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----
-title: "Writing HTTP Handlers"
-tags: posts
-toc: true
----
-
-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:
-```go
-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.
-```go
-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.
-
-```go
-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)
-	}
-}
-```
-
-```shell
-❯ 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.
-
-```go
-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.
-```go
-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.
-```go
-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).