How to Write a play-json Reads for Map[(Int, Int), A] in Scala

Learn how to effectively deserialize a JSON structure into a specialized Scala class using play-json by creating a custom Reads for a Map of tiles.
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This video is based on the question https://stackoverflow.com/q/62502859/ asked by the user 'jules' ( https://stackoverflow.com/u/7798989/ ) and on the answer https://stackoverflow.com/a/62505738/ provided by the user 'Matthias Berndt' ( https://stackoverflow.com/u/5582028/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding JSON Deserialization in Scala with play-json

In the world of Scala programming, deserializing JSON data into Scala objects can often lead to challenges, particularly with complex structures like Maps. Today, we'll solve a specific problem: how to correctly write a Reads instance for a Map[(Int, Int), Tile] that represents a game rack in a 5x5 grid.

The Problem

Imagine you have a JSON object that represents a gaming rack, which includes dimensions and tile placements. Here is an example of such a JSON structure:

[[See Video to Reveal this Text or Code Snippet]]

This JSON indicates a 5x5 playing field with one tile located at the position (1,2). To represent this in Scala, we define the following case classes:

[[See Video to Reveal this Text or Code Snippet]]

When attempting to decode the tiles into a Map[(Int, Int), Tile], however, you might encounter an error similar to:

[[See Video to Reveal this Text or Code Snippet]]

This error suggests that the play-json library doesn't natively know how to convert your desired JSON structure into the Map type you need.

Solution: The Custom Reads Instance

Instead of creating a custom Reads for the Map directly, which can be cumbersome, we can make use of a more structured approach by defining a new case class. Here's how we can do that:

Step 1: Define a New Case Class

We introduce a new case class called Tiles, which will specifically encapsulate the handling of tile placements:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Create the Reads Instance

Now, we will implement the Reads instance tailored to the Tiles case class. Here’s how:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Code:

Imports: We import necessary components from the play.api.libs.json package.

Reads.list: This constructs a Reads that transforms a JSON array into a Scala list.

Tuple Creation: For each tile in the JSON array, we create a tuple of ((row, col), tile).

Map Conversion: Finally, we convert the list of tuples into a map and wrap it into the Tiles case class.

Conclusion

By defining a specialized case class and corresponding Reads, we facilitate the deserialization of complex JSON structures with maps in Scala while avoiding the pitfalls of directly handling the Map type in a Reads. This structured approach not only simplifies the code but also enables the play-json library to find the necessary implicits without requiring additional imports.

Now you can deserialize your JSON rack representation into your Scala objects seamlessly. Happy coding!

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