Notes for Mill Support Implementation - 2025-04-02

Today's Accomplishments

1. Completed research on Mill build tool structure and extension development
2. Analyzed existing SBT infrastructure to understand equivalent implementation in Mill

3. Created proof-of-concept implementation of key components:

   • IWMillVersions - Centralized version management
   • IWMillDeps - Standardized dependency management
   • IWScalaModule - Standard module configuration
   • IWPublishModule - Publishing configuration

4. Created example project showing usage

5. Documented approach and usage in README.md

Key Observations

Advantages of Mill

1. More intuitive and readable build files

   • Regular Scala code vs. SBT's DSL
   • Method-based configuration vs. key-value pairs
   • Cleaner module definition and inheritance

2. Simpler extension development

   • Regular library vs. complex plugin system
   • Trait-based sharing of functionality
   • Easier to understand and maintain

3. Better performance

   • Aggressive caching of build tasks
   • More explicit task dependencies
   • Faster incremental compilation

4. Improved developer experience

   • Better IDE support with type-checked build files
   • More consistent error messages
   • Easier to debug and troubleshoot

Implementation Notes

1. IWMillVersions

   • Direct port of IWMaterialsVersions
   • Same version constants, same naming scheme
   • Can be used directly by library users

2. IWMillDeps

   • Uses Mill's ivy"org::name:version" syntax
   • Same helper methods as IWMaterialsDeps (useZIO, useZIOAll, etc.)
   • Returns Loose.Agg[Dep] instead of SBT's Seq[Def.Setting[_]]

3. IWScalaModule

   • Extends Mill's ScalaModule and ScalafmtModule
   • Sets default Scala version (3.6.3)
   • Configures standard compiler options
   • Enables SemanticDB for tooling
   • Includes nested IWTests for standard test configuration

4. IWPublishModule

   • Configures publishing to IW Maven repositories
   • Handles snapshots vs. releases based on version
   • Uses environment variables for credentials

Issues to Address

1. Scala.js Support

   • Need to implement IWScalaJSModule for JS compilation
   • Need to handle cross-platform dependencies properly
   • Need to support timezone and locale configuration

2. Mill Plugin Ecosystem

   • Need to identify and integrate with essential Mill plugins
   • Equivalent functionality to SBT plugins we currently use

3. Testing Framework

   • Currently using utest, but need to support ZIO Test
   • Need to configure proper test initialization

4. Build Integration

   • Need to test with Nix and direnv
   • Need to ensure compatibility with CI/CD pipelines

Next Development Steps

For Next Session (2025-04-03)

1. Create IWScalaJSModule

   • Support for Scala.js compilation
   • Cross-platform dependency handling
   • JS-specific configuration

2. Enhanced Test Support

   • Implement ZIO Test integration
   • Configure proper test initialization
   • Add test logging configurations

3. Multi-Module Example

   • Create example with multiple modules
   • Show inter-module dependencies
   • Demonstrate publishing configuration

For Future Sessions

1. Documentation

   • Create comprehensive migration guide
   • Document all traits and components
   • Provide examples for different use cases

2. Integration Testing

   • Test with real-world projects
   • Measure performance vs. SBT
   • Validate build equivalence

3. Release Planning

   • Define versioning strategy
   • Setup CI/CD for the library
   • Create announcement for the team

Migration Considerations

1. Incremental Approach

   • Start with new projects
   • Test on non-critical projects first
   • Gradually migrate existing projects

2. Dual Build Configuration

   • Initial phase: support both SBT and Mill
   • Update IWMaterialsVersions and IWMillVersions together
   • Maintain feature parity during transition

3. Knowledge Transfer

   • Create training materials
   • Run workshops for the team
   • Pair programming for initial migrations

4. Compatibility Monitoring

   • Keep track of SBT plugin updates
   • Ensure Mill implementation stays up to date
   • Regular testing of both build systems