[](https://hexdocs.pm/protean/) [](https://github.com/zachallaun/protean/actions/workflows/ci.yml)
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_Caveat emptor: Protean started as a library for personal learning and exploration. It should not yet be relied upon._
A library for managing state and side-effects with event-driven statecharts.
Protean is heavily inspired by [XState](https://xstate.js.org/docs/), a robust JavaScript/TypeScript statechart implementation, but strays to adhere to Elixir idioms and OTP conventions.
Protean also attempts to follow the [SCXML](https://www.w3.org/TR/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 assigns), side-effects (through actions), process management (through invoke), and more.
To learn more about statecharts, I recommend [statecharts.dev](https://statecharts.dev/).
## Goals
This project is currently an exploration of statecharts as they fit into the assigns 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
Add `Protean.Supervisor` under your application supervisor.
This starts a supervisor that is used by Protean internally to manage subprocesses.
```elixir
children = [
Protean.Supervisor,
# ...
]
```
This simple statechart has a single state that defines the behavior of a counter with an optional maximum and minimum.
```elixir
defmodule Counter do
use Protean
alias Protean.Action
@machine [
initial: :active,
assigns: [
count: 0,
min: nil,
max: nil
],
states: [
atomic(:active,
on: [
match("Inc", actions: :increment, guard: [not: :at_max]),
match("Dec", actions: :decrement, guard: [not: :at_min]),
match({"Set", _}, actions: :set_min_or_max),
match({"Log", _}, actions: :log)
]
)
]
]
@impl true
def handle_action(:increment, context, _event) do
context
|> Action.assign_in([:count], & &1 + 1)
end
def handle_action(:decrement, context, _event) do
context
|> Action.assign_in([:count], & &1 - 1)
end
def handle_action(:set_min_or_max, context, {"Set", {key, val}}) do
context
|> Action.assign(key, val)
end
def handle_action(:log, context, {"Log", attribute}) do
%{assigns: assigns} = context
IO.puts("#{attribute}: #{assigns[attribute]}")
context
end
@impl true
def guard(:at_max, %{assigns: %{max: max, count: count}}, _event) do
max && count >= max
end
def guard(:at_min, %{assigns: %{min: min, count: count}}, _event) do
min && count <= min
end
end
```
It can be started under a supervisor, but we'll start it directly.
```elixir
{:ok, pid} = Protean.start_link(Counter)
Protean.current(pid).assigns
# %{count: 0, min: nil, max: nil}
Protean.send(pid, "Inc")
# :ok
Enum.each(1..4, fn _ -> Protean.send(pid, "Inc") end)
Protean.current(pid).assigns
# %{count: 5, min: nil, max: nil}
Protean.call(pid, {"Set", {:max, 10}})
# %Protean.Context{
# assigns: %{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
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 XContext's [introduction to state machines and statecharts](https://xstate.js.org/docs/guides/introduction-to-state-machines-and-statecharts/) for that reason.
Refer to `Protean.Builder` for documentation on machine definitions.
When `use Protean` is invoked, functions and macros from `Protean.Builder` are imported automatically.
### The `@machine` attribute
By default, Protean assumes that your machine is defined on the `@machine` attribute.
```elixir
defmodule MyMachine do
use Protean
@machine [
initial: :my_initial_state,
states: [
atomic(:my_initial_state,
# ...
),
# ...
]
]
end
```
## 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.
```elixir
children = [
Counter
]
Supervisor.start_link(children, strategy: :one_for_one)
```
Protean machines also accept the same options as `Protean.start_link/2`.
See those docs for more details.
For instance, here's how you could start the `Counter` with a custom name:
```elixir
children = [
# Start the Counter machine
{Counter, name: MyCounter}
]
Supervisor.start_link(children, strategy: :one_for_one)
Protean.current(MyCounter)
# %Protean.Context{
# assigns: %{count: 0, max: nil, min: nil},
# event: "$protean.init",
# value: MapSet.new([["active", "#"]])
# }
```
## 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
* `call/3` - Send an event synchronously to a Protean machine and receive the machine context and any replies resulting from transition.
* `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. Like `Process.send_after/4`, returns a timer reference so that the send can be canceled with `Process.cancel_timer/2`.
### Additional functions specific to Protean machines
* `current/1` - Get the current machine context of a running Protean machine.
* `matches?/2` - Query the currently active state(s) of a machine.
* `subscribe/2` (and `unsubscribe/2`) - Subscribes the calling process to receive a message on every state transition.
## 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:
```elixir
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.
```elixir
# 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`.
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## Documentation
Documentation can be found [on hexdocs](https://hexdocs.pm/protean/).
Things are changing pretty regularly, however, and some documentation is certainly out-of-sync.
## Todo
- [ ] Add `stream` with similar options as `subscribe` so that you can get a stream of `{context, replies}` or just a stream of replies.
- [ ] Actions
- [ ] Spawn in assigns
- [ ] Raise action (queues event in internal queue)
- [ ] Respond action
- [ ] Forward action
- [ ] Escalate action
- [ ] Parallel states
- [ ] Correct transition conflict handling
- [ ] History states
- [ ] Fix all "FIXME"
- [ ] Fix all "TODO"
- [ ] Stricter machine config parsing with helpful error messages
- [ ] Differentiate between internal and external events (internal events should not trigger catch-all)
## Installation
This package is under active development and is pre-0.1. If you would like to experiment with it, install directly from the repository:
```elixir
def deps do
[
{:protean, github: "zachallaun/protean"}
]
end
```
