Adapters for middleware in Go

adapter.go

package main

// Adapter takes in a http.Handler, enhances it,
// and returns another http.Handler
type Adapter func(http.Handler) http.Handler

// Composes variadic adapters and returns a final
// adapter to be applied to the http.Handler
func Compose(adapters ...Adapter) Adapter {
  return func(h http.Handler) http.Handler {
    for _, adapter := range adapters {
      h = adapter(h)
    }
    
    return h
  }
}

main.go

package main

func main() {
  r := mux.NewRouter()
  
  // enhance a route handler with  middleware
  todosHandlerEnhancer := Compose(
    WithHeader("content-type", "application/json"),
    WithHeader("api-version", "v1"),
  )
  
  router.Handle("/todos", todosHandlerEnhancer(todosHandler))
  
  // apply for the entire router
  router.Handle("/", Logging(logger)(router))
}

func todosHandler(w http.ResponseWriter, r *http.Request) {
  // continue normal operation
}

middleware.go

package main

// Add a new header with the given key, value
func WithHeader(k, v string) Adapter {
  return func(h http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
      w.Header.Add(k, v)
      h.ServeHTTP(w, r)
    })
  }
}

// Add a new header that logs the request
func Logging(l *log.Logger) Adapter {
  return func(h http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
      l.Println(r.Method, r.URL.Path) // use your own logging solution 
      h.ServeHTTP(w, r)
    })
  }
}