Use class_eval for monkey patching classes, to avoid forgetting to load the target class and ending up with a more cryptic type error.

monkey.rb

# 1. in one file, assuming that Foo is loaded/auto-loaded
class Foo
  def monkey_patch
    # ...
  end
end

# 2. later, the real Foo (in another file) is loaded
class Foo < File
end

# 3. results in a TypeError
#=> TypeError: superclass mismatch for class Foo

#
# vs.
#

# 1. in one file, assuming that Foo is loaded/auto-loaded
Foo.class_eval do
  def monkey_patch
    # ...
  end
end

# 2. results in a NameError, since target class is not yet loaded, forces you to require Foo first.
#=> NameError: uninitialized constant Foo

# 3. the above exception makes you realize Foo is not loaded yet, so you add a require
require 'foo'
Foo.class_eval do
  def monkey_patch
    # ...
  end
end