Is "monkey patching" really that bad?

Question

Some languages like Ruby and JavaScript have open classes which allow you to modify interfaces of even core classes like numbers, strings, arrays, etc. Obviously doing so could confuse others who are familiar with the API but is there a good reason to avoid it otherwise, assuming that you are adding to the interface and not changing existing behavior?

For example, it might be nice to add a an Array.map implementation to web browsers which don't implement ECMAScript 5th edition (and if you don't need all of jQuery). Or your Ruby arrays might benefit from a "sum" convenience method which uses "inject". As long as the changes are isolated to your systems (e.g. not part of a software package you release for distribution) is there a good reason not to take advantage of this language feature?

Answer 1

Monkey-patching, like many tools in the programming toolbox, can be used both for good and for evil. The question is where, on balance, such tools tend to be most used. In my experience with Ruby the balance weighs heavily on the "evil" side.

So what's an "evil" use of monkey-patching? Well, monkey-patching in general leaves you wide open to major, potentially undiagnosable clashes. I have a class A. I have some kind of monkey-patching module MB that patches A to include method1, method2 and method3. I have another monkey-patching module MC that also patches A to include a method2, method3 and method4. Now I'm in a bind. I call instance_of_A.method2: whose method gets called? The answer to that can depend on a lot of factors:

1. In which order did I bring in the patching modules?
2. Are the patches applied right off or in some kind of conditional circumstance?
3. AAAAAAARGH! THE SPIDERS ARE EATING MY EYEBALLS OUT FROM THE INSIDE!

OK, so #3 is perhaps a tad over-melodramatic....

Anyway, that's the problem with monkey-patching: horrible clashing problems. Given the highly-dynamic nature of the languages that typically support it you're already faced with a lot of potential "spooky action at a distance" problems; monkey-patching just adds to these.

Having monkey-patching available is nice if you're a responsible developer. Unfortunately, IME, what tends to happen is that someone sees monkey-patching and says, "Sweet! I'll just monkey-patch this in instead of checking to see if other mechanisms might not be more appropriate." This is a situation roughly analogous to Lisp code bases created by people who reach for macros before they think of just doing it as a function.