ruby struct dynamic constant assignment

What is Dynamic Constant Assignment, and How Can You Avoid It?

Have you ever stumbled across the “dynamic constant assignment error” in Ruby? If so, you’re not alone. It’s a common pitfall that newer Ruby developers may encounter and its explanation can seem confusing. Here, we’ll clarify what dynamic constant assignment means in Ruby and how you can avoid it.

Understanding the Problem

In Ruby, constants are meant to be immutable - they should not be changed once assigned. Ruby enforces this by discouraging the assignment of constants within code blocks that might be executed multiple times, such as inside methods.

For example, consider the following code:

When this code is executed, Ruby will raise a “SyntaxError: dynamic constant assignment error” because we are attempting to assign a value to a constant within a method.

The Solution

To avoid this error, you should define constants outside of methods, typically within the class definition itself:

This ensures that the constant is defined once and cannot be modified within methods.

While the above solution is the recommended approach, there can be situations where you genuinely need to define a constant within a method. In such cases, you can use the const_set method:

However, this approach is discouraged and should be used sparingly.

Dynamic constant assignment is a potential issue in Ruby because it can lead to unintended changes in the behavior of your code. By following the guidelines outlined in this article, you can avoid this error and ensure the stability of your Ruby programs.

Remember to define constants outside of methods and use const_set only when absolutely necessary. Keep in mind that constants should be treated as immutable values and any modifications should be carefully considered.

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Ruby Struct Explained

Simplifying data structures.

Table of contents

Basic usage, when to use struct, when not to use struct, play with it, standard boilerplate, alternatives comparison.

In simple words, Ruby Struct is a built-in class that provides useful functionalities and shortcuts. You can use it for both logic and tests. I will quickly go through its features, compare with other similar stuff, and show some less-known but still useful information about it.

All my notes are based on years of hands-on experience at iRonin.IT - a top software development company , where we provide custom software development and IT staff augmentation services for a wide array of technologies.

As you can see, it behaves like a simple Ruby class. The above code is equivalent to:

What if we want to define the #full_name name on our Employee class? We can do it with Struct as well:

Struct is often used to make code cleaner, format more structured data from a hash, or as a replacement for real-world classes in tests.

Temporary data structure - the most popular example is a geocoding response where you want to form Address object with attributes instead of a hash with the geocoded data.

Cleaner code

Testing - as long as Struct responds to the same methods as the object used in tests, you can replace it if it does make sense. You can consider using it when testing dependency injection.

Avoid inheritance from Struct when you can. I intentionally assigned Struct to constant in the above example instead of doing this:

When your class inherits from Struct you may not realize that:

Arguments are not required - if one of the passed arguments it's an object then calling a method on it will cause an error

Attributes are always public - it is far from perfect encapsulation unless you desire such behavior

Instances are equal if their attributes are equal - Employee.new == Employee.new

Access the class attributes the way you want:

Use the equality operator:

Iterate over values or pairs:

Alternatives

Hash is also considered as an alternative to Struct . It is faster to use but has worse performance than its opponent (I will test it a little bit later in this article).

OpenStruct is slower but more flexible alternative. Using it you can assign attribute dynamically and it does not require predefined attributes. All you have to do is to pass a hash with attributes:

Although it may get annoying when you have to type it multiple times:

I used the following code to measure the performance of the above solutions: