google-go-style

Google Go Style — Skill

Derived from the Google Go Style Guide, © Google LLC, licensed CC-BY 3.0. This skill is a derivative digest, not a verbatim reproduction.

Codifies the Google Go Style Guide (canonical + normative + best-practices) into actionable rules.

The full guide is large. This file holds only what should be in your head on every Go change. For deeper rules, load the matching references/*.md. For unusual situations, WebFetch the source — the guide is normative.

Quick Rules

Apply on every line of Go you write or review.

Naming

• No Get prefix on getters. User(), not GetUser(). Exception: the underlying concept is "get" (HTTP GET).
• Don't repeat the package name in identifiers. bytes.Buffer, not bytes.BytesBuffer. widget.New, not widget.NewWidget.
• Receiver names: 1–2 letters, abbreviation of the type, consistent across all methods. Never this, self, me, or _ (unless unused). func (c *Config), not func (config *Config).
• Initialisms keep one case. URL, ID, HTTP, DB. Exported: UserID, ServeHTTP. Unexported: userID, urlPath. Never Url, Id, Http.
• Test doubles end in Stub / Fake / Spy / Mock (or describe behaviour: AlwaysCharges, AlwaysDeclines).
• No util / common / helper / model package names. They invite import renames at every call site. Name by what the package provides.
• No underscores in identifiers (except *_test.go test/benchmark/example names, and packages imported only by generated code).
• Local variable scope ↔ name length. i, c, db for tight loops; userCount, pollInterval for file scope. Don't drop letters to save typing (Sandbox, not Sbx).

Errors

• fmt.Errorf("doing X: %w", err) — wrap with %w at the end, so the chain prints newest→oldest as outer: middle: inner.
• %w only when callers need errors.Is / errors.As on the underlying error. Otherwise use %v.
• %w at the start is for sentinel wrappers only: fmt.Errorf("%w: invalid header", ErrParse). Category first, details after.
• Sentinel name: ErrFoo at package level: var ErrNotFound = errors.New("not found").
• Don't duplicate context the underlying error already carries. os.Open errors already include the path — fmt.Errorf("could not open settings.txt: %v", err) is wrong; use a higher-level annotation: fmt.Errorf("launch codes unavailable: %v", err).
• Don't add bare "failed: %v" wrappers. They add no information; just return err.
• Either log it or return it — not both. Pick one. Letting the caller log avoids spam.
• Error strings: lowercase, no trailing punctuation, no \n. fmt.Errorf("something bad happened"), not "Something bad happened.".
• Cross-process boundaries (gRPC/RPC): use canonical codes via status.Errorf(codes.X, ...) rather than wrapping internal errors raw with %w.
• error is the last return value. A function taking context.Context should usually return error.

For more, see references/errors.md.

Panics

• Don't panic for normal error handling. Return error and multiple return values.
• log.Fatal over panic for terminal conditions in main / init. Fatal does not run deferred functions; that's the point.
• Panics never cross package boundaries in public APIs. Convert to error at the API edge with a top-level defer recover() that re-panics on unknown payloads.
• MustX is for package-level vars and tests only. MustParse, template.Must. Not for runtime user input.
• Don't recover() to suppress crashes. Corrupted state propagates further; better is monitoring + crash + fix.
• panic("unreachable") after log.Fatalf is the idiom — the compiler doesn't know Fatal doesn't return.

For more, see references/panics.md.

Tests

• No assertion libraries / helpers (assert.Equal, require.NotNil). Use if got != want { t.Errorf(...) }. For complex types: cmp.Equal / cmp.Diff from go-cmp.
• t.Error over t.Fatal by default — keep going, report all failures in one run. t.Fatal only when continuing is meaningless (setup failed, cascading errors would mislead).
• Inside t.Run subtests: use t.Fatal to skip just that case. Outside subtests in a table loop: t.Error + continue.
• NEVER call t.Fatal (FailNow, Fatalf, SkipNow) from a goroutine other than the test's own. Use t.Error from worker goroutines; t.Fatal only after wg.Wait() from the main goroutine.
• Table-driven tests use named struct fields, not positional: {name: "empty", input: "", want: ""}.
• Test helpers that fail setup call t.Helper() + t.Fatalf. This makes the failure point to the test line, not the helper line.
• Failure message format: YourFunc(%v) = %v, want %v — function name, inputs, got, want, in that order.

For more, see references/tests.md.

Variables and strings

• := for non-zero init, var x T for zero values. i := 42, but var coords Point (not coords := Point{}).
• var t []string, not t := []string{} for empty slices. Empty slice and nil slice behave the same for len, cap, range, append.
• new(T) vs &T{}: both are fine for zero values; new reads as "zero value placeholder", &T{} is more common when fields are filled.
• Never == on nil for slices. Use len(s) == 0. APIs must not distinguish nil from empty.
• Strings: + for 2–3 short literals; fmt.Sprintf for formatting; strings.Builder in loops; text/template for templates.
• Pre-size with make([]T, 0, n) / make(map[K]V, n) only when n is known and the code is performance-sensitive. Default to zero-init.
• No shadowing in nested scopes. if *x { ctx, cancel := ...; } shadows ctx outside the if. Use ctx, cancel = ... with = and var cancel func() declared above.

For more, see references/strings-and-vars.md.

API design

• context.Context is the first parameter. func F(ctx context.Context, ...) error. Even in test helpers.
• Never store context.Context in a struct. Pass it through every method that needs it.
• No global mutable state. Pass dependencies through constructors / function parameters (DI).
• Take interfaces, return concrete types. The consumer of an interface defines it, not the implementer.
• Channel direction in signatures. <-chan T for receive-only, chan<- T for send-only.
• Many parameters → option struct (call site reads as labelled fields). 3+ optional / API expected to grow → option struct. Don't put context.Context in option structs.
• Variadic functional options (...Option) only when most callers pass nothing, options need parameters, or third parties define options.
• Avoid *string, *io.Reader parameters "to save bytes". Pass values; use pointers for large structs and protobuf messages.

For more, see references/api-design.md.

Documentation

• Doc comments are full sentences starting with the symbol's name. // Encode writes the JSON encoding of req to w.
• Document WHY, not WHAT. The code already says what.
• Document significant sentinel errors and error types the function returns.
• Document cleanup contracts (Close, Stop, deferred resources).
• Document concurrency-safety only when non-obvious (read-only methods are assumed safe; mutating ones are assumed unsafe — say so when that doesn't hold).
• Don't document that ctx.Done cancels the function — that's the contract of context.Context.

For more, see references/documentation.md.

Package layout

• internal/ for packages not part of the public API.
• Avoid util / common / helper. Name by domain.
• Imports grouped: stdlib / third-party / proto / blank-side-effect. Blank imports only in main or tests.
• No import . ever (in Google codebase).
• Proto imports get a pb suffix: foopb "path/to/foo_go_proto".

For more, see references/package-layout.md.

Decision Matrices

%v vs %w when wrapping an error

Situation	Choose	Why
Caller will errors.Is / errors.As to inspect the chain	%w	Preserves type/sentinel through the chain
Crossing an external boundary (RPC, IPC, storage); caller wants canonical codes	%v (or status.Errorf)	Don't leak internal error types over the wire
Logging or human-display only; no programmatic inspection	%v	%w adds chain semantics nobody uses
Same error is logged here AND returned upward	%v	Wrapping a logged-then-returned error confuses the chain
Sentinel categorisation first (ErrParse-style)	%w at start: "%w: detail"	Reader sees the category first
Adding context around a wrapped error (the common case)	%w at end: "context: %w"	Chain prints newest→oldest naturally: outer: middle: inner
Underlying error already carries this info	nothing — return err	Wrapping without adding info is noise
Just propagating without analysis	nothing — return err	Don't wrap for the sake of wrapping

Function arguments: positional vs option struct vs variadic options

Situation	Choose	Why
≤ 3 parameters, all required, all distinct types	Positional args	Smallest mechanism
Many parameters, most callers set most of them	Option struct (last param)	Self-documenting field names; grows without breaking call sites
Many parameters, most callers set none	Variadic options (...Option)	Zero overhead at simple call sites
Options need failure validation	Variadic options returning error	Can't validate in struct construction
Third-party packages must define options	Variadic options with exported Option type	Struct fields can't be extended
Same option set used by multiple functions	Option struct	Reuse + share + write helpers on the struct
context.Context	First positional arg, never in option struct	Convention

panic vs log.Fatal vs error return

Situation	Choose	Why
Library detects normal failure	error return	Caller decides
Library detects an "impossible" invariant violation	error return or log.Fatal	Caller can't recover anyway
Bad flag/config in main / init	log.Exit (no stack)	Stack trace useless; user wants the message
Internal package consistency check that has been verified by tests	log.Fatal	More reliable than panic (no defer deadlock risk)
Parser internals that always have a matching recover at the API edge	panic of a private type	Plumbing errors through deep recursion is noise
Package-level var initializer needs a value derived from a fallible call	MustX (panics)	Init-time only; init cannot return errors
HTTP handler crashes mid-request	never recover() to mask it	State is corrupted; let the process crash and restart

t.Error vs t.Fatal vs t.Errorf + continue

Situation	Choose
Multiple independent assertions in one Test*, all should run	t.Error / t.Errorf for each
Setup failure — rest of test cannot proceed	t.Fatal / t.Fatalf
First failure makes subsequent assertions misleading (e.g. encoded ≠ expected, can't decode meaningfully)	t.Fatalf then continue with t.Errorf
Table loop without subtests, this case is broken	t.Errorf + continue
Inside t.Run(...) subtest, this case is broken	t.Fatal (skips this subtest only)
Worker goroutine inside a test	t.Errorf only (NEVER t.Fatal)
Test helper called from main test goroutine	t.Helper() + t.Fatalf is fine

Variable declaration form

Situation	Choose	Example
Initializing with a known non-zero value	:=	i := 42
Need a zero value, ready for use	var x T	var coords Point, var s []string
Need a *T to a zero value	new(T) or &T{}	msg := new(pb.Bar)
Need a *T to a value with fields	&T{...}	c := &Config{Port: 8080}
Empty slice for return / accumulation	var s []T	not s := []T{}
Empty map (must be initialized to write)	make(map[K]V) or map[K]V{}	nil map can be read but not written
Pre-sized slice/map (perf-sensitive, size known)	make([]T, 0, n) / make(map[K]V, n)	Don't over-pre-allocate

Reference Files

Load on demand:

• references/errors.md — detailed error structure, wrapping rules, sentinels, RPC boundaries, errors.Is/As.
• references/naming.md — receivers, initialisms, repetition, test doubles, util-package antipattern, shadowing.
• references/api-design.md — option struct vs variadic options, channel direction, DI, interfaces, generics.
• references/tests.md — Test funcs vs helpers, table-driven, subtests, goroutines, acceptance testing, cmp.
• references/panics.md — when allowed (invariants, parsers with recover, init), when forbidden, log.Fatal vs log.Exit.
• references/documentation.md — godoc conventions, what to document, what to skip, contexts, errors, cleanup.
• references/package-layout.md — package size, file structure, imports, internal/, side-effect imports.
• references/strings-and-vars.md — concatenation choices, zero values, size hints, new vs &T{}, shadowing.

Authoritative Sources

When in doubt, especially for unusual situations, WebFetch the source section before writing the code. The Google Go Style Guide is the normative authority — this skill is a digest, not a replacement.

• https://google.github.io/styleguide/go/index — overview, normativity definitions
• https://google.github.io/styleguide/go/guide — canonical + normative core principles
• https://google.github.io/styleguide/go/decisions — normative detailed decisions
• https://google.github.io/styleguide/go/best-practices — non-normative patterns and discussions

For Go fundamentals not covered here, read Effective Go.