func main() {
 f, err := os.Open("filename.ext")
 if err != nil {
 log.Fatal(err)
 // 或者更简单的：
 // return err
 }
 ...
}

func main() {
 ...
 if f, err = os.Open("filename.ext"); err != nil{
 log.Fatal(err)
 }
 ...
}

type error interface {
 Error() string
}

buf := make([]byte, 100)
n, err := r.Read(buf)
buf = buf[:n]
if err == io.EOF {
 log.Fatal("read failed:", err)
}

err := os.Remove("/tmp/nonexist")
log.Println(err)

2017/02/08 14:09:22 remove /tmp/nonexist: no such file or directory

// PathError records an error and the operation and file path that caused it.
type PathError struct {
 Op string
 Path string
 Err error
}

func (e *PathError) Error() string { return e.Op + " " + e.Path + ": " + e.Err.Error() }

// file_unix.go 针对 *nix 系统的实现
// Remove removes the named file or directory.
// If there is an error, it will be of type *PathError.
func Remove(name string) error {
 // System call interface forces us to know
 // whether name is a file or directory.
 // Try both: it is cheaper on average than
 // doing a Stat plus the right one.
 e := syscall.Unlink(name)
 if e == nil {
 return nil
 }
 e1 := syscall.Rmdir(name)
 if e1 == nil {
 return nil
 }

 // Both failed: figure out which error to return.
 // OS X and Linux differ on whether unlink(dir)
 // returns EISDIR, so can't use that. However,
 // both agree that rmdir(file) returns ENOTDIR,
 // so we can use that to decide which error is real.
 // Rmdir might also return ENOTDIR if given a bad
 // file path, like /etc/passwd/foo, but in that case,
 // both errors will be ENOTDIR, so it's okay to
 // use the error from unlink.
 if e1 != syscall.ENOTDIR {
 e = e1
 }
 return &PathError{"remove", name, e}
}

err := xxxx()
if err != nil {
 swtich err := err.(type) {
 case *os.PathError:
 ...
 default:
 ...
 }
}

err := errors.New("whoops")
// or
err := errors.Errorf("whoops: %s", "foo")

cause := errors.New("whoops")
err := errors.Wrap(cause, "oh noes")

err := errors.New("whoops")
fmt.Printf("%+v", err)

type temporary interface {
 Temporary() bool
}

// IsTemporary returns true if err is temporary.
func IsTemporary(err error) bool {
 te, ok := errors.Cause(err).(temporary)
 return ok && te.Temporary()
}