Decider

This gem provides simple DSL for building Functional Event Sourcing Decider in Ruby. To learn more about the pattern read the original article by Jérémie Chassaing.

Special credits for Ismael Celis for inspiration.

Installation

gem install decide.rb

or add to Gemfile

gem "decide.rb"

Usage

require "decider"

State = Data.define(:value) do
  def max?
    value >= MAX
  end
  
  def min?
    value <= MIN
  end
end

module Commands
  Increase = Data.define
  Decrease = Data.define
end

module Events
  ValueIncreased = Data.define
  ValueDecreased = Data.define
end

MIN = 0
MAX = 100

ValueDecider = Decider.define do
  # define intial state
  initial_state State.new(value: 0)

  # decide command with state
  decide Commands::Increase do
    # return collection of events
    if state.max?
      []
    else
      [Events::ValueIncreased.new]
    end
  end

  decide Commands::Decrease do
    if state.min?
      []
    else
      [Events::ValueDecreased.new]
    end
  end

  # evolve state with events
  evolve Events::ValueIncreased do
    # return new state
    state.with(value: state.value + 1)
  end

  evolve Events::ValueDecreased do
    # state is immutable Data object
    state.with(value: state.value - 1)
  end

  terminal? do
    state <= 0
  end
end

state = ValueDecider.initial_state
events = ValueDecider.decide(Commands::Increase.new, state)
new_state = events.reduce(state) { |state, event| ValueDecider.evolve(state, events)

You can also compose deciders:

Left = Data.define(:value)
Right = Data.define(:value)

left = Decider.define do
  initial_state Left.new(value: 0)

  decide Commands::LeftCommand do
    [Events::LeftEvent.new(value: command.value)]
  end

  evolve Events::LeftEvent do
    state.with(value: state.value + 1)
  end

  terminal? do
    state <= 0
  end
end

right = Decider.define do
  initial_state Right.new(value: 0)

  decide Commands::RightCommand do
    [Events::RightEvent.new(value: command.value)]
  end

  evolve Events::RightEvent do
    state.with(value: state.value + 1)
  end

  terminal? do
    state <= 0
  end
end

Composition = Decider.compose(left, right)

state = Composition.initial_state
#> #<data Decider::Pair left=#<data Left value=0>, right=#<data Right value=0>>

events = Composition.decide(Decider::Left.new(Commands::LeftCommand.new(value: 1)), state)
#> [#<Decider::Left value=#<data value=1>]

state = events.reduce(state, &Composition.method(:evolve))
#> #<data Decider::Pair left=#<data value=1>, right=#<data value=0>>

Development

After checking out the repo, run bin/setup to install dependencies. Then, run rake test to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and the created tag, and push the .gem file to rubygems.org.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/[USERNAME]/decider.

License

The gem is available as open source under the terms of the MIT License.