I've spent the past two years developing some of the most popular libraries and applications written in Go. I've also made a lot of mistakes along the way. Recognizing that "The only real mistake is the one from which we learn nothing. -John Powell", I would like to share with you the mistakes that I have made over my journey with Go and how you can avoid them.

1. 7 Common
Mistakes
In Go
and when to avoid them

2. @Spf13
Author of
Hugo, Cobra,
Afero, Viper &
more

3. “As long as the
world is turning
and spinning, we're
gonna be dizzy and
we're gonna make
mistakes.”
–Mel Brooks

4. “Appreciate your
mistakes for what
they are: precious
life lessons that
can only be learned
the hard way. “
–Al Franken

5. “If I had to live my
life again, I'd make
the same mistakes,
only sooner.”
– Tallulah Bankhead

6. 7 Lessons
I Wish I
Learned
Sooner

7. Mistake 1:
Not
Accepting
Interfaces

8. State & Behavior
•Types can express state &
behavior
•State = data structure
•Behavior = methods

9. Interfaces
•One of Go’s most powerful
features
•Permits extensibility
•Defined by methods
•Adherance is only satisfied by
behavior

11. Example : []Byte
type Source struct {
Frontmatter []byte
Content []byte
}

12. Func Restricted To String
func StripHTML(s string) string {
output := ""
// Shortcut strings with no tags in them
if !strings.ContainsAny(s, "<>") {
output = s
} else {
s = strings.Replace(s, "n", " ", -1)
s = strings.Replace(s, "</p>", "n", -1)
s = strings.Replace(s, "<br>", "n", -1)
s = strings.Replace(s, "<br />", "n", -1)
…

13. Passes Implementation
Details Down The Stack
func (p *Page) Plain() string {
return helpers.StripHTML(string(p.Content))
}

14. Any Read() Works
func StripHTML(r Reader) string {
buf := new(bytes.Buffer)
buf.ReadFrom(r)
by := buf.Bytes()
if bytes.ContainsAny(s, []byte("<>")) {
by = bytes.Replace(by, []byte("n"), " ", -1)
by = bytes.Replace(by, []byte("</p>"), "n", -1)
by = bytes.Replace(by, []byte("<br>"), "n", -1)
by = bytes.Replace(by, []byte("<br />"), "n", -1)
…

15. type Source struct {
Frontmatter []byte
content []byte
}
type Reader interface {
Read(p []byte) (n int, err error)
}
func (s *Source) Content() (Reader) {
return bytes.NewReader(content)
}
Example : Read

16. type Source struct {
Frontmatter []byte
content []byte
}
type Reader interface {
Read(p []byte) (n int, err error)
}
func (s *Source) Content() (Reader) {
return bytes.NewReader(content)
}
Example : Read

17. type Source struct {
Frontmatter []byte
content []byte
}
type Reader interface {
Read(p []byte) (n int, err error)
}
func (s *Source) Content() (Reader) {
return bytes.NewReader(content)
}
Example : Read

18. func (p *Page) Plain() string {
return helpers.StripHTML(p.Content())
}
Implementation Details
Abstracted With A Method

19. Mistake 2:
Thinking Of
Errors As
Strings

20. Error Is An Interface
type error interface {
Error() string
}

21. Standard Errors
•errors.New(“error here”) is
usually sufficient
•Exported Error Variables
can be easily checked

22. Custom Errors
•Can provide context to
guarantee consistent feedback
•Provide a type which can be
different from the error value
•Can provide dynamic values
(based on internal error state)

23. func NewPage(name string) (p *Page,
err error) {
if len(name) == 0 {
return nil,
errors.New("Zero length page name")
}
Standard Error

24. Exported Error Var
var ErrNoName = errors.New("Zero length
page name")
func NewPage(name string) (*Page, error)
{
if len(name) == 0 {
return nil, ErrNoName
}

25. Exported Error Var
var ErrNoName = errors.New("Zero length page name")
func Foo(name string) (error) {
err := NewPage("bar")
if err == ErrNoName {
newPage("default")
} else {
log.FatalF(err)
}
}

26. Custom Errors : Os
// Portable analogs of some common system call errors.
var ErrInvalid = errors.New("invalid argument")
var ErrPermission = errors.New("permission denied")
// 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()
}

27. Custom Errors : Os
// Portable analogs of some common system call errors.
var ErrInvalid = errors.New("invalid argument")
var ErrPermission = errors.New("permission denied")
// 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()
}

28. Custom Errors : Os
func (f *File) WriteAt(b []byte, off int64) (n int, err error) {
if f == nil {
return 0, ErrInvalid
}
for len(b) > 0 {
m, e := f.pwrite(b, off)
if e != nil {
err = &PathError{"write", f.name, e}
break
}
n += m
b = b[m:]
off += int64(m)
}
return
}

29. Custom Errors : Os
func (f *File) WriteAt(b []byte, off int64) (n int, err error) {
if f == nil {
return 0, ErrInvalid
}
for len(b) > 0 {
m, e := f.pwrite(b, off)
if e != nil {
err = &PathError{"write", f.name, e}
break
}
n += m
b = b[m:]
off += int64(m)
}
return
}

30. Custom Errors : Os
func (f *File) WriteAt(b []byte, off int64) (n int, err error) {
if f == nil {
return 0, ErrInvalid
}
for len(b) > 0 {
m, e := f.pwrite(b, off)
if e != nil {
err = &PathError{"write", f.name, e}
break
}
n += m
b = b[m:]
off += int64(m)
}
return
}

31. Custom Errors : Os
func baa(f *file) error {
…
n, err := f.WriteAt(x, 3)
if _, ok := err.(*PathError) {
…
} else {
log.Fatalf(err)
}
}

32. Custom Errors : Os
…
if serr != nil {
if serr, ok := serr.(*PathError); ok &&
serr.Err == syscall.ENOTDIR {
return nil
}
return serr
…

33. Custom Errors : Os
…
if serr != nil {
if serr, ok := serr.(*PathError); ok &&
serr.Err == syscall.ENOTDIR {
return nil
}
return serr
…

34. Mistake 3:
Requring
Broad
Interfaces

35. Interfaces Are
Composable
•Functions should only accept interfaces
that require the methods they need
•Functions should not accept a broad
interface when a narrow one would
work
•Compose broad interfaces made from
narrower ones

37. Composing Interfaces
type File interface {
io.Closer
io.Reader
io.ReaderAt
io.Seeker
io.Writer
io.WriterAt
}

38. Requiring Broad Interfaces
func ReadIn(f File) {
b := []byte{}
n, err := f.Read(b)
...
}

39. Composing Interfaces
type File interface {
io.Closer
io.Reader
io.ReaderAt
io.Seeker
io.Writer
io.WriterAt
}

40. Requiring Narrow Interfaces
func ReadIn(r Reader) {
b := []byte{}
n, err := r.Read(b)
...
}

41. Mistake 4:
Methods Vs
Functions

42. Too Many Methods
•A lot of people from OO
backgrounds overuse
methods
•Natural draw to define
everything via structs and
methods

43. What Is A Function?
•Operations performed on N1
inputs that results in N2 outputs
•The same inputs will always
result in the same outputs
•Functions should not depend on
state

44. What Is A Method?
•Defines the behavior of a type
•A function that operates
against a value
•Should use state
•Logically connected

45. Functions Can Be Used
With Interfaces
•Methods, by definition, are
bound to a specific type
•Functions can accept
interfaces as input

47. Example From Hugo
func extractShortcodes(s string, p *Page, t
Template) (string, map[string]shortcode, error) {
...
for {
switch currItem.typ {
...
case tError:
err := fmt.Errorf("%s:%d: %s",
p.BaseFileName(),
(p.lineNumRawContentStart()
+ pt.lexer.lineNum() - 1), currItem)
}
}
...
}

48. Example From Hugo
func extractShortcodes(s string, p *Page, t
Template) (string, map[string]shortcode, error) {
...
for {
switch currItem.typ {
...
case tError:
err := fmt.Errorf("%s:%d: %s",
p.BaseFileName(),
(p.lineNumRawContentStart()
+ pt.lexer.lineNum() - 1), currItem)
}
}
...
}

49. Mistake 5:
Pointers Vs
Values

50. Pointers Vs Values
•It’s not a question of performance
(generally), but one of shared access
•If you want to share the value with
a function or method, then use a
pointer
•If you don’t want to share it, then
use a value (copy)

51. Pointer Receivers
•If you want to share a value with
it’s method, use a pointer
receiver
•Since methods commonly manage
state, this is the common usage
•Not safe for concurrent access

52. Value Receivers
•If you want the value copied
(not shared), use values
•If the type is an empty struct
(stateless, just behavior)…
then just use value
•Safe for concurrent access

53. Afero File
type InMemoryFile struct {
at int64
name string
data []byte
closed bool
}
func (f *InMemoryFile) Close() error {
atomic.StoreInt64(&f.at, 0)
f.closed = true
return nil
}

54. type Time struct {
sec int64
nsec uintptr
loc *Location
}
func (t Time) IsZero() bool {
return t.sec == 0 && t.nsec == 0
}
Time

55. Mistake 6:
Not Using
Io.Reader &
Io.Writer

56. Io.Reader & Io.Writer
•Simple & flexible interfaces
for many operations around
input and output
•Provides access to a huge
wealth of functionality
•Keeps operations extensible

57. Io.Reader & Io.Writer
type Reader interface {
Read(p []byte) (n int, err error)
}
type Writer interface {
Write(p []byte) (n int, err error)
}

59. func (page *Page) saveSourceAs(path string) {
b := new(bytes.Buffer)
b.Write(page.Source.Content)
page.saveSource(b.Bytes(), path)
}
func (page *Page) saveSource(by []byte, inpath
string) {
WriteToDisk(inpath, bytes.NewReader(by))
}
Stop Doing This!!

60. func (page *Page) saveSourceAs(path string) {
b := new(bytes.Buffer)
b.Write(page.Source.Content)
page.saveSource(b.Bytes(), path)
}
func (page *Page) saveSource(by []byte, inpath
string) {
WriteToDisk(inpath, bytes.NewReader(by))
}
Stop Doing This!!

61. func (page *Page) saveSourceAs(path string) {
b := new(bytes.Buffer)
b.Write(page.Source.Content)
page.saveSource(b.Bytes(), path)
}
func (page *Page) saveSource(by []byte, inpath
string) {
WriteToDisk(inpath, bytes.NewReader(by))
}
Stop Doing This!!
https://github.com/spf13/hugo/blob/master/hugolib/page.go#L582

62. func (page *Page) saveSourceAs(path string) {
b := new(bytes.Buffer)
b.Write(page.Source.Content)
page.saveSource(b, path)
}
func (page *Page) saveSource(b io.Reader, inpath
string) {
WriteToDisk(inpath, b)
}
Instead

63. Mistake 7:
Ignoring
Concurrent
Access

64. Consider Concurrency
•If you provide a library someone
will use it concurrently
•Data structures are not safe for
concurrent access
•Values aren’t safe, you need to
create safe behavior around them

65. Making It Safe
•Sync package provides behavior
to make a value safe (Atomic/
Mutex)
•Channels cordinate values
across go routines by permitting
one go routine to access at a
time

67. Maps Are Not Safe
func (m *MMFs) Create(name string) (File, error) {
m.getData()[name] = MemFileCreate(name)
m.registerDirs(m.getData()[name])
return m.getData()[name], nil
}

68. Maps Are Not Safe
func (m *MMFS) Create(name string) (File, error) {
m.getData()[name] = MemFileCreate(name)
m.registerDirs(m.getData()[name])
return m.getData()[name], nil
}

69. Maps Are Not Safe
panic: runtime error: invalid memory address or nil
pointer dereference
[signal 0xb code=0x1 addr=0x28 pc=0x1691a7]
goroutine 90 [running]:
runtime.panic(0x501ea0, 0x86b104)
/usr/local/Cellar/go/1.3.3/libexec/src/pkg/runtime/
panic.c:279 +0xf5
github.com/spf13/afero.
(*MemMapFs).registerDirs(0xc208000860, 0x0, 0x0)
/Users/spf13/gopath/src/github.com/spf13/afero/
memmap.go:88 +0x27

70. Maps Can Be Used Safely
func (m *MMFS) Create(name string) (File, error) {
m.lock()
m.getData()[name] = MemFileCreate(name)
m.unlock()
m.registerDirs(m.getData()[name])
return m.getData()[name], nil
}

71. Biggsest
Mistake:
Not Makimg
Mistakes

72. @Spf13
Author of
Hugo, Cobra,
Afero, Viper &
more