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

type Reader interface {
    Read(p []byte) (n int, err error)
}

type Writer interface {
    Write(p []byte) (n int, err error)
}

// Composed from small interfaces
type 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 needs
type Sender interface {
    Send(to, body string) error
}

type Service struct {
    sender Sender
}

The

email

Client

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 concrete
func NewService(store UserStore) *Service { ... }

// BAD — NEVER return interfaces from constructors
func 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 implementation
type UserRepository interface {
    FindByID(ctx context.Context, id string) (*User, error)
}
type userRepository struct { db *sql.DB }

// Good — start concrete, extract an interface later when needed
type 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 use
var buf bytes.Buffer
buf.WriteString("hello")

var mu sync.Mutex
mu.Lock()

// Bad — zero value is broken, requires constructor
type Registry struct {
    items map[string]Item // nil map, panics on write
}

// Good — lazy initialization guards the zero value
func (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

any

// Bad — loses type safety
func Contains(slice []any, target any) bool { ... }

// Good — generic, type-safe
func Contains[T comparable](slice []T, target T) bool { ... }

Key Standard Library Interfaces

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

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()

fmt.Stringer

ToString()

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

io.ReadWriter

Type Assertions & Type Switches

Safe Type Assertion

Type assertions MUST use the comma-ok form to avoid panics:

// Good — safe
s, ok := val.(string)
if !ok {
    // handle
}

// Bad — panics if val is not a string
s := 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\n", 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 Logger
s := 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 methods
type Server struct {
    http.Handler
}

// Named field — Server uses the store but doesn't expose its methods
type 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

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:"-"`
}

json:"name"

json:"name,omitempty"

json:"-"

json:",string"

db:"column"

yaml:"name"

xml:"name,attr"

validate:"required"

Pointer vs Value Receivers

(s *Server)

(s Server)

sync.Mutex

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

go vet

// noCopy may be added to structs which must not be copied after first use.
// See https://pkg.go.dev/sync#noCopy
type noCopy struct{}

func (*noCopy) Lock()   {}
func (*noCopy) Unlock() {}

type ConnPool struct {
    noCopy noCopy
    mu     sync.Mutex
    conns  []*Conn
}

go vet

ConnPool

sync.WaitGroup

sync.Mutex

strings.Builder

Always pass these structs by pointer:

// Good
func process(pool *ConnPool) { ... }

// Bad — go vet will flag this
func 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

v, ok := x.(T)

var _ Interface = (*Type)(nil)

ToString()

String()

any

[T comparable]