gistfile1.txt

result
nil
nil
nil

expected
{"foo": 0}
{"bar": 1}
{"?": "What is this crap?"}

serializer.rb

#!/usr/bin/env ruby

require 'pp'

class Serializer
  include Enumerable

  def initialize(io, &block)
    @io = io
    block.call(self)
  end

  def <<(obj)
    data = Marshal.dump(data)
    @io.write([data.size].pack("I"))
    @io.write(data)
  end

  def each
    while not @io.eof?
      size = @io.read(4)
      raise EOFError unless size
      data = @io.read(size.unpack("I").first)
      yield Marshal.load(data)
    end
  end
end

File.open("foo.dat", 'w') do |io|
  Serializer.new(io) do |s|
    s << {"foo": 0}
    s << {"bar": 1}
    s << {"?": "What is this crap?"}
  end
end

File.open("foo.dat") do |io|
  Serializer.new(io) do |s|
    s.each do |record|
      pp record
    end
  end
end