Mill vs SBT: What are the differences?

Introduction: Mill and SBT are both build tools used in the Scala ecosystem to manage and automate the build process of software projects. While they have some similarities, there are key differences between the two.

1. Dependency handling: In Mill, dependency handling is based on semantic versioning, which ensures that a project only pulls in compatible versions of its dependencies. This means that when a new release of a dependency is made, Mill will automatically determine the latest compatible version and update the project. On the other hand, SBT uses a version resolution mechanism called conflict resolution, which allows multiple versions of the same dependency to coexist within a project. SBT uses an algorithm to determine the most suitable version of a dependency based on different factors like dependency hierarchy and version constraints.

2. Configuration: In Mill, build configuration is defined using a Scala DSL (Domain Specific Language), which provides a concise and type-safe way to define build tasks and dependencies. SBT, on the other hand, uses a declarative configuration file written in Scala, allowing more flexibility and customization options but sometimes at the cost of verbosity and complexity.

3. Build time performance: Mill is designed with performance in mind and aims to provide faster build times compared to SBT. Mill achieves this by utilizing parallelization and caching, where it only rebuilds the necessary parts of a project, resulting in significantly faster incremental builds. SBT, while also offering incremental compilation, is known to be slower and tends to have longer build times, especially for larger projects with complex dependency graphs.

4. Out-of-the-box Features: SBT comes with a rich and extensive set of built-in features and plugins, making it a more mature and feature-complete build tool. It provides a wide range of functionalities like package management, continuous integration, testing frameworks integration, and much more. Mill, on the other hand, provides a more minimalist approach by focusing on the core build tasks, leaving room for customization and extensibility through its flexible plugin system.

5. IDE Support: SBT enjoys better IDE support compared to Mill. It has been around for a longer time and is widely adopted by the Scala community, ensuring better integration with popular Scala IDEs like IntelliJ IDEA, Eclipse, and Visual Studio Code. Mill, although gaining popularity, still has a relatively smaller community and might have limited or less stable IDE integrations.

6. Learning Curve: When it comes to the learning curve, SBT can be more challenging for newcomers due to its complex and versatile nature. It has a steeper learning curve, primarily because of its declarative configuration syntax and the need to understand its advanced features and concepts. Mill, on the other hand, aims to be more beginner-friendly with a simpler and more intuitive approach, making it easier for developers to get started and understand the build process.

In summary, the key differences between Mill and SBT lie in their dependency handling approach, configuration style, build time performance, out-of-the-box features, IDE support, and learning curve. Mill focuses on semantic versioning, offers a Scala DSL for configuration, aims for faster build times, has a minimalist feature set, might have limited IDE support, and provides a beginner-friendly experience. SBT, on the other hand, uses conflict resolution for dependency management, relies on a declarative configuration file, might have slower build times, offers a rich feature set, enjoys better IDE support, but has a steeper learning curve.