WAT.rb

# Example 1
module Logger
  extend self
  attr_accessor :output, :log_actions

  def log(&event)
    self.log_actions ||= []
    self.log_actions << event
  end

  def play
    output = []
    log_actions.each { |e| e.call(output) }
    puts output.join("\n")
  end
end

class Thing
  def initialize(id)
    Logger.log { |output| output << "created thing #{id}" }
  end
end

def do_something
  1000.times { |i| Thing.new(i) }
end

do_something
GC.start
Logger.play
Logger.log_actions = nil
GC.start
puts ObjectSpace.each_object(Thing).count # returns 1000
# because each block stores a Thing object in it's closure
# and there are a 1000 blocks in log_actions

# Example 2
module Logger
  extend self
  attr_accessor :output

  def log(&event)
    self.output ||= []
    event.call(output)
  end

  def play
    puts output.join("\n")
  end
end



class Thing
  def initialize(id)
    Logger.log { |output| output << "created thing #{id}" }
  end
end

def do_something
  1000.times { |i| Thing.new(i) }
end

do_something
GC.start
Logger.play
puts ObjectSpace.each_object(Thing).count # returns 0
# as expected

# Example 3
module Logger
  extend self
  attr_accessor :output, :log_actions

  def log(&event)
    self.log_actions ||= []
    self.log_actions << event
  end

  def play
    output = []
    log_actions.each { |e| e.call(output) }
    puts output.join("\n")
  end
end

class Thing
  def initialize(id)
    Logger.log { |output| output << "created thing #{id}" }
  end
end

def do_something
  1000.times { |i| Thing.new(i) }
end

do_something
GC.start
Logger.play
Logger.log_actions = nil
GC.start
puts ObjectSpace.each_object(Thing).count # still returns 1000
# Known memory leak - explaned in the first example
# but why are they kept in memory?