package main

import (
    "fmt"
    "net/http"
    "runtime/debug"
)

// The serverError helper writes an error message and stack trace to the errorLog,
// then sends a generic 500 Internal Server Error response to the user.
func (app *application) serverError(w http.ResponseWriter, err error) {
    trace := fmt.Sprintf("%s\n%s", err.Error(), debug.Stack())
    app.errorLog.Print(trace)

    http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
}

// The clientError helper sends a specific status code and corresponding description
// to the user. We'll use this later in the book to send responses like 400 "Bad
// Request" when there's a problem with the request that the user sent.
func (app *application) clientError(w http.ResponseWriter, status int) {
    http.Error(w, http.StatusText(status), status)
}

// For consistency, we'll also implement a notFound helper. This is simply a
// convenience wrapper around clientError which sends a 404 Not Found response to
// the user.
func (app *application) notFound(w http.ResponseWriter) {
    app.clientError(w, http.StatusNotFound)
}

package main

import (
    "fmt"
    "html/template"
    "net/http"
    "strconv"
)

func (app *application) home(w http.ResponseWriter, r *http.Request) {
    if r.URL.Path != "/" {
        app.notFound(w) // Use the notFound() helper
        return
    }

    files := []string{
        "./ui/html/base.tmpl",
        "./ui/html/partials/nav.tmpl",
        "./ui/html/pages/home.tmpl",
    }

    ts, err := template.ParseFiles(files...)
    if err != nil {
        app.serverError(w, err) // Use the serverError() helper.
        return
    }

    err = ts.ExecuteTemplate(w, "base", nil)
    if err != nil {
        app.serverError(w, err) // Use the serverError() helper.
    }
}

func (app *application) snippetView(w http.ResponseWriter, r *http.Request) {
    id, err := strconv.Atoi(r.URL.Query().Get("id"))
    if err != nil || id < 1 {
        app.notFound(w) // Use the notFound() helper.
        return
    }

    fmt.Fprintf(w, "Display a specific snippet with ID %d...", id)
}

func (app *application) snippetCreate(w http.ResponseWriter, r *http.Request) {
    if r.Method != http.MethodPost {
        w.Header().Set("Allow", http.MethodPost)
        app.clientError(w, http.StatusMethodNotAllowed) // Use the clientError() helper.
        return
    }

    w.Write([]byte("Create a new snippet..."))
}

package main

...

func (app *application) serverError(w http.ResponseWriter, err error) {
    trace := fmt.Sprintf("%s\n%s", err.Error(), debug.Stack())
    app.errorLog.Output(2, trace)

    http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
}

...