# Machinist

[![CI](https://github.com/norbajunior/machinist/actions/workflows/ci.yml/badge.svg)](https://github.com/norbajunior/machinist/actions/workflows/ci.yml)]
[![Hex.pm Version](https://img.shields.io/hexpm/v/machinist?color=blueviolet)](https://hex.pm/packages/machinist)]

This  is a small library that allows you to implement finite state machines with Elixir in a simple way. It provides a simple DSL to write combinations of transitions based on events.

* [Installation](#Installation)
* [Usage](#Usage)

## Installation

You can install `machinist` by adding it  to your list of dependencies in `mix.exs`:

```elixir
def deps do
  [
    {:machinist, "~> 0.2.0"}
  ]
end
```

## Usage

A good example is how we would implement the behaviour of a door. With `machinist` would be this way:

```elixir
defmodule Door do
  defstruct [state: :locked]

  use Machinist

  transitions do
    from :locked,   to: :unlocked, event: "unlock"
    from :unlocked, to: :locked,   event: "lock"
    from :unlocked, to: :opened,   event: "open"
    from :opened,   to: :closed,   event: "close"
    from :closed,   to: :opened,   event: "open"
    from :closed,   to: :locked,   event: "lock"
  end
end
```

By defining this rules with `transitions` and `from` macros, `machinist` generates and inject into the module `Door` `transit/2` functions like this one:

```elixir
def transit(%Door{state: :locked} = struct, event: "unlock") do
  {:ok, %Door{struct | state: :locked}}
end
```

So that we can transit between states by relying on the **state** + **event** pattern matching.

Let's see this in practice:

By default our `Door` is `locked`

```elixir
iex> door_locked = %Door{}
%Door{state: :locked}
```

So let's change its state to `unlocked` and `opened`

```elixir
iex> {:ok, door_unlocked} = Door.transit(door_locked, event: "unlock")
{:ok, %Door{state: :unlocked}}
iex> {:ok, door_opened} = Door.transit(door_unlocked, event: "open")
{:ok, %Door{state: :opened}}
```

If we try to make a transition that not follow the rules, we got an error:

```elixir
iex> Door.transit(door_opened, event: "lock")
{:error, :not_allowed}
```

### Setting different attribute name that holds the state

By default `machinist` expects the struct being updated holds a `state` attribute, if you hold state in a different attribute, just pass the name as an atom, as follows:

```elixir
transitions attr: :door_state do
  # ...
end
```

And then `machinist` will set state in that attribute

```elixir
iex> Door.transit(door, event: "unlock")
{:ok, %Door{door_state: :unlocked}}
```

### Implementing different versions of a state machine

Let's suppose we want to build a selection process app that handles applications of candidates and they may possibly going through different versions of the process. For example:

A Selection Process **V1** with the following sequence of stages: [Registration] -> [**Code test**] -> [Enrollment]

And a Selection Process **V2** with these ones: [Registration] -> [**Interview**] -> [Enrollment]

The difference here is in **V1** candidates must take a **Code Test** and V2 an **Interview**.

So, we could have a `%Candidate{}` struct that holds these attributes:

```elixir
defmodule SelectionProcess.Candidate do
  defstruct [:name, :state, test_score: 0]
end
```

And a `SelectionProcess` module that implements the state machine. Notice this time we don't want to implement the rules in the module that holds the state, in this case it makes more sense the `SelectionProcess` keep the rules, also because we want more than one state machine version handling candidates as mentioned before. This is our **V1** of the process:

```elixir
defmodule SelectionProcess.V1 do
  use Machinist

  alias SelectionProcess.Candidate

  @minimum_score 100

  transitions Candidate do
    from :new,           to: :registered,    event: "register"
    from :registered,    to: :started_test,  event: "start_test"
    from :started_test,  to: &check_score/1, event: "send_test"
    from :approved,      to: :enrolled,      event: "enroll"
  end

  defp check_score(%Candidate{test_score: score}) do
    if score >= @minimum_score, do: :approved, else: :reproved
  end
end
```

In this code we pass the `Candidate` module as a parameter to `transitions` to tell `machinist` that we expect `V1.transit/2` functions with a `%Candidate{}` struct as first argument and not the `%SelectionProcess.V1{}` which would be by default.

```elixir
def transit(%Candidate{state: :new} = struct, event: "register") do
  {:ok, %Candidate{struct | state: :registered}}
end
```

Also notice we provided the *function* `&check_score/1` to the option `to:` instead of an *atom*, in order to decide the state based on the candidate `test_score` value.

In the **version 2**, we replaced the `Code Test` stage by the `Interview` which has different state transitions:

```elixir
defmodule SelectionProcess.V2 do
  use Machinist

  alias SelectionProcess.Candidate

  transitions Candidate do
    from :new,                 to: :registered,          event: "register"
    from :registered,          to: :interview_scheduled, event: "schedule_interview"
    from :interview_scheduled, to: :approved,            event: "approve_interview"
    from :interview_scheduled, to: :repproved,           event: "reprove_interview"
    from :approved,            to: :enrolled,            event: "enroll"
  end
end
```

Now let's see how this could be used:

**V1:** A `registered` candidate wants to start its test.

```elixir
iex> candidate1 = %Candidate{name: "Ada", state: :registered}
iex> SelectionProcess.V1.transit(candidate1, event: "start_test")
%{:ok, %Candidate{state: :test_started}}
```

**V2:** A `registered` candidate wants to schedule the interview

```elixir
iex> candidate2 = %Candidate{name: "Jose", state: :registered}
iex> SelectionProcess.V2.transit(candidate1, event: "schedule_interview")
%{:ok, %Candidate{state: :interview_scheduled}}
```

That's great because we also can implement many state machines for only one entity and test different scenarios, evaluate and collect data for deciding which one is better.

`machinist` gives us this flexibility since it's just pure Elixir.

## How does the DSL works?

The use of `transitions` in combination with each `from` statement will be transformed in functions that will be injected into the module that is using `machinist`.

This implementation:

```elixir
defmodule Door do
  defstruct state: :locked

  use Machinist

  transitions do
    from :locked,   to: :unlocked, event: "unlock"
    from :unlocked, to: :locked,   event: "lock"
    from :unlocked, to: :opened,   event: "open"
    from :opened,   to: :closed,   event: "close"
    from :closed,   to: :opened,   event: "open"
    from :closed,   to: :locked,   event: "lock"
  end
end
```

is the same as:

```elixir
defmodule Door do
  defstruct state: :locked

  def transit(%__MODULE__{state: :locked} = struct, event: "unlock") do
    {:ok, %__MODULE__{struct | state: :unlocked}}
  end

  def transit(%__MODULE__{state: :unlocked} = struct, event: "lock") do
    {:ok, %__MODULE__{struct | state: :locked}}
  end

  def transit(%__MODULE__{state: :unlocked} = struct, event: "open") do
    {:ok, %__MODULE__{struct | state: :opened}}
  end

  def transit(%__MODULE__{state: :opened} = struct, event: "close") do
    {:ok, %__MODULE__{struct | state: :closed}}
  end

  def transit(%__MODULE__{state: :closed} = struct, event: "open") do
    {:ok, %__MODULE__{struct | state: :opened}}
  end

  def transit(%__MODULE__{state: :closed} = struct, event: "lock") do
    {:ok, %__MODULE__{struct | state: :locked}}
  end
  # a catchall function in case of unmatched clauses
  def transit(_, _), do: {:error, :not_allowed}
end
```

So, as we can see, we can eliminate a lot of boilerplate with `machinist` making it easier to maintain and less prone to errors.