Table of Contents

• Month 1 · Week 3 — Exercises
• How to use
• Prompts
• Results

Month 1 · Week 3 — Exercises

Practice for methods, interfaces, type switches, and the Go error toolkit. Each folder is its own package with a solution and table-driven tests, using only the standard library.

Exercise	Concept	Day	Run tests
shapes/	interfaces, implicit satisfaction, type switch	016–017	go test ./shapes
counter/	value vs pointer receivers, mutating method sets	015	go test ./counter
validate/	sentinel/custom errors, errors.Is/As/Join	018–019	go test ./validate

How to use

1. Try it yourself first — set the solution aside and re-implement from the prompt below.
2. Run go test ./... from this directory.
3. Compare with the provided solution; log differences in your day note's "Mistakes" section.

Prompts

• shapes: Define a Shape interface (Area(), Perimeter()), implement Rectangle and Circle, then write TotalArea([]Shape) and a Describe type switch (square is a rectangle with equal sides). Add compile-time var _ Shape = ... assertions.
• counter: Build a Counter (Inc, Add, Reset, Value) and a Stack (Push, Pop comma-ok, Len). Use pointer receivers so mutation persists, and write a test proving a value copy diverges from the original.
• validate: Create sentinel errors (ErrEmpty, ErrTooYoung) and a custom FieldError{Field, Err} with Unwrap. ValidateUser should accumulate all failures with errors.Join. Test with errors.Is (sentinels) and errors.As (extract the *FieldError).

Results

Exercise	Tests	Status
shapes	Area, Perimeter, TotalArea, Describe	✅ pass
counter	Counter, CopyDiverges, Stack	✅ pass
validate	Valid, Is, As, WrappingPreservesChain	✅ pass

Tip: run all of Week 3 at once with go test ./....

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⬅ Exercises