Day 006 — Packages & Modules

Month 1 · Week 1 · ⬅ Day 005 · Day 007 ➡ · Journal index

🎯 Learning Objective

Organize code into a reusable package, understand exported vs unexported names, and run a first test.

📚 Topics

• Packages, modules (go.mod), import paths, exported identifiers, doc comments, go test basics

📖 Reading / Sources

• Learning Go ch.10 (modules/packages)
• How to Write Go Code
• Effective Go — Names, Commentary

📝 Notes

• A package is a directory of .go files sharing package <name> (conventionally = directory name). A module is a tree of packages with a go.mod at the root.
• Capitalization is access control: identifiers starting with an uppercase letter are exported (public to importers); lowercase are package-private. No public/private keywords.
• Import by module path + subdir: import "github.com/nabin747/go-from-zero/examples/mathutil".
• Doc comments: a comment immediately above an exported symbol, starting with the symbol's name, becomes its documentation (go doc, pkg.go.dev).
• package main builds an executable; any other name builds a library.
• First taste of testing: *_test.go, func TestXxx(t *testing.T), run with go test ./.... Used table-driven style (preview of Month 2).

💻 Code Examples

package mathutil
// Max returns the larger of a and b.
func Max(a, b int) int { if a > b { return a }; return b } // exported (uppercase)
func clampNeg(n int) int { … }                              // private (lowercase)

Full code: examples/mathutil/ · Test: go test ./examples/mathutil

🏋️ Exercises / Practice

Exercise	Status	Link
Extract mathutil package (Max/Min/Abs/IsPrime/Sum)	✅	examples/mathutil
Table-driven tests for it	✅	examples/mathutil/mathutil_test.go

🐛 Mistakes Made

• Named a function max (lowercase) then tried to call it from another package — unexported, so it wasn't visible. Renamed to Max.
• Ran go test in the wrong directory (no Go files) — must be inside the module / use ./....

❓ Open Questions

• When to split into multiple packages vs keep flat? (Answer: split by responsibility, not by type; avoid premature packages — keep it flat until it hurts.)

🧠 Active Recall

1. Q: How do you make a function usable by other packages?
   A

   Start its name with an uppercase letter (export it).
2. Q: What distinguishes a module from a package?
   A

   A module is a collection of packages versioned together, rooted at a go.mod; a package is one directory of related .go files.

🪶 Feynman Reflection

Folders of Go code are "packages"; a project of packages with a go.mod is a "module." Whether something is public is decided by ONE rule: capital first letter = visible to others.

🕳️ Knowledge Gaps

• Internal packages (internal/) and semantic-version import rules — deferred to project work (Month 4).

✅ Summary

I can structure code into a documented, tested, reusable package and control its visibility.

⏭️ Next Steps / Prep for Tomorrow

• Day 007: Week 1 review + closed-book recall.

---

Time spent	Difficulty	Confidence
100 min	🟦🟦⬜⬜⬜	🟦🟦🟦⬜⬜

Suggested commit: refactor(examples): extract mathutil package with tests (day 006)