Скачать или смотреть Using Kotlin Generics to Create a Type-Safe and Ergonomic Collection of Heterogeneous Elements

Discover how to efficiently manage a heterogeneous collection of elements in Kotlin using generics. Learn to implement type-safe operations without the cumbersome visitor pattern.
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This video is based on the question https://stackoverflow.com/q/68259178/ asked by the user 'mat-tso' ( https://stackoverflow.com/u/16381916/ ) and on the answer https://stackoverflow.com/a/68259651/ provided by the user 'Alexey Romanov' ( https://stackoverflow.com/u/9204/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Introduction

When writing applications in Kotlin, managing collections of heterogeneous types can sometimes become cumbersome. If you've ever found yourself tangled in boilerplate code while trying to operate on a list of generic objects, you're not alone. The visitor pattern, often suggested for such situations, can make the code less ergonomic and less intuitive. In this guide, we'll explore how to streamline working with a heterogeneous collection using Kotlin’s powerful generics features, allowing for more concise and readable code.

The Problem

The primary challenge lies in operating on a list of generic objects—elements that could be of various types. The last known solution utilized a visitor pattern, which while functional, required significant effort from the user, as they had to subclass and implement visitor interfaces each time. This added unnecessary complexity and boilerplate code.

Key Requirements

The elements must be storable in a homogeneous list.

The operation on the elements must remain separate from the class definitions to adhere to concerns of different packages.

Ideally, we want to avoid the unnecessary overhead of visitor implementations.

The Solution

Basic Structure

To solve this problem, we can simplify our approach to the design using a generic data class which directly encapsulates the behavior we want to leverage. Below is an example structure that fulfills the requirements:

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With this setup, each ElementA instance has two components: a produce function that returns an object of type T, and a doSomething function that takes a parameter of type T and performs an action on it.

Using Type Aliasing

To maintain flexibility, we can utilize Kotlin’s type aliases. Although this step is optional, it simplifies working with elements:

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This alias allows us to refer to the ElementA class generically, supporting any type in practice.

Streamlined Operation

Now let's redefine our exampleUse function which operates on a list of these elements. Instead of the cumbersome visitor approach, we can leverage a local function, resulting in a more concise syntax:

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Making it Even More Ergonomic

To enhance user experience further, we can convert our use function into an extension function, thereby allowing it to be called directly on ElementA instances:

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In this structure, users can now interact with each element seamlessly using the use() function which outputs a more idiomatic and concise Kotlin code structure.

Conclusion

By implementing the improvements discussed, we can handle heterogeneous collections of Kotlin elements in a highly ergonomic and type-safe manner. This method not only reduces boilerplate code but also elevates the overall readability and maintainability of our applications.

Final Thoughts

If you find yourself struggling with the complexities of managing different types in your Kotlin applications, consider using generics in the way discussed above. You'll find that it not only streamlines your code but also enhances the productivity of your development process. Happy coding!