Why metaprogram when you can program?

When I sought to learn Ruby, it was for three reasons. I’d heard of this cool thing called blocks, and that they had a lot of great use cases. I read there was this thing called metaprogramming and it was easier and more practical than learning Lisp. Plus, I knew several smart, nice people who were doing Ruby so it was probably a good thing to pay attention to. As it turns out, I will never go back to a language without the first and last. I can’t live without blocks, and I can’t live without smart, kind, fun people.

Metaprogramming requires a little more nuance. I understand metaprogramming well enough to get clever with it, and I understand it well enough to mostly understand what other people’s metaprogramming does. I still struggle with the nomenclature (eigenclass, metaclass, class Class?) and I often fall back to trial and error or brute-force tinkering to get things working.

On the other hand, I think I’ve come far enough that I can start to smell out when metaprogramming is done in good taste. See, every language has a feature that is terribly abused because it’s the cool, clever thing in the language: operator overloading in Scala, monadic everything in Haskell, XML in Java, and metaprogramming in Ruby.

Adam’s Handy Guide to Metaprogramming

This guide won’t teach you how to metaprogram, but it will teach you when to metaprogram.

I want you to think twice the next time you reach for the metaprogramming hammer. It’s a great tool for building developer-friendly APIs, little languages, and using code as data. But often, it’s a step too far. Normal, everyday programming will do you just fine.

There are two principles at work here.

Don’t metaprogram when you can just program

Exhaust all your all tricks before you reach for metaprogramming. Use Ruby’s mixins and method delegation to compose a class. Dip into your Gang of Four book and see if there isn’t a pattern that solves your problem.

Lots of metaprogramming is in support of callback-oriented programming. Think “before”/”after”/”around” hooks. You can do this by defining extension points in the public API for your class and mixing other modules into the class that implement logic around those public methods.

Another common form is configuring an object or framework. Think about things that declare models, connections, or queries. Use method chaining to build or configure an object that acts as a parameter list for another method or object.

Use the weakest form of metaprogramming possible

Once you’ve exhausted your patterns and static Ruby tricks, it’s time to play a game: how little metaprogramming can you do and get the job done?

Various forms of metaprogramming are weaker or stronger than others. The weaker ones are harder to screw up and less likely to require a deep understanding of Ruby. The stronger ones have trade-offs that require careful application and possibly need a lot of explanation to newcomers to your codebase.

Now, I will present to you a partial ordering of metaprogramming forms, in order of weak to strong. We can bicker on their specific placement, but I’m pretty certain that the first one is far better to use frequently than the last.

  • Blocks - I hesitate to call this a form of metaprogramming. But, it is sometimes abused, and it is sometimes smart to use blocks instead of tricks further down this list. That said, if you find yourself needing more than one block parameter to a method, you should consider a parameter object that holds those blocks instead.
  • Dynamic message send on a static object - You set a symbol on an object and later it will send that symbol as a method selector to an object that doesn’t change at runtime. This is weak because the only thing that varies is the method that gets called. On the other hand, you could have just used a block.
  • Dynamic message send on a dynamic object - You set a symbol and a receiver object, at some point they are combined into a method call. This is stronger than the previous form because you’ve got two points of variability, which means two things to hunt down and two more things to hold in your brain.
  • Class.new - I love this method so much. But, it’s a source of potential hurt when trying to understand a new piece of code. Classes magically poofing into existence at runtime makes code harder to read and navigate with simple tools. At the very least, have the civility to assign classes created this way to a constant so they feel like a normal class. Downsides, err, aside, I love this method so much, having it around is way better than not.
  • define_method - I like this method a lot too. Again, it’s way better to have it around than not. It’s got two modes of use, one gnarly and one not-so-bad. If you look at how its used in Rails, you’ll see a lot of instances where its passed a string of code, sometimes with interpolations inside said string. This is the gnarly form; unfortunately, it’s also faster on MRI and maybe other runtimes. There is another form, where you pass a block to define_method and the block becomes the body of the newly defined method. This one is far easier to read. Don’t even ask me the differences in how variables are bound in that block; Evan Phoenix and Wilson Bilkovich tried to explain it to me once and I just stared at them like a yokel.
  • class_eval - We’re getting into the big guns of metaprogramming now. The trick with class_eval is that its tricky to understand exactly which class (the metaclass or the class itself) the parameters to class_eval apply to. The upside is that’s mostly a write-time problem. It’s easy to look at code that uses class_eval and figure out what it intends to do. Just don’t put that stuff in front of me in an interview and expect me to tell you where the methods land without typing the damn thing into IRB.
  • instance_eval - Same tricks as class_eval. This may have simpler semantics, but I always find myself falling back to tinkering with IRB, your mileage may vary. The one really tricky thing you can do with instance_eval (and the class <<some_obj trick) is put methods on specific instances of an object. Another thing that’s better to have around than not, but always gives me pause when I see it or think I should use it.
  • method_missing - Behold, the easiest form of metaprogramming to grasp and thus the most widely abused. Don’t feel like typing out methods to delegate or want to build an API that’s easy to use but impossible to document? method_missing that stuff! Builder objects are a legitimate use of method_missing. Everything else requires deep zen to justify. Remember: friends don’t let friends write objects that indiscriminately swallow messages.
  • eval - You almost certainly don’t need this; almost everything else is better off as a weaker form of metaprogramming. If I see this, I expect that you’re doing something really, really clever and therefore have a well-written justification and a note from your parents.

Bonus principle!

At some point you will accidentally type “meatprogram” instead of “metaprogram”. Cherish that moment!

It’s OK to write a few more lines of code if they’re simple, concise, and easy to test. Use delegation, decorators, adapters, etc. before you metaprogram. Exhaust your GoF tricks. Read up on SOLID principles and understand how they change how you program and give you much of the flexibility that metaprogramming provides without all the trickery. When you do resort to trickery, use the simplest trickery you can. Document it, test it, and have someone review it.

When it comes to metaprogramming, it’s not about how much of the language you use. It’s about what the next person to see the code whispers under their breath. Don’t let your present self make future enemies.

Adam Keys @therealadam