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

// Bad — an interface return hides every other method of the concrete type from callers
func NewService(store UserStore) ServiceInterface { ... }