Is "monkey patching" really that bad?

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.

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?

As a lone developer on an isolated problem there are no issues with extending or altering native objects. Also on larger projects this is a team choice that should be made.

Personally I dislike having native objects in javascript altered but it's a common practice and it's a valid choice to make. If your going to write a library or code that is meant to be used by other's I would heavily avoid it.

It is however a valid design choice to allow the user to set a config flag which states please overwrite native objects with your convenience methods because there's so convenient.

To illustrate a JavaScript specific pitfall.

Array.protoype.map = function map() { ... };

var a = [2];
for (var k in a) {
    console.log(a[k]);
} 
// 2, function map() { ... }

This issue can be avoided by using ES5 which allows you to inject non-enumerable properties into an object.

This is mainly a high level design choice and everyone needs to be aware / agreeing on this.

Wikipedia has a short summary of the pitfalls of monkey-patching:

http://en.wikipedia.org/wiki/Monkey_patch#Pitfalls

There's a time and place for everything, also for monkey-patching. Experienced developers have many techniques up their sleeves and learn when to use them. It's seldom a technique per se that's "evil", just inconsiderate use of it.

It's perfectly reasonable to use "monkey patching" to correct a specific, known problem where the alternative would be to wait for a patch to fix it. That means temporarily taking on responsibility for fixing something until there's a "proper", formally released fix that you can deploy.

A considered opinion by Gilad Bracha on Monkey Patching: http://gbracha.blogspot.com/2008/03/monkey-patching.html