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+---
+title: "Writing HTTP Handlers"
+tags: posts
+---
+
+I'm sharing how I write handlers in Go. 
+
+I write them like this for reasons that are probably fairly contextual. I've written a few applications and had to swap REST libraries or even swapped REST for GRPC, so things that make that easier speak to me a great deal. 
+
+I've used `ints` instead of the `http.StatusXXXX` and omitted `JSON` tags in an attempt to try save up screen space.
+
+To begin with, you might have something like this:
+```
+package main
+
+import (
+	"fmt"
+	"log"
+	"net/http"
+)
+
+func handler(w http.ResponseWriter, r *http.Request) {
+	fmt.Fprintf(w, "Hello, World!")
+}
+
+func main() {
+	http.HandleFunc("/", handler)
+	log.Fatal(http.ListenAndServe(":8080", nil))
+}
+
+```
+
+Then you might get told off because you've just registered routes with the default mux, which isn't very testable.
+
+So you tweak it a little bit.
+```
+package main
+
+import (
+	"fmt"
+	"log"
+	"net/http"
+)
+
+func handler(w http.ResponseWriter, r *http.Request) {
+	fmt.Fprintf(w, "Hello, World!")
+}
+
+func newMux() *http.ServeMux {
+	mux := http.NewServeMux()
+	mux.HandleFunc("/", handler)
+
+	return mux
+}
+
+func Run() error {
+	mux := newMux()
+	return http.ListenAndServe(":8080", mux)
+}
+
+func main() {
+	if err := Run(); err != nil {
+		log.Fatal(err)
+	}
+}
+```
+
+`newMux()` gives you a `mux` to use when testing. 
+
+`Run` keeps `main` nice and clean, so you can just return errors as needed instead of going `log.Fatal` and just generally being messy.
+
+But now you need to do something real, you want to store and fetch data.
+
+```
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"log"
+	"net/http"
+	"strconv"
+)
+
+func NewMux() *http.ServeMux {
+	mux := http.NewServeMux()
+	s := Server{
+		data: make(map[int]Content),
+	}
+
+	s.Register(mux)
+	return mux
+}
+
+func Run() error {
+	mux := NewMux()
+	return http.ListenAndServe(":8080", mux)
+}
+
+type Server struct {
+	data map[int]Content
+}
+
+func (s *Server) Register(mux *http.ServeMux) {
+	mux.HandleFunc("GET /{id}", s.Get)
+	mux.HandleFunc("POST /", s.Post)
+}
+
+func (s *Server) Get(w http.ResponseWriter, r *http.Request) {
+	idStr := r.PathValue("id")
+	id, err := strconv.Atoi(idStr)
+	if err != nil {
+		w.WriteHeader(400)
+		w.Write([]byte(fmt.Sprintf("failed to parse id: %v", err)))
+		return
+	}
+	data, ok := s.data[id]
+	if !ok {
+		w.WriteHeader(404)
+		w.Write([]byte("not found"))
+		return
+	}
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(200)
+	json.NewEncoder(w).Encode(data)
+}
+
+type ContentPostReq struct {
+	Foo string
+}
+
+func (s *Server) Post(w http.ResponseWriter, r *http.Request) {
+	req := ContentPostReq{}
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		w.WriteHeader(400)
+		w.Write([]byte(fmt.Sprintf("failed to parse request: %v", err)))
+		return
+	}
+	id := len(s.data)
+	content := Content{
+		ID:  id,
+		Foo: req.Foo,
+	}
+	s.data[id] = content
+
+	w.WriteHeader(200)
+	json.NewEncoder(w).Encode(content)
+}
+
+type Content struct {
+	ID  int
+	Foo string
+}
+
+func main() {
+	if err := Run(); err != nil {
+		log.Fatal(err)
+	}
+}
+```
+
+```
+❯ curl -X POST localhost:8080 --header "Content-Type: application/json" -d '{"foo":"bar"}'
+{"ID":0,"Foo":"bar"}
+❯ curl -X GET localhost:8080/0
+{"ID":0,"Foo":"bar"}
+```
+
+Erm, well, okay. Quite a bit has changed here, but I'm sure you can read it. We now save and fetch very, very safely from a map and return the response as `JSON`. I've done some things for brevity because I want to get to the main point.
+
+This API is inconsistent. It sometimes returns `JSON`, and the others return strings. Overall, it's just a mess.
+
+So let's try to standardise things.
+First, let's design some form of REST spec.
+
+```
+type JSONResp[T any] struct {
+	Resources []T
+	Errs      []ErrorResp
+}
+
+type ErrorResp struct {
+	Status int
+	Msg    string
+}
+```
+We want to be able to support fetching multiple resources at once, if we can only fetch some resources, let's return them under `resources` and show the errors under `errs`
+
+Now, add some helpful functions to handle things.
+```
+func Post[In any, Out any](successCode int, fn func(context.Context, In) ([]Out, []ErrorResp)) func(http.ResponseWriter, *http.Request) {
+	return func(w http.ResponseWriter, r *http.Request) {
+		var v In
+
+		if err := json.NewDecoder(r.Body).Decode(&v); err != nil {
+			writeJSONResp[Out](w, http.StatusBadRequest, nil, []ErrorResp{
+				{
+					Status: http.StatusBadRequest,
+					Msg:    fmt.Sprintf("failed to parse request: %v", err),
+				},
+			})
+
+			return
+		}
+
+		res, errs := fn(r.Context(), v)
+		writeJSONResp(w, successCode, res, errs)
+	}
+}
+
+func writeJSONResp[T any](w http.ResponseWriter, successCode int, res []T, errs []ErrorResp) {
+	body := JSONResp[T]{
+		Resources: res,
+		Errs:      errs,
+	}
+
+	status := successCode
+	for _, e := range errs {
+		if e.Status > status {
+			status = e.Status
+		}
+	}
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	json.NewEncoder(w).Encode(body)
+}
+```
+And we've standardised all `POST` requests!
+
+This function can be used by all `POST` requests, ensuring they adhere to the spec. It also removes the repetitive code around marshalling and unmarshalling to `JSON` and handles errors in a consistent manner.
+The handler functions accept a `context` param and their expected struct input.
+```
+func (s *Server) Register(mux *http.ServeMux) {
+...
+	mux.HandleFunc("POST /", Post(201, s.Post))
+}
+
+func (s *Server) Post(ctx context.Context, req ContentPostReq) ([]Content, []ErrorResp) {
+	id := len(s.data)
+	content := Content{
+		ID:  id,
+		Foo: req.Foo,
+	}
+	s.data[id] = content
+
+	return []Content{content}, nil
+}
+```
+As you can see, the post function is fairly cleaner now.
+
+You can extend this to all the other request types. If you have query or path parameters, you could either pass in the request, write a custom struct tag parser, or find someone else who has already done it: [https://github.com/gorilla/schema](https://github.com/gorilla/schema).