribute(env.module, :events)
    quote do
      def __events__ do
        unquote(Enum.reverse(fields))
      end
    end
  end
end

defmodule Door do
  use FSM
  trans :push, "closed", "opened"
  trans :pull, "opened", "closed"
end

state0 = "closed"
state1 = Door.push(state0) # "opened"
state2 = Door.pull(state1) # "closed"

Door.__events__ # [:push, :pull]

fsm1.exs

defmodule FSM do
  defmacro __using__(opts) do
    quote do
      import FSM.DSL, only: [trans: 3]
    end
  end
end

defmodule FSM.DSL do
  defmacro trans(name, from, to) do
    quote do
      def unquote(name)(unquote(from)) do
        unquote(to)
      end
    end
  end
end

defmodule Door do
  use FSM
  trans :push, "closed", "opened"
  trans :pull, "opened", "closed"
end

state0 = "closed"
state1 = Door.push(state0) # "opened"
state2 = Door.pull(state1) # "closed"

fsm2.exs

defmodule FSM do
  defmacro __using__(opts) do
    quote do
      import FSM.DSL, only: [trans: 3]
      Module.register_attribute(__MODULE__, :events, accumulate: true)
      @before_compile FSM.DSL
    end
  end
end

defmodule FSM.DSL do
  defmacro trans(name, from, to) do
    quote do
      @events unquote(name)
    end
  end

  defmacro __before_compile__(env) do
    fields = Module.get_attribute(env.module, :events)
    quote do
      def __events__ do
        unquote(Enum.reverse(fields))
      end
    end
  end
end

defmodule Door do
  use FSM
  trans :push, "closed", "opened"
  trans :pull, "opened", "closed"
end

Door.__events__ # [:push, :pull]