The Plan

• Value vs. pointer semantics, zero values, structs
• Slices and maps and their cost models
• Errors as values: wrapping, errors.Is / errors.As

Deliverable: A CLI that reads input, transforms it, and reports failures with wrapped errors — no panics on bad input.

• Implicit satisfaction and small interfaces
• Accept interfaces, return structs
• Fakes for tests instead of mocks

Deliverable: Refactor week 1's CLI behind one or two small interfaces, with a hand-written fake that lets you test it with no real I/O.

• Goroutines, channels, select, context cancellation
• Mutexes vs. channels; the race detector
• Bounded concurrency with a worker pool

Deliverable: A concurrent fetch-and-process worker pool with context cancellation that passes `go test -race`.

• net/http ServeMux routing (1.22 patterns)
• Middleware, JSON encoding, and request context
• Graceful shutdown

Deliverable: A JSON API over net/http with logging middleware (log/slog) and a clean shutdown on SIGTERM.

• database/sql and a Postgres driver (pgx)
• Type-safe queries; a repository behind an interface
• A generic helper where it genuinely removes duplication

Deliverable: Back the API with a Postgres store behind a repository interface, plus one generic helper (e.g. a Map over rows) justified by ≥2 uses.

• Table-driven tests and subtests; -race in CI
• A benchmark + a pprof pass to justify one optimization
• Multi-stage Docker build to a distroless image

Deliverable: Ship it: table-tested, race-clean in CI, one profiled-and-justified optimization, in a small distroless container.