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---name: golang-structs-interfacesdescription: 'Golang struct and interface design patterns — composition, embedding, type assertions, type switches, interface segregation, dependency injection via interfaces, struct field tags, and pointer vs value receivers. Use this skill when designing Go types, defining or implementing interfaces, embedding structs or interfaces, writing type assertions or type switches, adding struct field tags for JSON/YAML/DB serialization, or choosing between pointer and value receivers. Also use when the user asks about "accept interfaces, return structs", compile-time interface checks, or composing small interfaces into larger ones.'user-invocable: truelicense: MITcompatibility: Designed for Claude Code or similar AI coding agents, and for projects using Golang.metadata: author: samber version: "1.1.3" openclaw: emoji: "🧩" homepage: https://github.com/samber/cc-skills-golang requires: bins: - go install: []allowed-tools: Read Edit Write Glob Grep Bash(go:*) Bash(golangci-lint:*) Bash(git:*) Agent AskUserQuestion--- **Persona:** You are a Go type system designer. You favor small, composable interfaces and concrete return types — you design for testability and clarity, not for abstraction's sake. > **Community default.** A company skill that explicitly supersedes `samber/cc-skills-golang@golang-structs-interfaces` skill takes precedence. # Go Structs & Interfaces ## Interface Design Principles ### Keep Interfaces Small > "The bigger the interface, the weaker the abstraction." — Go Proverbs Interfaces SHOULD have 1-3 methods. Small interfaces are easier to implement, mock, and compose. If you need a larger contract, compose it from small interfaces: → See `samber/cc-skills-golang@golang-naming` skill for interface naming conventions (method + "-er" suffix, canonical names) ```gotype Reader interface { Read(p []byte) (n int, err error)} type Writer interface { Write(p []byte) (n int, err error)} // Composed from small interfacestype ReadWriter interface { Reader Writer}``` Compose larger interfaces from smaller ones: ```gotype ReadWriteCloser interface { io.Reader io.Writer io.Closer}``` ### Define Interfaces Where They're Consumed Interfaces Belong to Consumers. Interfaces MUST be defined where consumed, not where implemented. This keeps the consumer in control of the contract and avoids importing a package just for its interface. ```go// package notification — defines only what it needstype Sender interface { Send(to, body string) error} type Service struct { sender Sender}``` The `email` package exports a concrete `Client` struct — it doesn't need to know about `Sender`. ### Accept Interfaces, Return Structs Functions SHOULD accept interface parameters for flexibility and return concrete types for clarity. Callers get full access to the returned type's fields and methods; consumers upstream can still assign the result to an interface variable if needed. ```go// Good — accepts interface, returns concretefunc NewService(store UserStore) *Service { ... } // BAD — NEVER return interfaces from constructorsfunc NewService(store UserStore) ServiceInterface { ... }``` ### Don't Create Interfaces Prematurely > "Don't design with interfaces, discover them." NEVER create interfaces prematurely — wait for 2+ implementations or a testability requirement. Premature interfaces add indirection without value. Start with concrete types; extract an interface when a second consumer or a test mock demands it. ```go// Bad — premature interface with a single implementationtype UserRepository interface { FindByID(ctx context.Context, id string) (*User, error)}type userRepository struct { db *sql.DB } // Good — start concrete, extract an interface later when neededtype UserRepository struct { db *sql.DB }``` ## Make the Zero Value Useful Design structs so they work without explicit initialization. A well-designed zero value reduces constructor boilerplate and prevents nil-related bugs: ```go// Good — zero value is ready to usevar buf bytes.Bufferbuf.WriteString("hello") var mu sync.Mutexmu.Lock() // Bad — zero value is broken, requires constructortype Registry struct { items map[string]Item // nil map, panics on write} // Good — lazy initialization guards the zero valuefunc (r *Registry) Register(name string, item Item) { if r.items == nil { r.items = make(map[string]Item) } r.items[name] = item}``` ## Avoid `any` / `interface{}` When a Specific Type Will Do Since Go 1.18+, MUST prefer generics over `any` for type-safe operations. Use `any` only at true boundaries where the type is genuinely unknown (e.g., JSON decoding, reflection): ```go// Bad — loses type safetyfunc Contains(slice []any, target any) bool { ... } // Good — generic, type-safefunc Contains[T comparable](slice []T, target T) bool { ... }``` ## Key Standard Library Interfaces | Interface | Package | Method || ------------- | --------------- | ------------------------------------- || `Reader` | `io` | `Read(p []byte) (n int, err error)` || `Writer` | `io` | `Write(p []byte) (n int, err error)` || `Closer` | `io` | `Close() error` || `Stringer` | `fmt` | `String() string` || `error` | builtin | `Error() string` || `Handler` | `net/http` | `ServeHTTP(ResponseWriter, *Request)` || `Marshaler` | `encoding/json` | `MarshalJSON() ([]byte, error)` || `Unmarshaler` | `encoding/json` | `UnmarshalJSON([]byte) error` | Canonical method signatures MUST be honored — if your type has a `String()` method, it must match `fmt.Stringer`. Don't invent `ToString()` or `ReadData()`. ## Compile-Time Interface Check Verify a type implements an interface at compile time with a blank identifier assignment. Place it near the type definition: ```govar _ io.ReadWriter = (*MyBuffer)(nil)``` This costs nothing at runtime. If `MyBuffer` ever stops satisfying `io.ReadWriter`, the build fails immediately. ## Type Assertions & Type Switches ### Safe Type Assertion Type assertions MUST use the comma-ok form to avoid panics: ```go// Good — safes, ok := val.(string)if !ok { // handle} // Bad — panics if val is not a strings := val.(string)``` ### Type Switch Discover the dynamic type of an interface value: ```goswitch v := val.(type) {case string: fmt.Println(v)case int: fmt.Println(v * 2)case io.Reader: io.Copy(os.Stdout, v)default: fmt.Printf("unexpected type %T\n", v)}``` ### Optional Behavior with Type Assertions Check if a value supports additional capabilities without requiring them upfront: ```gotype Flusher interface { Flush() error} func writeData(w io.Writer, data []byte) error { if _, err := w.Write(data); err != nil { return err } // Flush only if the writer supports it if f, ok := w.(Flusher); ok { return f.Flush() } return nil}``` This pattern is used extensively in the standard library (e.g., `http.Flusher`, `io.ReaderFrom`). ## Struct & Interface Embedding ### Struct Embedding Embedding promotes the inner type's methods and fields to the outer type — composition, not inheritance: ```gotype Logger struct { *slog.Logger} type Server struct { Logger addr string} // s.Info(...) works — promoted from slog.Logger through Loggers := Server{Logger: Logger{slog.Default()}, addr: ":8080"}s.Info("starting", "addr", s.addr)``` The receiver of promoted methods is the _inner_ type, not the outer. The outer type can override by defining its own method with the same name. ### When to Embed vs Named Field | Use | When || --- | --- || **Embed** | You want to promote the full API of the inner type — the outer type "is a" enhanced version || **Named field** | You only need the inner type internally — the outer type "has a" dependency | ```go// Embed — Server exposes all http.Handler methodstype Server struct { http.Handler} // Named field — Server uses the store but doesn't expose its methodstype Server struct { store *DataStore}``` ## Dependency Injection via Interfaces Accept dependencies as interfaces in constructors. This decouples components and makes testing straightforward: ```gotype UserStore interface { FindByID(ctx context.Context, id string) (*User, error)} type UserService struct { store UserStore} func NewUserService(store UserStore) *UserService { return &UserService{store: store}}``` In tests, pass a mock or stub that satisfies `UserStore` — no real database needed. ## Struct Field Tags Use field tags for serialization control. Exported fields in serialized structs MUST have field tags: ```gotype Order struct { ID string `json:"id" db:"id"` UserID string `json:"user_id" db:"user_id"` Total float64 `json:"total" db:"total"` Items []Item `json:"items" db:"-"` CreatedAt time.Time `json:"created_at" db:"created_at"` DeletedAt time.Time `json:"-" db:"deleted_at"` Internal string `json:"-" db:"-"`}``` | Directive | Meaning || ----------------------- | ------------------------------------------- || `json:"name"` | Field name in JSON output || `json:"name,omitempty"` | Omit field if zero value || `json:"-"` | Always exclude from JSON || `json:",string"` | Encode number/bool as JSON string || `db:"column"` | Database column mapping (sqlx, etc.) || `yaml:"name"` | YAML field name || `xml:"name,attr"` | XML attribute || `validate:"required"` | Struct validation (go-playground/validator) | ## Pointer vs Value Receivers | Use pointer `(s *Server)` | Use value `(s Server)` || --- | --- || Method modifies the receiver | Receiver is small and immutable || Receiver contains `sync.Mutex` or similar | Receiver is a basic type (int, string) || Receiver is a large struct | Method is a read-only accessor || Consistency: if any method uses a pointer, all should | Map and function values (already reference types) | Receiver type MUST be consistent across all methods of a type — if one method uses a pointer receiver, all methods should. ## Preventing Struct Copies with `noCopy` Some structs must never be copied after first use (e.g., those containing a mutex, a channel, or internal pointers). Embed a `noCopy` sentinel to make `go vet` catch accidental copies: ```go// noCopy may be added to structs which must not be copied after first use.// See https://pkg.go.dev/sync#noCopytype noCopy struct{} func (*noCopy) Lock() {}func (*noCopy) Unlock() {} type ConnPool struct { noCopy noCopy mu sync.Mutex conns []*Conn}``` `go vet` reports an error if a `ConnPool` value is copied (passed by value, assigned, etc.). This is the same technique the standard library uses for `sync.WaitGroup`, `sync.Mutex`, `strings.Builder`, and others. Always pass these structs by pointer: ```go// Goodfunc process(pool *ConnPool) { ... } // Bad — go vet will flag thisfunc process(pool ConnPool) { ... }``` ## Cross-References - → See `samber/cc-skills-golang@golang-naming` skill for interface naming conventions (Reader, Closer, Stringer)- → See `samber/cc-skills-golang@golang-design-patterns` skill for functional options, constructors, and builder patterns- → See `samber/cc-skills-golang@golang-dependency-injection` skill for DI patterns using interfaces- → See `samber/cc-skills-golang@golang-code-style` skill for value vs pointer function parameters (distinct from receivers) ## Common Mistakes | Mistake | Fix || --- | --- || Large interfaces (5+ methods) | Split into focused 1-3 method interfaces, compose if needed || Defining interfaces in the implementor package | Define where consumed || Returning interfaces from constructors | Return concrete types || Bare type assertions without comma-ok | Always use `v, ok := x.(T)` || Embedding when you only need a few methods | Use a named field and delegate explicitly || Missing field tags on serialized structs | Tag all exported fields in marshaled types || Mixing pointer and value receivers on a type | Pick one and be consistent || Forgetting compile-time interface check | Add `var _ Interface = (*Type)(nil)` || Using `ToString()` instead of `String()` | Honor canonical method names || Premature interface with a single implementation | Start concrete, extract interface when needed || Nil map/slice in zero value struct | Use lazy initialization in methods || Using `any` for type-safe operations | Use generics (`[T comparable]`) instead |