I am going to show how to monkey-patch graciously using Ruby.

The original idea is to implement a method Module#def_after so that I can easily make something to be done after the original method. Like this:

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class Foo
	def bar
		print 'before'
	end
end

class Foo
	def_after :bar do
		puts ' & after'
	end
end

Foo.new.bar # => before & after

The implementation is a little easy:

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class Module
	def def_after method_name, &refine_block
		old = instance_method method_name
		define_method method_name do |*args, **opts, &block|
			old.bind_call self, *args, **opts, &block
			refine_block.(*args, **opts, &block)
		end
	end
end

However, there is a little problem. The self in refine_block depends on how and where refine_block is defined instead of just being the instance receiving the method.

Since an instance method (UnboundMethod object) defined in a Module can bind any other object, we can use Module#define_method and send refine_block as a block parameter, and then bind the instance method to self:

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class Proc
	def bind receiver
		Module.new.module_exec self do |block|
			instance_method define_method :_, &block
		end.bind receiver
	end
	def bind_call receiver, *args, **opts, &block
		bind(receiver).(*args, **opts, &block)
	end
end
class Module
	def def_after method_name, &refine_block
		old = instance_method method_name
		define_method method_name do |*args, **opts, &block|
			old.bind_call self, *args, **opts, &block
			refine_block.bind_call self, *args, **opts, &block
		end
	end
end

The self can successfully be converted. You can test it yourself.

Here is still a problem. When the new instance method is defined, its visibility is public, while the original visibility may be private or protected.

Use the following means to get the visibility beforehand and set the visibility afterward:

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class Module
	def method_visibility method_name
		%i[public protected private].find do |visibility|
			__send__ :"#{visibility}_method_defined?", method_name
		end
	end
	def def_after method_name, &refine_block
		visibility = method_visibility method_name
		old = instance_method method_name
		define_method method_name do |*args, **opts, &block|
			old.bind_call self, *args, **opts, &block
			refine_block.bind_call self, *args, **opts, &block
		end
		__send__ visibility, method_name
	end
end

There can be some improvement. If we need to refine a singleton method, calling def_after on its singleton_class will lead to calling obj.singleton_class.instance_method(sym).bind_call(obj, *), which is way too complex. The straightforward way to do it is to call obj.method(sym).call(*).

With this inspiration, we can implement Object#def_after:

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class Object
	def def_after method_name, &refine_block
		visibility = singleton_class.method_visibility method_name
		old = method method_name
		define_singleton_method method_name do |*args, **opts, &block|
			old.(*args, **opts, &block)
			refine_block.bind_call self, *args, **opts, &block
		end
		singleton_class.__send__ visibility, method_name
	end
end

Then there comes a new problem. A Module also has singleton methods, while Module#def_after can only change its instance methods instead of its singleton methods. The way to solve this is to judge whether is_a? Module in Object#def_after, and add a keyword argument singleton:

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class Object
	def def_after method_name, singleton: false, &refine_block
		singleton ||= !is_a?(Module)
		# mod: the module containing the old method
		# get_method: the method to get the Method/UnboundMethod obj
		# def_method: the method to define a new method
		mod, get_method, def_method = singleton ?
				[singleton_class, method(:method), method(:define_singleton_method)] :
				[self, method(:instance_method), method(:define_method)]
		# get visibility
		visibility = mod.method_visibility method_name
		# get old
		old = get_method.(method_name)
		# override
		def_method.(method_name) do |*args, **opts, &block|
			old = old.bind self unless old.is_a? Method
			old.(*args, **opts, &block)
			refine_block.bind_call *args, **opts, &block
		end
		# set visibility
		mod.__send__ visibility, method_name
	end
end

What about parsing a callable object as an argument instead of through refine_block? Parsing a Symbol can also be useful. Like this:

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Object.def_after :display, :puts

Then Object#def_after will be a little complex:

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class Object
	# pat: when refine_block is nil, it is used to represent a refinement
	# singleton: force singleton when self is a Module
	def def_after method_name, pat = nil , singleton: false, &refine_block
		singleton ||= !is_a?(Module)
		# mod: the module containing the old method
		# get_method: the method to get the Method/UnboundMethod obj
		# def_method: the method to define a new method
		mod, get_method, def_method = singleton ?
				[singleton_class, method(:method), method(:define_singleton_method)] :
				[self, method(:instance_method), method(:define_method)]
		# get visibility
		visibility = mod.method_visibility method_name
		# get pat
		pat = refine_block || {
			to_sym:  ->symbol { get_method.(symbol.to_sym) },
			to_proc: :to_proc.to_proc,
			call:    ->callable { callable.method :call }
		}.each do |duck, out|
			break out.(pat) if pat.respond_to? duck
		end
		# get old
		old = get_method.(method_name)
		# override
		def_method.(method_name) do |*args, **opts, &block|
			# bind old
			old = old.bind self unless old.is_a? Method
			# bind pat
			pat = pat.bind self unless pat.is_a? Method
			# call the new method
			old.(*args, **opts, &block)
			pat.(*args, **opts, &block)
		end
		# set visibility
		mod.__send__ visibility, method_name
	end
end

We are still not satisfied. We need to define a lot of methods like def_after, such as def_before, def_if… Maybe we should define Object::def_ and use it like this:

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class Object
	# use this binding to eval to avoid excessive local variables
	def self.class_binding
		binding
	end
	{
		after:  'result = old.(*); pat.(*); result',
		after!: 'old.(*); pat.(*)',
		before: 'pat.(*); old.(*)',
		with:   'pat.(old.(*), *)',
		chain:  'pat.(old, *)',
		and:    'old.(*) && pat.(*)',
		or:     'old.(*) || pat.(*)',
		if:     'pat.(*) && old.(*)',
		unless: 'pat.(*) || old.(*)'
	}.each do |sym, code|
		str = "def_(:#{sym}) { |old, pat, *| #{code} }"
		str.gsub! ?*, '*args, **opts, &block'
		class_binding.eval str
	end
	singleton_class.undef_method :def_, :class_binding
end

The main difficulty is to implement Object::def_. We can accomplish this just by editing the Object#def_after we defined before:

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class Object
	# the method is going to be undefined soon
	def self.def_ sym, &def_block
		# pat: when refine_block is nil, it is used to represent a refinement
		# singleton: force singleton when self is a Module
		define_method :"def_#{sym}" do |method_name, pat = nil, singleton: false,
		                                &refine_block|
			singleton ||= !is_a?(Module)
			# mod: the module containing the old method
			# get_method: the method to get the Method/UnboundMethod obj
			# def_method: the method to define a new method
			mod, get_method, def_method = singleton ?
					[singleton_class, method(:method), method(:define_singleton_method)] :
					[self, method(:instance_method), method(:define_method)]
			# get visibility
			visibility = mod.method_visibility method_name
			# get pat
			pat = refine_block || {
				to_sym:  ->symbol { get_method.(symbol.to_sym) },
				to_proc: :to_proc.to_proc,
				call:    ->callable { callable.method :call }
			}.each do |duck, out|
				break out.(pat) if pat.respond_to? duck
			end
			# get old
			old = get_method.(method_name)
			# override
			def_method.(method_name) do |*args, **opts, &block|
				# bind old
				old = old.bind self unless old.is_a? Method
				# bind pat
				pat = pat.bind self unless pat.is_a? Method
				# call the new method
				def_block.(old, pat, *args, **opts, &block)
			end
			# set visibility
			mod.__send__ visibility, method_name
		end
	end
end

The final source code can be found here.

The reason why I do not use Module#refine and Module#using is that they currently have too much limitations and even bugs. I have already found as many as two bugs (#16107 and #16617). Although both of them have been fixed (or will be fixed), I cannot be sure that there will not be further and fatal bugs. I do not think these features are very reliable in recent Ruby versions.