Go: Errors

package main

import (
	"errors"
	"fmt"
	"log"
	"log/slog"
	"os"
)

// 1.
var (
	ErrReadingFile = errors.New("no such file or directory")
	ErrClosingFile = errors.New("could not close file")
)

// 2.
func ReadFile(file string) (string, error) {
	f, err := os.ReadFile(file)
	if err != nil {
		return "", err
	}

	contents := string(f)
	return contents, nil
}

// 3.
func ReadFileBetter(file string) (string, error) {
	f, err := os.ReadFile(file)
	if err != nil {
		return "", fmt.Errorf("main: ReadFileBetter: %w", err)
	}

	contents := string(f)
	return contents, nil
}

func closeFile(f *os.File, name string) {
	msg := fmt.Sprintf("closing %v", name)
	slog.Debug("debug", "msg", msg)
	err := f.Close()
	if err != nil {
		slog.Error("error", "msg", err)
	}
}

// 4.
func ReadFileBest(file string) (string, error) {
	f, err := os.OpenFile(file, os.O_RDONLY, 0644)
	if err != nil {
		return "", fmt.Errorf("main: ReadFileBest: %w: %w", ErrReadingFile, err)
	}
	defer closeFile(f, f.Name())

	b := make([]byte, 8)
	_, err = f.Read(b)
	// uncomment this error to see how the different errors are handled
	// err = errors.New("this error should never happen")
	if err != nil {
		return "", fmt.Errorf("main: ReadFileBest: %w", err)
	}

	contents := string(b)
	return contents, nil
}

func main() {
	// 5.
	logFile, err := os.OpenFile("./log.json", os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0644)
	if err != nil {
		log.Fatal(err)
	}
	defer func() {
		if err := logFile.Close(); err != nil {
			slog.Error("error", "msg", err)
		}
	}()
	// to see the logs in the terminal, change the writer to be os.Stdout or os.Stderr
	logger := slog.New(slog.NewJSONHandler(logFile, &slog.HandlerOptions{
		Level: slog.LevelDebug,
		ReplaceAttr: func(groups []string, a slog.Attr) slog.Attr {
			if a.Key == slog.TimeKey {
				t := a.Value.Time()
				a.Value = slog.StringValue(t.Format("2006-01-02 15:04:05"))
			}
			return a
		},
	}))

	slog.SetDefault(logger)

	f := "../example.txt"

	// 6.
	contents, err := ReadFile(f)
	if err != nil {
		log.Println(err)
		// or log fatal
		// log.Fatal(err)
	}
	if contents != "" {
		fmt.Println(contents)
	}

	// 7.
	contents2, err := ReadFileBetter(f)
	if err != nil {
		slog.Error("error", "msg", err)
	}
	if contents2 != "" {
		fmt.Println(contents2)
	}

	// 8.
	contents3, err := ReadFileBest(f)
	if err != nil {
		if errors.Is(err, ErrReadingFile) {
			slog.Error("error", "msg", err)
			slog.Info("info", "hint", "check the file path is correct relative to program")
		} else {
			slog.Error("error", "msg", err)
			panic(err)
		}
	}
	if contents3 != "" {
		fmt.Println(contents3)
	}
}