pass data from http handler back to middleware

main.go

package main

import (
	"context"
	"fmt"
	"net/http"
)

type capture struct {
	value string
}

func handler(w http.ResponseWriter, r *http.Request) {
	r.Context().Value("key").(*capture).value = "bar"
	w.WriteHeader(http.StatusOK)
}

func middleware(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		r = r.WithContext(context.WithValue(r.Context(), "key", &capture{}))
		next.ServeHTTP(w, r)
		val := r.Context().Value("key").(*capture).value
		fmt.Print(val)
	})
}

func main() {
	http.Handle("/", middleware(http.HandlerFunc(handler)))
	http.ListenAndServe(":8080", nil)
}