HTTP middlewares in Go from the first principles

main.go

package main

import (
	"context"
	"fmt"
	"log"
	"net/http"
	"sync/atomic"
)

// hello is your HTTP web application.
func hello() http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		fmt.Fprintf(w, "hello world (rid: %q)\n", RID(r.Context()))
	})
}

// addRID is a middleware that grabs the request ID value from the specified
// header, and puts into the request context passed to the upstream handler.
func addRID(ridHeader string, handler http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		handler.ServeHTTP(
			w,
			r.WithContext(context.WithValue(
				r.Context(),
				ridKey,
				r.Header.Get(ridHeader))))
	})
}

// RID returns the request ID from the context.
func RID(ctx context.Context) string {
	rid, _ := ctx.Value(ridKey).(string)
	return rid
}

var ridKey = &struct{ name string }{"request id"}

// requestCounter is a middleware that counts requests.
func requestCounter(handler http.Handler) http.Handler {
	var count uint64
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		log.Printf("request count: %v", atomic.AddUint64(&count, 1))
		handler.ServeHTTP(w, r)
	})
}

func main() {
	http.ListenAndServe(
		"localhost:8888",
		requestCounter(
			addRID("X-RID",
				hello())))
}