Day 021 — Week 3 Review + Recall

Month 1 · Week 3 · ⬅ Day 020 · Day 022 ➡ · Journal index

🎯 Learning Objective

Consolidate Week 3 (methods, interfaces, type assertions, and the full error toolkit) with closed-book recall, and capture what to carry forward.

📚 Topics

• Methods & value vs pointer receivers · method sets
• Interfaces & implicit satisfaction · the nil-interface trap
• Type assertions / type switches (comma-ok)
• Errors as values · %w wrapping · sentinel/custom errors · errors.Is/As/Join
• panic/recover and when to use them

📖 Reading / Sources

• Re-read my notes for [[Day 015]]–[[Day 020]]
• Effective Go — Errors
• Skimmed Go blog — Errors are values

Full write-up: Week 3 Review

📝 Notes

• This is a light day — the detailed synthesis, re-quiz, metrics, and next-week goals live in the week review.
• One-line summary of the week: types gain behavior via methods, interfaces decouple via implicit satisfaction, and errors are values you wrap and inspect — reserving panic for real bugs.

🏋️ Exercises / Practice

Exercise	Status	Link
All Week 3 exercises green (go test ./...)	✅	exercises/month-01/week-3
Closed-book re-quiz (below)	✅	—

🐛 Mistakes Made

• On recall I again said err == ErrSentinel first instead of errors.Is — flagged for spaced repetition.

❓ Open Questions

• Designing a clean public error API (which sentinels/types to export) — parked for the REST-API project.

🧠 Active Recall (answer without looking)

1. Q: Why does a T value not satisfy an interface whose method uses a pointer receiver?
   

   A

   Pointer-receiver methods belong to the method set of `*T`, not `T`. Only `*T` (and an addressable `T` via auto-`&`) satisfies the interface.

2. Q: Name the three errors helpers and when each is used.
   

   A

   `errors.Is` (is a specific sentinel in the chain?), `errors.As` (extract a typed error from the chain), `errors.Join` (combine multiple errors so any can be matched).

🪶 Feynman Reflection

Week 3 was about giving types behavior and handling failure well. Methods let a type act; interfaces let unrelated types be used interchangeably without ceremony; type switches recover concrete types when needed; and errors-as-values plus wrapping make failures readable and inspectable instead of thrown.

🕳️ Knowledge Gaps

• Reflex for errors.Is over ==; nil-interface trap under pressure — drilling both next week.

✅ Summary

Week 3 consolidated: I can add methods with the right receiver, design and satisfy interfaces implicitly, assert/switch on types safely, and build/inspect error chains, reserving panic for genuine bugs.

⏭️ Next Steps / Prep for Tomorrow

• Start Week 4 (generics, testing depth, or stdlib deep-dive per the roadmap). See the week review for goals.

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Time spent	Difficulty	Confidence
75 min	🟦⬜⬜⬜⬜	🟦🟦🟦🟦⬜

Suggested commit: docs(journal): week 3 review and recall (day 021)