Effective Go

Author: The Go Authors (Go team) | Length: ~33 pages / ~16.5k words | Sections: 17 | Generated: 2026-08-06

Source: go.dev/doc/effective_go. The chapters distill the 2009 document (core language, not generics/modules). references/modern.md covers the 1.18–1.22 syntax and stdlib that agents otherwise emit from outdated training data.

How to Use This Skill

• Writing or editing Go - read references/modern.md first. Apply each rule whose Since is ≤ the target package's owning go.mod version and whose preconditions are met; go.work does not override that language version. Those rules win over older snippets in the chapters.
• Without arguments - load core frameworks below. Do not load all 17 chapters.
• With a topic - ask about slices, channels, interfaces, defer, etc.; load only the matching chapter (and modern.md when the topic is syntax/stdlib).
• With a chapter - ask for ch09 (Data) or ch15 (Concurrency); load that chapter file.
• Browse - ask "what chapters do you have?" to see the full index.

When you ask about a topic not in Core Frameworks below, I read the relevant chapter file before answering.

Core Frameworks & Mental Models

Share memory by communicating. Go's slogan: "Do not communicate by sharing memory; instead, share memory by communicating." Pass shared values on channels; only one goroutine owns a value at a time, so data races can't occur by design. Prefer this over mutex-guarded variables; reserve mutexes for cases like reference counts. (Ch 15)

gofmt is the single source of truth for formatting. Never hand-align; run gofmt (file) or go fmt (package). Tabs indent; no line-length limit; no parentheses on if/for/switch. If gofmt's output looks wrong, restructure the code - don't work around it. (Ch 3)

Capitalization is visibility. First letter upper-case = exported; lower-case = package-private. This is semantic, not stylistic. (Ch 5)

Name for how it reads at the call site. Package names: lower-case, single word, match the directory base, favor brevity. Don't stutter - bufio.Reader not BufReader; ring.New when one exported type; once.Do(setup) not DoOrWaitUntilDone. Getters drop Get (Owner for field owner); setters use Set (SetOwner). One-method interfaces are method+-er (Reader, Writer); reuse canonical names/signatures (String, not ToString). Use MixedCaps, never underscores. (Ch 5)

Keep the happy path down the page. if/switch take an init statement (if err := f(); err != nil). When a guard body ends in return/break/continue, drop the else - eliminate error cases as they arise. := lets one err flow through a chain of guards in the same scope. (Ch 7)

for is the only loop; switch is a condition ladder. for init; cond; post {} / for cond {} / for {}. On Go 1.22+, for i := range n replaces for i := 0; i < n; i++. range also iterates collections and decodes strings to runes; drop the second value for key-only, use _ for value-only. On 1.22+, loop variables declared with := are per-iteration; omit redundant v := v copies for those variables. With =, keep a copy or pass the value as a goroutine argument when capturing an iteration's value. switch {} (no expression) switches on true - the idiomatic if-else-if. No fallthrough; comma-separated cases. Label a loop and break Loop to exit it from inside a switch. Type switch: switch t := x.(type) on any, not interface{}. (Ch 7, modern.md)

Multiple returns replace in-band errors and out-params. Return (value, error). Named result parameters document the return and enable naked return. (Ch 8)

defer guarantees cleanup. defer f.Close() right after os.Open - never forgets a close on any return path, and keeps release next to acquire. Deferred args evaluate at the defer; calls run LIFO. It's function-scoped, not block-scoped - that's its power. (Ch 8)

new zeros and returns a pointer; make initializes and returns a value. make is for slices/maps/channels only. Design types whose zero value is usable (bytes.Buffer, sync.Mutex) - it composes transitively. Construct with composite literals (&File{fd: fd}; &File{} == new(File)). Prefer slices over arrays. append returns a (possibly reallocated) slice - always reassign (x = append(x, ...)). Map lookups return the zero value for missing keys; use the comma-ok idiom when absence matters. (Ch 9)

Master fmt and avoid the String() recursion trap. %v/%+v/%#v/%T/%q; define String() string (the Stringer interface) to control formatting. A String() that calls Sprintf("%s", m) on the same string-like type recurses forever - convert to string first or use a non-string verb. (Ch 9, 10)

iota generates constant families; init verifies/registers. const ( _ = iota; KB = 1<<(10*iota); MB ... ). init runs after vars and imports - use it for sanity checks and registration. (Ch 10)

Pointer receiver to mutate or satisfy interfaces. Value methods run on values and pointers; pointer methods run only on pointers (the compiler auto-takes the address of addressable values). A *T receiver is how *ByteSlice satisfies io.Writer. (Ch 11)

Interfaces are implicit, small, and capability-shaped. A type satisfies an interface by implementing its methods - no implements. Convert between same-underlying types to reuse method sets (no copy). Type-assert with comma-ok (v, ok := x.(T)); never bare. When a type only implements an interface, export the interface and return it from the constructor. Any type with methods - struct, int, channel, function - can satisfy one; http.HandlerFunc(f) adapts a function into an http.Handler. (Ch 12)

_ makes intent explicit. Discard unwanted returns, do presence-only map checks, silence unused decls temporarily, import for side effects (import _ "pkg"), and enforce conformance at compile time (var _ I = (*T)(nil)). Never use it to discard an error. (Ch 13)

Embedding is composition, not inheritance. Embed a type to promote its methods and satisfy interfaces for free (io.ReadWriter = Reader + Writer; bufio.ReadWriter embeds *Reader/*Writer). The promoted method's receiver is the inner type - no virtual dispatch. Reach in via the type name to refine. (Ch 14)

Channels synchronize; buffered channels limit throughput. Unbuffered = synchronous handshake. make(chan int, N) is a semaphore of capacity N. Gate goroutine creation (or use a fixed pool) to bound resources - don't just limit running count. Channels are first-class - embed a reply channel for parallel demux. select/default gives non-blocking ops (leaky-buffer free lists). Concurrency (structure) ≠ parallelism (speed). (Ch 15)

Return descriptive errors; reserve panic for the unrecoverable. Implement error with a richer struct (PathError{Op, Path, Err}) when callers may inspect cause. Inspect with errors.Is / errors.As (wrap with %w); do not use err == target or a bare type-assert on a wrapped error. recover works only in deferred functions - isolate failing goroutines or convert internal panics to errors at the package boundary. Never expose panics to clients. (Ch 16, modern.md)

Chapter Index

Topic Index

• append -> ch09
• blank identifier (_) -> ch07, ch09, ch10, ch13
• buffered channel / semaphore -> ch15
• channel -> ch15
• comma-ok idiom -> ch09, ch12
• composite literal -> ch09
• concurrency vs parallelism -> ch15
• constant / iota -> ch10
• conversion -> ch12
• defer -> ch08, ch16
• doc comment -> ch04
• embedding -> ch14
• error -> ch16, modern.md
• errors.Is / errors.As -> modern.md
• exported / capitalization -> ch05
• fmt verbs -> ch09
• for / range / range-over-int -> ch07, modern.md
• function literal / closure -> ch15
• getter -> ch05
• gofmt -> ch03
• goroutine -> ch15
• Handler / HandlerFunc / ServeMux patterns -> ch12, ch17, modern.md
• loopvar (1.22 per-iteration) -> ch07, ch15, modern.md
• if (init, omit else) -> ch07
• init function -> ch10
• interface -> ch12
• make vs new -> ch09
• map -> ch09
• method (pointer/value receiver) -> ch11
• MixedCaps -> ch05
• multiple return values -> ch08
• named results / naked return -> ch08
• panic / recover -> ch16
• package names -> ch05
• rune -> ch07, ch09
• select -> ch15
• semicolons -> ch06
• slice / slices.Contains -> ch09, modern.md
• Stringer -> ch09
• switch (on true / type switch) -> ch07, ch12
• type assertion -> ch12, ch16
• zero value -> ch09, ch10

Supporting Files

• references/modern.md - 1.18–1.22 emit rules (any, errors.Is, slices, range n, loopvar, ServeMux)
• glossary.md - all key terms with definitions
• patterns.md - all techniques and idioms, by situation
• cheatsheet.md - decision rules, quick tables, and tells

Scope & Limits

Chapters cover Effective Go - how to think, name, and structure Go as of the 2009 document. references/modern.md covers the small 1.18–1.22 set that agents otherwise generate from outdated training data. This skill still does not cover generics-as-language (constraints, type sets), module authoring, slog, iterators, or anything 1.23+. Honor the module go line: do not emit APIs newer than it. For hands-on work, use this skill alongside the project's own conventions.