This document discusses changes and improvements to reflection in Go 1.7. It provides examples of using reflection to dynamically declare anonymous structs, check if a value implements an interface, and get struct field tags. It emphasizes that reflection should only be used when necessary, and otherwise standard interfaces and types are preferred.

1. Don’t Use
reflect
Go 1.7 Release Party
Aug 8, 2016
Daisuke Maki @lestrrat

2. Don’t Use
reflect
Go 1.7 Release Party
Aug 8, 2016
Daisuke Maki @lestrrat

3. Don’t Use
reflect
Go 1.7 Release Party
Aug 8, 2016
Daisuke Maki @lestrrat

4. New Go Book!

6. 無念！

7. Reflect

8. Dynamically
Change Behavior

9. All the cool kids are doing it

10. … in Go?

11. Performance

12. From The Book

14. Line Noise

15. @P=split//,".URRUUc8R";@d=split//,"nrekcah xinU / lreP rehtona tsuJ";sub p{
@p{"r$p","u$p"}=(P,P);pipe"r$p","u$p";++$p;($q*=2)+=$f=!fork;map{$P=$P[$f^ord
($p{$_})&6];$p{$_}=/ ^$P/ix?$P:close$_}keys%p}p;p;p;p;p;map{$p{$_}=~/^[P.]/&&
close$_}%p;wait until$?;map{/^r/&&<$_>}%p;$_=$d[$q];sleep rand(2)if/S/;print

17. Line Noise
in Go?

18. reflect.TypeOf((*Maybe)(nil)).Elem()

19. reflect.TypeOf((*Maybe)(nil)).Elem()
WTF

20. reflect.TypeOf((*Maybe)(nil)).Elem()

21. reflect.TypeOf((*Maybe)(nil)).Elem()
pointer to nil “Maybe”

22. reflect.TypeOf((*Maybe)(nil)).Elem()

23. reflect.TypeOf((*Maybe)(nil)).Elem()
Dereference the pointer reflect.Type

24. It’s an
Interface Type!

25. stringer = reflect.
TypeOf((*Stringer)(nil)).Elem()
if rv.Implements(stringer) {
…
}
Check If A Value Implements An Interface

26. • reflect.Type/Value usually requires a value to create
• But if you don’t want to create a variable just to create
an interface reflect.Type…
• Create a nil pointer, then dereference it!
reflect.TypeOf((*Maybe)(nil)).Elem()

27. Why Use reflect?

28. 意訳「そこにreflectがあるから」

29. Sometimes, you
just have to

30. Otherwise, DON’T

31. New in Go1.7

32. 禁句

33. 「この機能、誰が嬉しいんだ？」

34. rv := reflect.ValueOf(…)
fv := rv.Field(1<<63-1) // 1.7 PANICs
if fv == reflect.Value{} {
// <1.6 silently returns zero value
}
Bug Fix

35. rv := reflect.ValueOf(…)
fv := rv.Field(1<<63-1) // 1.7 PANICs
if fv == reflect.Value{} {
// <1.6 silently returns zero value
}
Bug Fix

36. rv := reflect.ValueOf(…)
fv := rv.Field(1<<63-1) // 1.7 PANICs
if fv == reflect.Value{} {
// <1.6 silently returns zero value
}
Bug Fix

37. type Foo struct {
foo int // not exported
}
Bug? Fix

38. _, ok := reflect.
TypeOf(Foo{}).
MethodByName(“foo”)
fmt.Printf(“ok = %tn”)
Bug? Fix

39. _, ok := reflect.
TypeOf(Foo{}).
MethodByName(“foo”)
fmt.Printf(“ok = %tn”)
Bug? Fix
// go1.6 -> true

40. _, ok := reflect.
TypeOf(Foo{}).
MethodByName(“foo”)
fmt.Printf(“ok = %tn”)
Bug? Fix
// go1.7 -> false

41. // struct { Foo int; Bar string }
typ := reflect.StructOf([]reflect.StructField{
reflect.StructField{ Name: “Foo”, Type: reflect.Int},
reflect.StructField{ Name: “Bar”, Type: reflect.String},
})
Dynamically Declare
Anonymous Structs

42. rv := reflect.New(typ)
// v.Foo = 100
fv := rv.Elem().
FieldByName(“Foo”).
Set(reflect.ValueOf(100))
// &struct { Foo int; Bar string }{Foo:100, Bar:""}
fmt.Printf(“%#vn”, rv.Interface())
Dynamically Declare
Anonymous Structs

43. field := fv.FieldByName(“Foo”)
field.Tag.Get(“json”) // string
field.Tag.Lookup(“json”) // string, bool
Can Tell Difference Between
empty tag and non-existing tag

44. Happy hacking
with Go1.7

45. …and Don’t Use
reflect! :D