Kotlin 101: Type Classes Quickly Explained

In this article, we explore the concept of type classes in Kotlin, a powerful tool that allows developers to abstract logic for different data types. We'll take data validation as an example to show how type classes can be used to write generic and reusable code. Our implementation will be based on the Arrow Kt library, which will exploit Kotlin's context receivers. So, without further ado, let's get the party started.

Setting the Stage

We'll use version 1.9.22 of Kotlin and version 1.2.1 of the Arrow library. We'll also use Kotlin's context receivers, which are still an experimental feature. We need to modify the Gradle configuration:

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tasks.withType<KotlinCompile>().configureEach {
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    kotlinOptions {
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        freeCompilerArgs = freeCompilerArgs + "-Xcontext-receivers"
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    }
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}

The Problem

In this article, we'll simulate a system for validating user portfolios in a fintech startup. Data validation is crucial in software development, especially in financial data transactions. Ensuring data conforms to expected formats and rules is vital for maintaining the system's integrity.

So, first, let's define the data we want to validate. The first DTO represents the creation of a new portfolio:

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data class CreatePortfolioDTO(val userId: String, val amount: Double)

The above code could be more optimal and maintainable. We can abstract the validation process in a dedicated function using type classes, which is a common pattern in functional programming.