Ruby closure memory leak labs (Based on https://practicingruby.com/articles/closures-are-complicated)

0_lazy_logger.rb

module LazyLogger
  extend self

  attr_accessor :output

  def log(&callback)
    @log_actions ||= []
    @log_actions << callback
  end

  def flush
    @log_actions.each { |e| e.call(output) }
  end
end

1_closure_memory_leak.rb

class User
  def initialize(id)
    @id = id
    LazyLogger.log { |out| out << "Created User with ID #{id}" }
  end
end

LazyLogger.output = ""
1000.times { |i| User.new(i) }

GC.start
p ObjectSpace.each_object(User).count # => 1000

2_lambda_closure_without_memory_leak.rb

class User
  Log = -> id do
    -> out { out << "Created User with ID #{id}" }
  end

  def initialize(id)
    @id = id
    LazyLogger.log &Log.curry[id]
  end
end

LazyLogger.output = ""
1000.times { |i| User.new(i) }

GC.start
p ObjectSpace.each_object(User).count # => 0

3_private_const_lambda_closure_without_memory_leak.rb

class Module
  def private_const(name, value)
    const_set(name, value)
    private_constant(name)
  end
end

class User
  private_const :Log, -> (id, out) do
    out << "Created User with ID #{id}"
  end

  def initialize(id)
    @id = id
    LazyLogger.log &Log.curry[id]
  end
end

LazyLogger.output = ""
1000.times { |i| User.new(i) }

GC.start
p ObjectSpace.each_object(User).count # => 0

4_mod_without_memory_leak.rb

module Logger
  extend self

  attr_accessor :output

  def log(msg)
    output << "#{msg}\n"
  end
end

class User
  def initialize(id)
    @id = id
    Logger.log("Created User with ID #{id}")
  end
end

Logger.output = ""
1000.times { |i| User.new(i) }

GC.start
p ObjectSpace.each_object(User).count #=> 0

Note: Other programming languages have similar issues with closures. e.g:

Javascript: https://auth0.com/blog/four-types-of-leaks-in-your-javascript-code-and-how-to-get-rid-of-them/

C#: http://www.davejsaunders.com/2017/05/06/memory-leak-lambdas.html