primitive fsm with hardcoded rules

README.md

Usage

den@den-vb:~/projects$ iex fsm.ex
Erlang/OTP 20 [erts-9.3.1] [source] [64-bit] [smp:2:2] [ds:2:2:10] [async-threads:10] [hipe] [kernel-poll:false]

Interactive Elixir (1.7.2) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> Fsm.transition!(%{status: "draft", data: "payload"}, :rejected)  
%{data: "payload", status: "rejected"}
iex(2)> Fsm.transition!(%{status: "rejected", data: "payload"}, :signed)  
** (RuntimeError) Transition from 'rejected' to 'signed' is not allowed
    fsm.ex:13: Fsm.transition!/2

fsm.ex

defmodule Fsm do
  @rules %{
    draft: [:signed, :rejected],
    rejected: [:draft],
    signed: []
  }

  def transition!(%{status: status} = struct, transition) when is_atom(transition) do
    transitions = Map.fetch!(@rules, String.to_existing_atom(status))
    if transition in transitions do
      %{struct | status: Atom.to_string(transition)}
    else
      raise "Transition from '#{status}' to '#{transition}' is not allowed"
    end
  end
end