golang-structs-interfaces

What this skill does

**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)

```go
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:

```go
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.

```go
// package notification — defines only what it needs
type 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 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.

```go
// 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:

```go
// 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` 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 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

| 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:

```go
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:

```go
// 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:

```go
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:

```go
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:

```go
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

| 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 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:

```go
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:

```go
type Order struct {
    ID        string    `json:"id"         db:"id"`
    UserID    string    `json:"user_id"    db:"user_id"`
    T