A benefit of class_eval is that it will fail if the class doesn’t already exist. This prevents you from creating new classes accidentally. When you call a method, you’re actually sending a message to an object. When we invoke or call this method using the dot-notation, what we are really saying is that we are passing a message to the String, and it decides how to respond to that message. In this case, it responds with a downcased version of itself. In a layered architecture, the bottom-most layer could be depended upon by many other libraries, which sit on top it.

After determining the class of the object it is about to format, the method dynamically calls the corresponding formatter using send. The Ruby language does a wonderful job at hiding code from users. What we did not see here is that Ruby created a metaclass for the object “Ana” and the “Ana.dance” method is held within the metaclass. We can easily identify that “dance” is an instance method and that “breathe” is a class method considering the addition of the “self” prefix. Metaprogramming simply refers to the technique of writing code that can manipulate or generate code. If this article piqued your interest, keep an eye out for the next installment in this series.

Recall that almost everything in Ruby is an object, meaning that Ruby also generated a metaclass for the “Person” class. In fact, the class method “breathe” is contained inside of it. As-of-yet-undefined methods is quite useful for Ruby metaprogramming.

Ruby developers can define an attached file on a model with the has_attached_file method. To understand Ruby metaprogramming in an easy-to-understand style, read the Ruby metaprogramming book by Paolo Parotta. When we use the send method, the name of the method and arguments are sent as a message to the calling object. This is where we answer your question; we explain the concept of Ruby metaprogramming, why and when to use it, its best practices, and real-world examples in this article. Classes are objects, and class names are just constants.

Ruby metaprogramming is indeed intimidating, but mostRuby on Rails developershaven’t come across its real-world examples. This is where we enlist some of them, so take a look at it for a better understanding of metaprogramming in the real world. Metaprogramming especially comes in handy when handling methods that are similar except for identifiers. You can utilize define_method to create methods and when identifiers are added in the later phase the methods will be present beforehand. A common term you may come across when looking into metaprogramming in Ruby is the monkey patch. The monkey patch is a way for a program to extend or modify system classes locally in an object-oriented language.

There are two other methods (starting Ruby 1.9) instance_exec and class_exec. They only evaluate blocks and they accept arguments and pass them to the block. So the block code is evaluated Offshore Software Development Services in the context of the specified object, with parameter from the outside. Metaclass, also called eigenclass or singleton class, is generated during the execution of the program.

Defining Methods Using “class_eval” and “instance_eval”

By using this piece of code we can create thousands of new methods starting with “newApp_” through the help of define_method . In this article, we’ll be looking at a few different examples of metaprogramming in Ruby, specifically in using the Sinatra and Rails frameworks. The Class constructor and the define_method allows you to generate classes and methods on the fly, as the program is running. All instance variables are instance variables of self, and all methods without an explicit receiver are called on self.

Since the class had previously been defined at run-time, the compiler adds the method to the class​ and no new class is created. Ruby opens the prefined Decision class and appends the method disagree in the same class. After peeking into how Ruby utilizes metaprogramming internally, we, as developers, may want to exploit the versatility of metaprogramming. Indeed, we have multiple methods in Ruby that can achieve just that.

Ruby Metaprogramming Real-World Examples

After learning all the relatively vague and theoretical definitions of what metaprogramming is, let’s really familiarize ourselves with this rather powerful concept. An object’s instance variables live in the object itself, and an object’s methods live in the object’s class. In Ruby, however, most language constructs are still there.

The block form of class_eval is preferable to the string-interpolated form. So we can see that this is the method that prints to STDOUT. Method, we see that this is not quite the same code as we had. Instead of opening up the class using the class keyword we use a class_eval.

• First we will add a new method to the Module class called delegar .
• DSL is a topic in itself but these basic concepts are a prerequisite to understanding advanced topics.
• Metaprogramming is often presented as a very nebulous and dangerous concept, one that is difficult to explain and hard to wrap your head around, and thus should be avoided.
• With this piece of code we can create literally thousands of new methods starting with “coding_”, and that fact is what makes our code DRY.
• Using metaprogramming in your code forces the client to use too many nested blocks, and unfortunately, you can see many open-source projects that use RSpec commit this sin.

They get transformed into byte-code or CIL (C#), or plain machine-code . You can’t modify these representations once the program has been compiled. We’ll specifically look at how we can read and analyze our code in Ruby, how we can call methods dynamically, and how we can generate new methods during the runtime of our program. After you master these building blocks of metaprogramming and truly understand its essence, you can proceed to something more complex, for example create your own Domain-specific Language .

Ruby Metaprogramming Explained: Key Aspects and Real-World Examples

So, it must be agnostic to any of the layers above to be reusable. Having the dependency direction pointing upward is wrong, and one of the horrible sins a programmer can commit. Even though it’s related to the introspective aspect of Ruby more than metaprogramming, I think the impact is huge and worth mentioning. If you don’t fallback to super, then it might lead you to bugs that are really hard to track. Remember, method_missing is where the bugs go to hide.

However, if you know what you are doing, monkey-patching can be very powerful. For example, the ActiveSupport gem in Rails makes heavy use of monkey-patching to open Ruby core classes and define new functionality on them. What a gem.Metaprogramming is writing programs that write programs. For example, when a User model is defined in Rails with email as an attribute, a method called find_by_email will be generated.

Calling a method on a class is the same as calling a method on an object. Evaluates a block in the context of an existing class. This allows you to reopen the class and define additional behavior on it.

Otherwise we just call super to do the work of reporting a method that is actually missing. And we are simply using define_method to create that new https://topbitcoinnews.org/ method. With this piece of code we can create literally thousands of new methods starting with “coding_”, and that fact is what makes our code DRY.

Two phase commit transactions with MongoDB & .Net Core

How do you create an organization that is nimble, flexible and takes a fresh view of team structure? These are the keys to creating and maintaining a successful business that will last the test of time. In the previous example, you can see how Rails uses super to propagate a call that the current method does not know how to handle. In the StringInquirer class above, if the method does not end with a question mark, then it allows the call to propagate further up by calling super. The method_missing implementation checks to make sure the method ends with a question mark. If it does, it chops that question mark off from the method name and compares it to the current object .

This isn’t much different than using the usual def to create a method but it does allow us to keep our code DRY. Because you’re sacrificing readability and comprehension of code without gaining anything in return. The list of HTTP verbs is stable — they almost never change. But by using metaprogramming, you have increased the complexity by dynamically defining methods for the HTTP verbs.

The following code is part of the backend/spec/controllers/spree/admin/payments_controller_spec.rb. Taking a look at the project we see that awesome_print.rb has two main files, inspector and formatter, and then a number of core extensions which do various monkey patches on core ruby classes. That also includes core language concepts like classes and methods.