Unlock the Power of Scala 3 Enums: Learn How to Provide a Generic Circe Decoder

Step 1: Create a Generic Decoder Trait

To start, let’s create a trait that will serve as the foundation for our generic decoder. We’ll call it `EnumDecoder`:

import io.circe._

trait EnumDecoder[E <: Enumeration] {
  implicit def enumDecoder(implicit e: E): Decoder[E#Value] = {
    Decoder[String].emap { s =>
      Try(e.withName(s)).toOption.toRight(DecodingFailure(s"Invalid ${e.getClass.getName} value: $s", Nil))
    }
  }
}

In this trait, we define an implicit decoder that takes an `Enumeration` value as an implicit parameter. The decoder uses the `Decoder[String]` to decode the JSON string and then attempts to find an Enum value with the corresponding name using `e.withName(s)`. If no matching value is found, a `DecodingFailure` is returned.

How Does it Work?

The magic happens when we use the `enumDecoder` implicit value in our decoder pipeline. Circe will automatically pick up the implicit decoder and use it to decode the Enum value.

Let’s create an example Enum, `Color`, to demonstrate this:

object Color extends Enumeration {
  type Color = Value
  val Red, Green, Blue = Value
}

Now, we can use our generic decoder to decode a JSON string into a `Color` value:

import io.circe.parser._

object Example {
  def main(args: Array[String]): Unit = {
    val json = """{ "color": "Red" }"""
    val decoder = EnumDecoder[Color.type]
    val result = parser.parse(json).flatMap(_.as[Color.Color](decoder.enumDecoder))
    println(result) // Right(Red)
  }
}

Step 2: Extend the Generic Decoder for More Advanced Use Cases

While our generic decoder works beautifully for simple Enums, we might need to handle more complex scenarios, such as Enums with custom names or additional metadata. To address this, we can extend our `EnumDecoder` trait to provide more flexibility.

Example: Enums with Custom Names

Sometimes, Enums have custom names that don’t match the JSON data. We can add an extension to our `EnumDecoder` to handle this:

trait EnumDecoderWithCustomNames[E <: Enumeration] extends EnumDecoder[E] {
  implicit def enumDecoderWithCustomNames(implicit e: E): Decoder[E#Value] = {
    val customNames = e.values.map(v => v.toString -> v).toMap
    Decoder[String].emap { s =>
      customNames.get(s).orElse(e.values.find(v => v.toString.equalsIgnoreCase(s))).toRight(
        DecodingFailure(s"Invalid ${e.getClass.getName} value: $s", Nil)
      )
    }
  }
}

In this extension, we define a new decoder that takes into account custom names for the Enum values. We create a map of custom names to Enum values and use it to look up the correct value when decoding the JSON string.

Example: Enums with Additional Metadata

Another scenario might involve Enums with additional metadata, such as descriptive labels or icons. We can extend our `EnumDecoder` to handle this:

trait EnumDecoderWithMetadata[E <: Enumeration] extends EnumDecoder[E] {
  implicit def enumDecoderWithMetadata(implicit e: E): Decoder[E#Value] = {
    val metadata = e.values.map(v => v.toString -> v).toMap
    Decoder[Json].emap { json =>
      val stringValue = json.as[String].right.get
      metadata.get(stringValue).orElse(e.values.find(v => v.toString.equalsIgnoreCase(stringValue))).toRight(
        DecodingFailure(s"Invalid ${e.getClass.getName} value: $json", Nil)
      )
    }
  }
}

In this extension, we define a new decoder that takes into account additional metadata associated with the Enum values. We use the `Decoder[Json]` to decode the JSON object and then extract the Enum value based on the metadata.

Conclusion

In this article, we’ve embarked on a journey to provide a generic Circe decoder for Scala 3 Enum values. We’ve explored the benefits of a generic decoder, created a basic decoder trait, and extended it to handle more advanced use cases.

By following these steps, you’ll be able to decode Enums with ease, ensuring type safety, flexibility, and code reusability in your Scala applications.

Remember, the key to a successful generic decoder is to design it with flexibility and extensibility in mind. By doing so, you’ll be able to adapt to changing requirements and handle complex Enum use cases with ease.

Best Practices and Next Steps

As you continue to work with Enums and Circe in Scala, keep the following best practices in mind:

• Keep your decoders separate from your Enum definitions to maintain a clean and modular design.
• Use implicit values to enable automatic decoder selection by Circe.
• Test your decoders extensively to ensure correct behavior and error handling.
• Consider using a type class approach to provide a more flexible and composable decoder system.

With these guidelines and the knowledge gained from this article, you’re ready to embark on your own journey of providing a generic Circe decoder for Scala 3 Enum values. Happy coding!

Frequently Asked Question

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