golang-structs-interfaces

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)

type 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:

type 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.

// 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.

// 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.

// 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:

// 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):

// 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

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:

var _ 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:

// 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:

switch 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", v)}

Optional Behavior with Type Assertions

Check if a value supports additional capabilities without requiring them upfront:

type 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:

type 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

// 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:

type 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:

type 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:"-"`}

Pointer vs Value Receivers

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:

// 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:

// 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