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interface-compliance.md

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Verify Interface Compliance

Verify interface compliance at compile time where appropriate. This includes:

  • Exported types that are required to implement specific interfaces as part of their API contract
  • Exported or unexported types that are part of a collection of types implementing the same interface
  • Other cases where violating an interface would break users
BadGood
 
type Handler struct {
  // ...
}



func (h *Handler) ServeHTTP(
  w http.ResponseWriter,
  r *http.Request,
) {
  ...
}
 
type Handler struct {
  // ...
}

var _ http.Handler = (*Handler)(nil)

func (h *Handler) ServeHTTP(
  w http.ResponseWriter,
  r *http.Request,
) {
  // ...
}

The statement var _ http.Handler = (*Handler)(nil) will fail to compile if *Handler ever stops matching the http.Handler interface.

The right hand side of the assignment should be the zero value of the asserted type. This is nil for pointer types (like *Handler), slices, and maps, and an empty struct for struct types.

type LogHandler struct {
  h   http.Handler
  log *zap.Logger
}

var _ http.Handler = LogHandler{}

func (h LogHandler) ServeHTTP(
  w http.ResponseWriter,
  r *http.Request,
) {
  // ...
}