package main

import (
    "net/http"
)

// The serverError helper writes a log entry at Error level (including the request
// method and URI as attributes), then sends a generic 500 Internal Server Error
// response to the user.
func (app *application) serverError(w http.ResponseWriter, r *http.Request, err error) {
    var (
        method = r.Method
        uri    = r.URL.RequestURI()
    )

    app.logger.Error(err.Error(), "method", method, "uri", uri)
    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)
}

package main

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

func (app *application) home(w http.ResponseWriter, r *http.Request) {
    w.Header().Add("Server", "Go")
    
    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, r, err) // Use the serverError() helper.
        return
    }

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

...

package main

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

func (app *application) serverError(w http.ResponseWriter, r *http.Request, err error) {
    var (
        method = r.Method
        uri    = r.URL.RequestURI()
        // Use debug.Stack() to get the stack trace. This returns a byte slice, which
        // we need to convert to a string so that it's readable in the log entry.
        trace  = string(debug.Stack())
    )

    // Include the trace in the log entry.
    app.logger.Error(err.Error(), "method", method, "uri", uri, "trace", trace)

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