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Learn how to easily customize error responses in Http4s applications to return JSON instead of plain text.
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Enhancing Error Responses in Http4s with Custom JSON

When developing applications with Http4s, there may be times when you encounter error responses that don't align with your application's design or requirements. A common example is hitting an unknown route, which typically returns a 404 error page with a Content-Type: text/plain and a simple message: "Not found". This is not always user-friendly and might not provide the details you want in a structured format.

In this guide, we’ll explore how to customize these error responses to return them in a more structured and useful format, specifically in JSON. This is particularly useful when you want a consistent API response format across your services.

Problem Overview

By default, when a user tries to access a route that doesn’t exist, Http4s returns a plain text response. This behavior might not be suitable for APIs that expect JSON responses. To transform these responses to return JSON structured errors, we need to implement a solution that alters the HTTP responses generated by our application.

Here’s a quick overview of what we want to achieve:

Return error messages in JSON format.

Ensure the Content-Type of the response is application/json.

Optionally, translate other error statuses to JSON, such as "400 Bad Request".

Initial Implementation

The first step involves mapping the application responses, specifically targeting those with client error status codes (like 404). Below is an example code snippet illustrating how to do this in Http4s:

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

Creating the Error Translator

The ErrorTranslator object will be responsible for checking the response's Content-Type and adjusting it accordingly:

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

In this code:

We're checking the Content-Type of the response.

If it’s not application/json, we wrap the body of the response in a structured JSON format.

Proposed Solution with mapF Function

An alternate and perhaps straightforward solution involves utilizing the mapF function. This method will allow you to respond with a specific JSON object in case of a 404 error. Below is how you could implement that:

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

In the above example:

We create a Response named jsonNotFound which creates a body containing the JSON formatted error message.

We ensure this body has the correct content type by setting the header appropriately.

Conclusion

By implementing this error response customization, you can significantly enhance the way your Http4s application communicates errors. Instead of generic plain text responses, your API will return structured JSON, making it easier for clients to understand and handle errors effectively.

In this post, we explored two viable methods for achieving custom error responses in Http4s. With these strategies, you can improve the user experience of your application while adhering to best practices for API development.

If you have further questions or need assistance with Http4s, don’t hesitate to reach out and share your experiences!