handler_test.go

package main

import (
	"net/http"
	"testing"

	webtest "github.com/podhmo/go-webtest"
	"github.com/podhmo/go-webtest/try"
)

func TestAPI(t *testing.T) {
	c := webtest.NewClientFromHandler(http.HandlerFunc(Add))
	var want interface{}
	try.It{
		Code: http.StatusOK,
		Want: &want,
	}.With(t, c, "GET", "/api/add?x=10&y=20&z=foo")
}

main.go

package main

import (
	"fmt"
	"log"
	"net/http"
	"strconv"
)

// Add
func Add(w http.ResponseWriter, req *http.Request) {
	qs := req.URL.Query()
	var ans int
	for _, vs := range qs {
		for _, v := range vs {
			v, err := strconv.Atoi(v)
			if err != nil {
				w.WriteHeader(http.StatusBadRequest)
				fmt.Fprintf(w, `{"error": %q}`, err.Error())
				return
			}
			ans += v
		}
	}
	fmt.Fprintf(w, `{"ans": %d}`, ans)
}

func main() {
	mux := &http.ServeMux{}
	mux.HandleFunc("/api/add", Add)
	log.Fatal(http.ListenAndServe(":8080", mux))
}

Makefile

server:
	go run main.go
client:
	http -b GET ":8080/api/add?x=10&y=20"