atelier/languages/go.md

Go — Language Application

How Atelier's domain principles apply in Go specifically. Derives from domains/ docs.

Type System (C1 Correctness, Data P7 Type Fidelity)

• Named types for domain concepts: type UserId string, not bare string.
• No interface{}/any without justification: Go 1.18+ generics reduce the need.
• any requires a type assertion or switch: never use the value without narrowing.

type UserId string
type OrderId string
// UserId and OrderId are distinct; cannot be mixed
func GetUser(id UserId) (*User, error) { ... }

Error Handling (Errors P1 Errors are Data)

• Errors are values: error is an interface, not an exception. Handle explicitly.
• Sentinel errors with errors.Is:

var ErrNotFound = errors.New("not found")
if errors.Is(err, ErrNotFound) { ... }

• Wrap with context: fmt.Errorf("get user %d: %w", id, err).
• Never _ = err: swallowed error (Errors P2). Handle or return.
• Custom error types with errors.As:

type ValidationError struct {
    Field string
    Msg  string
}
func (e *ValidationError) Error() string { return e.Field + ": " + e.Msg }

Concurrency (Concurrency — Go's strength)

• Goroutines + channels for message passing (P5 Lock Minimization).
• context.Context for cancellation and timeout: every function that does I/O takes a ctx.
• sync.Mutex scoped minimally: not held across I/O (P3 Lock Scope).
• Bounded channels: make(chan T, N), not unbounded (P9 Bounded Queues).

func fetchWithTimeout(ctx context.Context, url string) (*Response, error) {
    ctx, cancel := context.WithTimeout(ctx, 5*time.Second)
    defer cancel()
    return doFetch(ctx, url)
}

Immutability (Concurrency P1)

• Pass by value for small structs; pass by pointer for large or mutable.
• No mutation of method receivers: use a value receiver, not a pointer receiver, for read-only methods.
• Copy-on-write for shared state: return a new struct, not a mutated one.

Nullability (C1)

• Pointers can be nil; values cannot. Be explicit: *User (nullable) vs User (not).
• nil check before deref: a nil deref is a panic.
• Return (T, error), not (*T, nil): avoid the "nil pointer" trap.

Testing (Testing)

• testing package + testify/assert or stdlib only.
• Table-driven tests: []struct{ name string; input X; want Y }.
• t.Parallel() for independent tests (P2 Independence).
• httptest for HTTP handlers; sqlite or testcontainers for DB.

Observability (Observability P1)

• slog (stdlib, Go 1.21+) or zap/zerolog: structured logs.
• context.Context carries trace_id: propagated via middleware.
• No fmt.Println: use the logger.

Tooling (DevOps P2)

• go vet + golangci-lint: lint.
• gofmt/goimports: format (automated, not debated).
• go test -race in CI: race detector (Concurrency P6 No Silent Races).
• go mod tidy + committed go.sum: reproducible builds.