Enum State Machine for Crystal

enum_state_machine is a type-safe finite state machine for Crystal where the states are defined using enum's. I created it because I wanted the minimal FSM data type that was function and easy to use.

Installation

1. Add the dependency to your shard.yml:

   dependencies:
     enum_state_machine:
       github: nogginly/enum_state_machine.cr

2. Run shards install

Usage

Here's a single example that illustrates how to use enum_state_machine to setup a toy Player class with two statuses, Action and Health, and their respective state machines.

require "enum_state_machine"

module MyGame
  enum Action
    Idle
    Walk
    Run
    Jump
    Attack
  end

  enum Health
    Alive
    Dead
  end

  class Player
    include EnumStateMachine

    state_machine(Action, initial: Action::Idle) do
      event :idle,        # from any state except these
        except_from: {Action::Idle, Action::Jump}, to: Action::Idle
      event :jump,        # from one of many states
        from: {Action::Walk, Action::Run, Action::Idle}, to: Action::Jump
      event :attack,      # from one state
        from: Action::Idle, to: Action::Attack
      event :run, to: Action::Run
    end

    state_machine(Health, initial: Health::Alive) do
      event :die,         # from one state
        from: Health::Alive, to: Health::Dead
      event :resurrect,   # from a state, with a guard
        from: Health::Dead, to: Health::Alive,
        guard: ->{ idle? }
    end
  end

Each event do_this adds the following capability methods:

• do_this attempts to perform the transition
• do_this(&) which yields after successfully performing transition
• may_do_this? returns true of the transition may be performed

Each state machine for an enum Feeling adds the following methods:

• feeling which returns the current value of the Feeling state
• for each enumeration of Feeling, say Feeling::Kind and Feeling::Sad
  • kind? which returns true of the current value of feeling is Feeling::Kind
  • sad?
  • and so on.

To do

In no particular order:

• More examples for the various ways to define and use enum_state_machine
• Document possible compile errors
• Event transition failure handler (like guard)
• Multiple transitions for a single event
• Test suite

Contributing

Bug reports and sugestions are welcome. Otherwise, at this time, this project is closed for code changes and pull requests. I appreciate your understanding.

This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the code of conduct.

License

This shard is available as open source under the terms of the MIT License.

Contributors

• nogginly - creator and maintainer