Metaprogramming
Ruby Internals
Summarized using AI
Ruby Internals
by Patrick FarleyIn this video, Patrick Farley delivers a detailed presentation on Ruby internals during the MountainWest RubyConf 2008, primarily focusing on the MRI (Matz's Ruby Interpreter) while also mentioning JRuby and Rubinius. Farley aims to emphasize the importance of understanding Ruby internals, particularly for developers using meta-programming techniques in their applications.
Key points discussed in the presentation include:
- Understanding Ruby Internals: Farley stresses that while exploring Ruby internals may seem daunting, it provides valuable insights into how Ruby functions. He discusses the potential pitfalls developers may encounter if they do not grasp these concepts fully.
- Module Behavior & Method Sharing: He illustrates the unexpected behavior of modules in Ruby, particularly the limitations of including module methods as class methods, hinting at deeper implications of method dispatch in Ruby.
- Meta-Programming Techniques: Farley highlights the prevalent use of meta-programming in Ruby, suggesting that many developers rely on these advanced features without understanding why they work, which can lead to debugging challenges.
- Method Dispatch: The core of Ruby's behavior lies in method dispatch, where method calls are resolved based on object classes and inheritance hierarchies. He explains how method tables and class pointers affect method resolution in Ruby.
- Examples and Humor: Throughout the talk, he integrates examples, such as the behavior of methods defined in classes and how instance-specific behaviors can be added—a Ninja class example underscores this point.
- Books for Further Learning: Farley recommends resources such as the 'Ruby Hacking Guide' and notes the necessity of understanding Ruby's underlying mechanics to avoid programming by coincidence.
- Conclusion: He concludes by emphasizing that a deeper understanding of Ruby's method dispatch and object behaviors is vital, particularly in complex applications and when utilizing Ruby's dynamic capabilities.
Farley's session underscores not just the complexities of Ruby's behavior but also the responsibility developers have to understand the tools they use, promoting better programming practices within the Ruby community.
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Hello, everyone. I'm Patrick Farley, a developer with ThoughtWorks, and today I'm going to discuss Ruby internals.
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To clarify what I mean by Ruby internals,
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I have some JRuby slides and some Rubinius slides, but mostly, I will focus on MRI stuff.
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Most of you are likely using MRI, which is Matt's Ruby interpreter, especially if you're on a MacBook Pro.
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Now, I think it's fair to ask why you should care about Ruby internals.
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Digging into this topic takes some effort, so what value does it really provide?
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To illustrate this, I'll look at some Ruby code.
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I'll focus on the weirder side of Ruby, but it's still pretty standard stuff.
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For instance, I am defining a module here and creating a method, self.bar, within the module Foo.
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Anyone who has worked with Ruby knows that there’s trouble ahead.
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If I’m using modules purely for namespace purposes, like creating a function library called Foo, I should be okay.
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However, if I intend to use modules for code sharing, meaning multiple inheritance,
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that’s where I’ll run into trouble.
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Because anyone familiar with Ruby realizes that when I include this module in a new class, let's say Baz,
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I won’t get bar as a class method.
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We learn this early on when we start with Ruby: you can’t bring class methods along when you include a module.
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It’s peculiar to think about why that is.
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Similarly, in this example, I'm defining a basic module called Laughable.
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It has a method, laugh, and this should seem familiar to many people.
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I extend the class Clown with Laughable, which gives me a class method called laugh.
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Interestingly, if I create a client object and then extend the Clown object with Laughable,
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I get an instance method on Clown.
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Again, this might seem straightforward, but it’s a bit unusual.
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Ruby provides both length and size methods on arrays, yet it appears to overload extends based on context.
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There are various syntaxes to define a class method.
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Many know that when you open up a singleton class and define a method, that magically becomes a class method.
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The question remains: why is that the case?
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Let’s look at a specific behavior in Rails.
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When you call id on nil, it returns four.
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By default, Ruby was returning four.
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Rails assumes that most likely you didn’t intend to introduce the number four into your codebase this way.
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You probably don’t want to do like three over id of nil.
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So, Rails warns you: it throws an error and says, "By the way, you probably don’t want to do that."
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It's surprising how this works.
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By a show of hands, how many of you have read the Pragmatic Programmer?
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The Ruby community is fantastic.
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I acknowledge that I'm preaching to the choir a bit here.
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In the Pragmatic Programmer, there’s a section that discusses programming by coincidence, featuring a programmer named Fred.
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Fred writes code that works, then writes a bit more code that works, and eventually, his code stops functioning.
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He struggles to figure out why and faces challenges in debugging.
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The insight from that practice is that Fred doesn't actually know why his code worked in the first place.
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My viewpoint is that a significant amount of this occurs in the Rails community.
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Not just in Rails but in the broader Ruby community as well.
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When diving into more advanced features of the language, many developers are using meta-programming techniques by example but lack the understanding of why those techniques work.
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This is a craftsmanship issue.
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It's our responsibility to thoroughly understand the tools we utilize, rather than waiting until we're in a panic like Fred.
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We've all encountered moments with meta-programming when things don’t work, leading us to believe that meta-programming might be bad.
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The intent isn't to discourage using meta-programming, but it is an essential tool to have in our tool belt.
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If you're developing Ruby-based Domain Specific Languages or more intricate applications, understanding why meta-programming works is crucial.
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Much of what you encounter concerning modules, mixins, metaclasses, and singleton classes is fundamentally about method dispatch.
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Matt had many options for implementing specific behavior or mixins;
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however, he made the elegant decision that method dispatch would always function the same way in MRI with no exceptions.
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This decision elegantly constrains the implementation options for instance-specific behavior, class methods, modules, and mixins.
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If you grasp how method dispatch operates in Ruby, it becomes much easier to learn how all these other aspects work.
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Remember, method dispatch always functions the same way.
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Regarding book recommendations, I recommend the Ruby Hacking Guide by Minero Aoki.
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Although it's written in Japanese, there are some partial translations available online.
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It covers much of the material I’ll discuss today.
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Although somewhat dated, it still contains valuable insights.
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I encourage you to Google for partial translations, and if you know Japanese, consider contributing to the translation effort.
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I'd also like to mention another book that has received positive feedback.
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Now, let's delve into method dispatch.
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I have a Ninja class featuring a method called b_awesome, which returns the symbol guitar_solo.
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I know this sounds silly; you may wonder why guitar_solo is a symbol.
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I've created an instance of Ninja named Bob and I call the b_awesome method.
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The question is, how does the b_awesome method get connected to the method information found?
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Interestingly, I made a point to add props to my slides before the sombrero concept appeared.
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The sombrero concept was a humorous notion, but it was difficult to visualize.
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Unfortunately, my efforts to create a visual representation of that idea fell short.
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I tried my best to insert an image but faced issues;
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needless to say, I gave up on that.
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Now, the following is a c-struct r object that represents Ruby objects.
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In Ruby, everything is an object, and I'm referring to standard objects now.
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Here's what it looks like without any reduction: a couple of data elements.
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The first is this basic struct, which we'll describe, and the next is this st_table iv table.
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So, while it seems complex, let’s break it down.
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Heroes fans, ever notice how certain characters reflect specific traits?
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There are analogies in the relationship between C and Ruby programmers.
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They often express confusion when they initially experience C programming concepts.
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Given the second part, the st_table instance variable table or sc_table acts as a hash implementation.
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The iv stands for instance variables, so Ruby knows an object’s instance variables, stored in a hash.
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The other aspect Ruby recognizes is tied to this r_basic.
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There are only three things everyone knows about an object: its instance variables, its class, and its properties collection.
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Importantly, behavioral aspects are missing; Ruby functions differently from prototype-based languages.
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This is a useful perspective when thinking about metaclasses, singleton classes, and so on.
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Understanding that Ruby’s design encourages storing behavior at a class level is key.
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When discussing Ninja and b_awesome, this behavior must reside somewhere.
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Let’s take a look at the struct that represents a class.
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It's interesting to consider classes as objects in MRI.
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From a usage perspective, they are, but from an implementation level, it's a different story.
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Classes—on the surface—stem from a unique struct, as you're probably seeing.
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The first two elements you'll see are familiar and an instance variable table.
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But two important distinctions exist: a method table containing behavior and a superclass pointer.
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Trajectory of method dispatch is crucial.
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When sending a message to an object, the interpreter dereferences the class pointer.
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It looks for the method in the method table hash.
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If not found, it processes the super pointer iteratively.
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The interpreter keeps referencing until ultimately hitting null and declaring method missing.
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Visually, this works like this.
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The class pointer dereferences and searches for the method within that hierarchy.
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After understanding method dispatch, we can discuss singleton classes and instance-specific behavior.
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Consider a new Ninja named Joseph.
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But here’s the twist: Joseph is a pirate hiding as a ninja.
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He has a specialized method called speak.
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If you're familiar with Ruby, you might recognize this as defining joseph.speak.
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This is one way to add instance-specific behavior.
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In this case, I’ve added the method directly to Joseph.
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When he speaks, he says 'arr', while Bob, being a ninja, can’t articulate anything.”},{
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