Add Writer monad implementation with comprehensive tests

[robertvansteen]

Summary

This PR introduces a new Writer monad to the library, enabling computations that carry values alongside accumulated logs. The Writer monad is useful for tracking side effects, audit trails, and other logging scenarios while maintaining functional purity.

Key Changes

• New Writer Monad: Added src/Writer/Writer.php with a complete implementation featuring:

  • map() - Transform the value while preserving the log
  • andThen() - Chain computations with automatic log combination
  • tell() - Append entries to the log
  • mapLog() - Transform the accumulated log
  • inspect() - Execute side effects without changing the value
  • reset() - Clear the log to a new value
  • listen() - Access both value and log in a computation
  • run() - Extract both value and log as a tuple

• Helper Function: Added src/Writer/functions.php with a Writer() factory function that creates Writers with array-based logs using array spread for combining entries

• Comprehensive Tests: Added tests/Writer/WriterTest.php with 11 test cases covering all operations and immutability guarantees, plus type assertion tests in tests/Writer/types.php

• Code Quality: Applied consistent formatting across the codebase:

  • Removed unnecessary blank lines in imports
  • Standardized closure syntax (fn() vs fn ())
  • Added trailing commas in multi-line parameter lists
  • Fixed string concatenation formatting in type assertions

• Autoload Configuration: Updated composer.json to include the new Writer functions file in autoload

Implementation Details

The Writer monad uses a generic combiner function to support flexible log types (arrays, strings, monoids, etc.). The implementation maintains immutability throughout all operations and properly chains logs when using andThen(). The default array-based Writer uses array spread syntax for efficient log combination.

https://claude.ai/code/session_01DZ1sjcUYedPJhLMyCBw7ZW