View Source Protean behaviour (Protean v0.0.1)
Caveat emptor: Protean is a library for personal learning and exploration, not (yet) for doing Serious Work.
An experimental Elixir library for managing state and side-effects through the use of event-driven statecharts. It is heavily inspired by XState, a robust JavaScript/TypeScript statechart implementation, but strays to adhere to Elixir idioms and OTP conventions. Protean also attempts to follow the SCXML standard, though not completely.
What are statecharts? They are an extension to finite state machines that allow you to model complex behavior in a declarative, data-driven manner. They include nested and parallel states, enhanced/augmented state (through context), side-effects (through actions), process management (through invoke), and more. To learn more about statecharts, I recommend statecharts.dev.
goals
Goals
This project is currently an exploration of statecharts as they fit into the context of Elixir and OTP. XState adopted the actor model in its implementation, so Elixir seemed like a natural fit. However, it may be that Elixir/OTP makes these abstractions unnecessary.
example
Example
Add Protean.Supervisor
under your application supervisor.
This starts a supervisor that is used by Protean internally to manage subprocesses.
children = [
Protean.Supervisor,
# ...
]
This simple statechart has a single state that defines the behavior of a counter with an optional maximum and minimum.
defmodule Counter do
use Protean
alias Protean.Action
defmachine(
initial: "active",
context: [
count: 0,
min: nil,
max: nil
],
states: [
active: [
on: [
{"Inc", actions: :increment, guard: [not: :at_max]},
{"Dec", actions: :decrement, guard: [not: :at_min]},
{{"Set", _}, actions: :set_min_or_max},
{{"Log", _}, actions: :log}
]
]
]
)
@impl Protean
def action(:increment, state, _event), do: Action.assign_in(state, [:count], & &1 + 1)
def action(:decrement, state, _event), do: Action.assign_in(state, [:count], & &1 - 1)
def action(:set_min_or_max, state, {"Set", {key, val}}) do
state
|> Action.assign(key, val)
end
def action(:log, state, {"Log", attribute}) do
%{context: context} = state
IO.puts("#{attribute}: #{context[attribute]}")
state
end
@impl Protean
def guard(:at_max, %{context: %{max: max, count: count}}, _event) do
max && count >= max
end
def guard(:at_min, %{context: %{min: min, count: count}}, _event) do
min && count <= min
end
end
It can be started under a supervisor, but we'll start it directly.
{:ok, pid} = Protean.start_link(Counter)
Protean.current(pid).context
# %{count: 0, min: nil, max: nil}
Protean.send(pid, "Inc")
# :ok
Enum.each(1..4, fn _ -> Protean.send(pid, "Inc") end)
Protean.current(pid).context
# %{count: 5, min: nil, max: nil}
Protean.call(pid, {"Set", {:max, 10}})
# %Protean.State{
# context: %{count: 5, max: 10, min: nil},
# event: {"Set", {:max, 10}},
# value: MapSet.new([["active", "#"]])
# }
Enum.each(1..20, fn _ -> Protean.send(pid, "Inc") end)
Protean.send(pid, {"Log", :count})
# count: 10
defining-a-statechart
Defining a statechart
Protean machines are event-driven statecharts, which means that, unlike ordinary finite-state machines, they can have complex, nested, potentially parallel states. This is more easily visualized than read, and I highly recommend looking at XState's introduction to state machines and statecharts for that reason.
todo-states
TODO: States
todo-transitions
TODO: Transitions
todo-guards-and-automatic-transitions
TODO: Guards and automatic transitions
todo-actions
TODO: Actions
todo-invoked-processes
TODO: Invoked processes
starting-supervised-machines
Starting supervised machines
Just like GenServer
, Protean machines will be most often started under a supervision tree.
Invoking use Protean
will automatically define a child_spec/1
function that allows you to start the process directly under a supervisor.
children = [
Counter
]
Supervisor.start_link(children, strategy: :one_for_one)
Protean machines also accept the same options as Protean.start_link/3
.
See those docs for more details.
For instance, here's how you could start the Counter
with a custom name:
children = [
# Start the Counter machine
{Counter, name: MyCounter}
]
Supervisor.start_link(children, strategy: :one_for_one)
Protean.current(MyCounter)
# %Protean.State{
# context: %{count: 0, max: nil, min: nil},
# event: "$protean.init",
# value: MapSet.new([["active", "#"]])
# }
interacting-with-protean-machines
Interacting with Protean machines
Under the hood, a Protean machine is a GenServer
, and Protean
exposes a similar set of functions for interacting with one.
You can see the individual docs for the functions in this module for details on their behavior, but here are some highlights.
familiar-functions
Familiar functions
call/3
- Send an event synchronously to a Protean machine and receive the machine state resulting from any transitions in response.send/2
- Send an event asynchronously to a Protean machine. Always returns:ok
.send_after/3
- Send an event to a Protean machine after a given delay. LikeProcess.send_after/4
, returns a timer reference so that the send can be canceled withProcess.cancel_timer/2
.
additional-functions-specific-to-protean-machines
Additional functions specific to Protean machines
current/1
- Get the current machine state of a running Protean machine.matches?/2
- Query the currently active state(s) of a machine.ask/3
- Likecall/3
, but potentially returns an "answer" value in addition to the machine state.subscribe/2
(andunsubscribe/2
) - Subscribes the calling process to receive a message on every state transition.
protean-supervisor
Protean Supervisor
Protean uses a DynamicSupervisor
to manage internally spawned processes (often spawned through the use of :invoke
).
The simplest thing to do is to add Protean.Supervisor
in your application supervision tree:
def start(_type, _args) do
children = [
Protean.Supervisor,
# ...
]
Supervisor.start_link(children, strategy: :one_for_one)
end
This will start the supervisor under the name Protean.Supervisor
and no additional configuration will be required.
If you would like to start multiple supervisors, or a different type of supervisor (like a fancy PartitionSupervisor
), you can pass the new name as an option when starting a machine.
Here's how that might look using the counter example from before.
# in your supervision tree
children = [
{Protean.Supervisor, name: ProteanSupervisor1},
{Protean.Supervisor, name: ProteanSupervisor2}
]
# starting the counter
Protean.start_link(Counter, supervisor: ProteanSupervisor1)
In the above example, any processes that are spawned by the Protean interpreter running Counter
will use ProteanSupervisor1
.
Link to this section Summary
Types
Any message sent to a Protean machine.
Option values for start*
functions.
A running Protean machine process.
Options for subscribe/2
.
Callbacks
Used to execute actions in response to machine transitions.
Used to determine whether a transition should take place.
Used to define invoked services at runtime. Returns a value or child spec usable by the invoke type.
Functions
Makes a synchronous call to the machine and waits for it to execute any transitions that result from the given event, returning an answer and the machine state.
Makes a synchronous call to the machine and waits for it to execute any transitions that result from the given event, returning a possible answer and the new machine state.
See Protean.Interpreter.Server.ask/3
.
Makes a synchronous call to the machine and waits for it to execute any transitions that result from the given event, returning the new machine state.
See Protean.Interpreter.Server.call/3
.
Synchronously retrieve the current machine state.
Returns true if the machine is currently in the given state.
Sends an asynchronous event to the machine.
Sends an event to the machine after time
in milliseconds has passed.
Start a Protean machine linked to the current process.
See Protean.Interpreter.Server.stop/1
.
Subscribes the caller to a running machine, returning a reference.
Unsubscribes the caller from the machine.
Link to this section Types
@type event() :: term()
Any message sent to a Protean machine.
@type option() :: {:machine, Protean.Machine.t()} | {:handler, module()} | {:parent, server() | pid()} | {:supervisor, Supervisor.name()} | GenServer.option()
Option values for start*
functions.
@type server() :: GenServer.server()
A running Protean machine process.
@type subscribe_option() :: {:monitor, boolean()}
Options for subscribe/2
.
Link to this section Callbacks
@callback action(action :: term(), Protean.State.t(), event()) :: Protean.State.t()
Used to execute actions in response to machine transitions.
Receives the current machine state and event triggering the action as arguments and must return
the machine state. It is possible to attach actions to the machine state to indicate that they
should be performed immediately following this action. See Protean.Action
.
example
Example
defmachine(
# ...
states: [
# ...
state: [
on: [
{{:data, _any}, target: "data_received", actions: "assign_and_send_data"}
]
]
]
)
@impl Protean
def action("assign_and_send_data", state, {:data, data}) do
%{service: pid} = state.context
PubSub.broadcast!(@pubsub, @topic, data)
state
|> Protean.Action.send({"data_received", data}, to: pid)
|> Protean.Action.assign(:last_received_data, data)
end
@callback guard(action :: term(), Protean.State.t(), event()) :: boolean()
Used to determine whether a transition should take place.
example
Example
defmachine(
# ...
states: [
editing_user: [
on: [
{
{:user_commit, _},
guard: "user_valid?",
actions: ["broadcast"],
target: "viewing_user"
},
{
{:user_commit, _},
guard: {:not, "user_valid?"},
actions: ["show_invalid_user_error"]
}
]
]
]
)
@impl Protean
def guard("user_valid?", state, {_, user}) do
User.changeset(%User{}, user).valid?
end
@callback invoke(action :: term(), Protean.State.t(), event()) :: term()
Used to define invoked services at runtime. Returns a value or child spec usable by the invoke type.
example
Example
defmachine(
# ...
states: [
# ...
awaiting_task: [
invoke: [
task: "my_task",
done: "task_complete"
]
],
task_complete: [
# ...
]
]
)
@impl Protean
def invoke("my_task", _state, {"trigger", data}) do
{__MODULE__, :run_task, [data]}
end
Link to this section Functions
Makes a synchronous call to the machine and waits for it to execute any transitions that result from the given event, returning an answer and the machine state.
Behaves like ask/3
, but raises if an answer is not returned.
@spec ask!(server(), event(), timeout()) :: {term(), Protean.State.t()}
Makes a synchronous call to the machine and waits for it to execute any transitions that result from the given event, returning a possible answer and the new machine state.
Returns one of:
{{:ok, answer}, state}
- Returned if any actions executed as a result of the event set an answer through the use ofAction.answer/2
.{nil, state}
- Returned if no actions execute or if no executed actions set an answer.
Answers are only returned to the caller if they result from the given event. If an asynchronous
call, through send/2
for example, would have resulted in an answer, it will be "lost".
@spec ask(server(), event(), timeout()) :: {{:ok, term()}, Protean.State.t()} | {nil, Protean.State.t()}
See Protean.Interpreter.Server.ask/3
.
Makes a synchronous call to the machine and waits for it to execute any transitions that result from the given event, returning the new machine state.
Shares semantics with GenServer.call/3
. See those docs for timeout
behavior.
@spec call(server(), event(), timeout()) :: Protean.State.t()
See Protean.Interpreter.Server.call/3
.
@spec current(server()) :: Protean.State.t()
Synchronously retrieve the current machine state.
TODO: Allow optional timeout as with call/3
.
@spec matches?(Protean.State.t(), descriptor :: term()) :: boolean()
@spec matches?(server(), descriptor :: term()) :: boolean()
Returns true if the machine is currently in the given state.
Note that calling matches?/2
on a machine process is a synchronous operation that is
equivalent to:
machine |> Protean.current() |> Protean.matches?(descriptor)
Sends an asynchronous event to the machine.
Shares semantics with GenServer.cast/2
.
@spec send_after(server(), event(), non_neg_integer()) :: reference()
Sends an event to the machine after time
in milliseconds has passed.
Returns a timer reference that can be canceled with Process.cancel_timer/1
.
@spec start_link(module(), [option()]) :: GenServer.on_start()
Start a Protean machine linked to the current process.
This is often used to start the machine as part of a supervision tree. See
GenServer.start_link/3
for description of return value.
The semantics are similar to GenServer.start_link/3
and accepts the same options, with the
addition of some specific to Protean.
options
Options
:machine
- defaults tomodule.machine()
-%Protean.Machine{}
that will be executed by the Protean interpreter.:handler
- defaults tomodule
- callback module used for actions, guards, invoke, etc. See "Callbacks".:parent
- defaults toself()
- process id of the parent that will receive events from the machine if aProtean.Action.send(..., to: :parent)
action is used or when the machine reaches a state with:type
of:final
.:supervisor
- defaults toProtean.Supervisor
- name of the supervisor process that will be used to start processes resulting from running the machine. See "Supervisor".- Any option accepted by
GenServer.start_link/3
.
See Protean.Interpreter.Server.stop/1
.
TODO
@spec subscribe(server(), [subscribe_option()]) :: reference()
Subscribes the caller to a running machine, returning a reference.
Processes subscribed to a machine will receive messages whenever the machine transitions. (Note
that a machine can transition to the same state it was in previously.) By default, subscribed
processes also monitor the machine (see Process.monitor/1
). This behavior can be changed by
passing monitor: false
.
Messages on transition will be delivered in the shape of:
{:state, state, ref}
where:
state
is theProtean.State
resulting from the transition;ref
is a monitor reference.
As with monitor, if the process is already dead when calling Protean.subscribe/2
, a :DOWN
message is delivered immediately.
Unsubscribes the caller from the machine.