What is monkey patching and why is it so abhorrent?

Question

Is a monkey patch when you extend a class?

class Hash
    def delete_blanks!
       delete_if { |k, v| v.is_nil? }
    end
end

Then you can do this:

h = { red: 'stop', yellow: 'ready', purple: nil, green: 'go'}
h.delete_blanks! #=> { red: 'stop', yellow: 'ready', green: 'go' }

Is that a monkey patch? What about this:

class ActiveRecord::Base
    def foo
        "bar"
    end
end

What's so bad about that?

I'm not being argumentative, I'm ready to assume it is bad, but how should I go about emulating this behaviour without a monkey patch? Should I send the method?

Answer 1

I wouldn't say monkey patching is bad, it's more like a "should avoid" practice, there are definitely scenarios where monkey patching is useful.

Another way to solve this problem is through inheritance so you could have something like:

class SuperHash < Hash
  def delete_blanks!
    delete_if { |k, v| v.is_nil? }
  end
end