Failure and success should preserve original types

[kibertoad]

Changes

Improve helper types

Checklist

• Apply one of following labels; major, minor, patch or skip-release
• I've updated the documentation, or no changes were necessary
• I've updated the tests, or no changes were necessary

Summary by CodeRabbit

• Tests

  • Added comprehensive test suite for the Either type covering success and failure paths, including type-level preservation and type guards.

• Refactor

  • Updated generic type parameters to improve type inference and type safety.

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Walkthrough

The pull request adds a comprehensive unit test suite for the Either type and updates its factory functions to use const generic parameters. The tests verify both success and failure paths with proper type narrowing, while the implementation changes enhance type precision without affecting runtime behavior.

Changes

Cohort / File(s)	Summary
Either Type Tests src/errors/either.spec.ts	Adds 81 lines of comprehensive unit tests covering success path (Right), failure path (Left), type preservation for literals and objects, and type narrowing with guards.
Either Type Implementation src/errors/either.ts	Updates failure and success factory functions to use const generic parameters (<const T> and <const U>) for improved type precision without changing runtime behavior.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically describes the main change: adding const generic parameters to preserve literal types in the failure and success factory functions.
Description check	✅ Passed	The description follows the required template with all sections present, though the 'Changes' section is vague ('Improve helper types' lacks specificity about what was improved and why).
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/preserve-type

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[CarlosGamero]

A bit late, but LGTM! Thanks Igor 🧡