go-kata/05-filesystems-packaging/13-iofs-config-loader/README.md

Kata 13: The Filesystem-Agnostic Config Loader

Target Idioms: io/fs abstraction, fs.WalkDir, Testability via fstest.MapFS, embed readiness
Difficulty: 🟡 Intermediate

🧠 The "Why"

In Go, passing "/etc/app/config" all over the place hard-couples your logic to the OS. Idiomatic Go uses fs.FS so you can:

• load from disk,
• load from embedded files,
• load from a ZIP filesystem,
• unit test without touching the real filesystem.

🎯 The Scenario

Your CLI loads configuration fragments from a directory tree, merges them, and prints a final config report.

🛠 The Challenge

Implement:

• func LoadConfigs(fsys fs.FS, root string) (map[string][]byte, error)

1. Functional Requirements

• Walk root recursively and read all *.conf files.
• Return a map of path -> content.
• Reject invalid paths cleanly.

2. The "Idiomatic" Constraints (Pass/Fail Criteria)

• Must accept fs.FS (not os paths) in the core API.
• Must use fs.WalkDir and fs.ReadFile.
• Must NOT use os.Open / filepath.Walk inside the core loader.
• Unit tests must use testing/fstest.MapFS.

🧪 Self-Correction (Test Yourself)

• If you can’t test without real files: you failed.
• If your loader only works on disk: you failed the abstraction goal.

📚 Resources

• https://pkg.go.dev/io/fs
• https://go.dev/src/embed/embed.go