Bulb decider in Ruby

example.rb

# Example based on https://github.com/thinkbeforecoding/dddeu-2023-deciders/blob/main/run.fsx

require "bundler/inline"

gemfile do
  source "https://rubygems.org"

  gem "minitest"
  gem "decide.rb", "~> 0.4.1"
end

require "minitest/autorun"
require "decider"

module Bulb
  module Commands
    Fit = Data.define(:max_uses)
    SwitchOn = Data.define
    SwitchOff = Data.define
  end

  module Events
    Fitted = Data.define(:max_uses)
    SwitchedOn = Data.define
    SwitchedOff = Data.define
    Blew = Data.define
  end

  module States
    NotFitted = Data.define
    Working = Data.define(:status, :remaining_uses)
    Blown = Data.define
  end

  Decider = ::Decider.define do
    initial_state States::NotFitted.new

    terminal? { _1.is_a? Blown } 

    decide Commands::Fit do |command, state|
      case state
      in States::NotFitted
        [Events::Fitted.new(command.max_uses)]
      else
        raise "Bulb has already been fitted"
      end
    end

    decide Commands::SwitchOn do |command, state|
      case state
      in States::Working(status: :off, remaining_uses:) if remaining_uses > 0
        [Events::SwitchedOn.new]
      in States::Working(status: :off)
        [Events::Blew.new]
      else
        []
      end
    end

    decide Commands::SwitchOff do |command, state|
      case state
      in States::Working(status: :on)
        [Events::SwitchedOff.new]
      else
        []
      end
    end

    evolve Events::Fitted do |state, event|
      States::Working.new(status: :off, remaining_uses: event.max_uses)
    end

    evolve Events::SwitchedOn do |state, event|
      state.with(status: :on, remaining_uses: state.remaining_uses - 1)
    end

    evolve Events::SwitchedOff do |state, event|
      state.with(status: :off)
    end

    evolve Events::Blew do |state, event|
      States::Blown.new
    end
  end
end

class TestDecide < Minitest::Test
  def decider = Bulb::Decider

  def given(events)
    @state = events.reduce(decider.initial_state, &decider.method(:evolve))

    self
  end

  def when(command)
    @events = decider.decide(command, @state)

    self
  end

  def expect(expected)
    assert_equal(expected, @events)
  end

  def test_fit
    given(
      []
    ).when(
      Bulb::Commands::Fit.new(max_uses: 5)
    ).expect(
      [Bulb::Events::Fitted.new(max_uses: 5)]
    )
  end

  def test_switch_on
    given(
      [Bulb::Events::Fitted.new(max_uses: 5)]
    ).when(
      Bulb::Commands::SwitchOn.new
    ).expect(
      [Bulb::Events::SwitchedOn.new]
    )
  end

  def test_switch_on_already_on
    given(
      [
        Bulb::Events::Fitted.new(max_uses: 5),
        Bulb::Events::SwitchedOn.new
      ]
    ).when(
      Bulb::Commands::SwitchOn.new
    ).expect(
      []
    )
  end

  def test_switch_off
    given(
      [
        Bulb::Events::Fitted.new(max_uses: 5),
        Bulb::Events::SwitchedOn.new
      ]
    ).when(
      Bulb::Commands::SwitchOff.new
    ).expect(
      [Bulb::Events::SwitchedOff.new]
    )
  end

  def test_switch_off_already_off
    given(
      [
        Bulb::Events::Fitted.new(max_uses: 5),
        Bulb::Events::SwitchedOn.new,
        Bulb::Events::SwitchedOff.new
      ]
    ).when(
      Bulb::Commands::SwitchOff.new
    ).expect(
      []
    )
  end

  def test_blew
    given(
      [
        Bulb::Events::Fitted.new(max_uses: 1),
        Bulb::Events::SwitchedOn.new,
        Bulb::Events::SwitchedOff.new
      ]
    ).when(
      Bulb::Commands::SwitchOn.new
    ).expect(
      [Bulb::Events::Blew.new]
    )
  end
end