Charlie on the MTA in Elixir

loop.erl

-module(loop).
% If you are not from greater Boston watch this video of the Song "Charlie on the MTA" by the Kingston Trio
% And this will make more sense https://www.youtube.com/watch?v=MbtkL5_f6-4
-export([charlie/1, init/0, run/0]).


-type line() :: 'red'|'green_e'|'blue'|'orange'.

-type station() :: 'kendal'|'charles/mgh'|'park_st'|'government_center'|'heath'.

-type events() :: {'board', station(), line(), 10} |
                  {'change', 'red','green_e', 'park_st'} |
                  {'travel', station(), station()} |
                  {'exit', station(), 5} |
                  {'sandwich', station()}.


-record(state, {
                station :: station(),
                money   :: integer(),
                line    :: line()
               }).

-spec(charlie(#state{}) -> no_return()).
charlie(State = #state{money = Money, station = none}) ->
    io:format("Charlie ~p~n", [State]),
    receive
        {board, Station, Line, Fare}->
            charlie(#state{station = Station,
                           money = Money - Fare,
                           line = Line}
                    )
    end;

charlie(State = #state{station = Station, line = Line, money = Money}) ->
    io:format("Charlie ~p~n", [State]),
    receive
        {travel, Station, NewStation} ->
            charlie(State#state{station = NewStation});

        {change, Line, NewLine, Station} ->
            charlie(State#state{line = NewLine});

        {sandwich , Station} ->
            charlie(State);

        {exit, Station, Fare} when  Money - Fare > 0 ->
            charlie(State#state{station=none, line=none})
    end.



init() ->
    charlie(#state{station = none,
                   money = 10,
                   line = none
             }).


run() ->
    Pid = spawn(?MODULE, init, []),
    Events = [{'board', 'kendal','red', 10},
              {'travel', 'kendal','charles/mgh'},
              {'travel', 'charles/mgh', 'park_st'},
              {'change', 'red','green_e', 'park_st'},
              {'travel', 'park_st', 'heath'},
              {'exit', 'heath', 5},
              {'travel', 'heath', 'government_center'},
              {'sandwich', 'government_center'}],
    [Pid ! Event || Event <- Events],
    Pid.

loop.ex

defmodule Loop do
# If you are not from greater Boston watch this video of the Song "Charlie on the MTA" by the Kingston Trio
# And this will make more sense https://www.youtube.com/watch?v=MbtkL5_f6-4

  defstruct station: :none, money: 0, line: :none

  @type line() :: :red | :green_e | :blue | :orange
  @type station() :: :kendal | :"charles/mgh" | :park_st | :government_center | :heath

  @type event() ::
          {:board, station(), line(), 10}
          | {:change, :red, :green_e, :park_st}
          | {:travel, station(), station()}
          | {:exit, station(), 5}
          | {:sandwich, station()}

  @spec loop(%Loop{}) :: no_return()
  def loop(state = %Loop{money: money, station: :none, line: :none}) do
    IO.puts("State #{inspect(state)}")

    receive do
      {:board, station, line, fare} ->
        loop(%Loop{state | money: money - fare, station: station, line: line})
    end
  end

  def loop(state = %Loop{station: station, money: money, line: line}) do
    IO.puts("State #{inspect(state)}")

    receive do
      {:travel, ^station, newstation} ->
        loop(%Loop{state | station: newstation})

      {:change, ^line, newline, ^station} ->
        loop(%Loop{state | line: newline})

      {:sandwhich, ^station} ->
        loop(state)

      {:exit, ^station, fare} when money - fare > 0 ->
        loop(%Loop{state | station: :none, line: :none})
    end
  end

  def init() do
    loop(%Loop{station: :none, money: 10, line: :none})
  end

  @spec run() :: pid()
  def run() do
    pid = spawn(Loop, :init, [])

    events = [
      {:board, :kendal, :red, 10},
      {:travel, :kendal, :"charles/mgh"},
      {:travel, :"charles/mgh", :park_st},
      {:change, :red, :green_e, :park_st},
      {:travel, :park_st, :heath},
      {:exit, :heath, 5},
      {:travel, :heath, :government_center},
      {:sandwich, :government_center}
    ]

    for evt <- events, do: send(pid, evt)
    pid
  end
end