understanding the interface

Go interfaces are an essential part of the language, but many do not understand exactly when or how to use them.

In this talk, Francesc goes from the basic theory of interfaces to best practices, covering patterns usually seen on Go codebases.

Transcript

1. understanding the interface @francesc
2. what is an interface?
3. "In object-oriented programming, a protocol or interface is a common means for unrelated objects to communicate with each other" - wikipedia
4. "In object-oriented programming, a protocol or interface is a common means for unrelated objects to communicate with each other" - wikipedia
5. "In object-oriented programming, a protocol or interface is a common means for unrelated objects to communicate with each other" - wikipedia
9. what is a Go interface?
10. abstract types concrete types
11. concrete types in Go - they describe a memory layout - behavior attached to data through methods int32 int64 int16 int8 type Number int func (n Number) Positive() bool { return n > 0 }
12. int *os.File *strings.Reader *gzip.Writer []bool
13. type Positiver interface { Positive() bool } abstract types in Go - they describe behavior - they define a set of methods, without specifying the receiver io.Reader io.Writer fmt.Stringer
14. type Reader interface { Read(b []byte) (int, error) } type Writer interface { Write(b []byte) (int, error) } two interfaces
15. int *os.File *strings.Reader *gzip.Writer []bool io.Reader io.Writer
16. type ReadWriter interface { Read(b []byte) (int, error) Write(b []byte) (int, error) } union of interfaces
17. type ReadWriter interface { Reader Writer } union of interfaces
18. int *os.File *strings.Reader *gzip.Writer []bool io.Reader io.Writer io.ReadWriter
19. int *os.File *strings.Reader *gzip.Writer []bool io.Reader io.Writer io.ReadWriter ?
20. interface{}
21. “interface{} says nothing” - Rob Pike in his Go Proverbs
23. why do we use interfaces?
24. - writing generic algorithms - hiding implementation details - providing interception points why do we use interfaces?
25. a) func WriteTo(f *os.File) error b) func WriteTo(w io.ReadWriteCloser) error c) func WriteTo(w io.Writer) error d) func WriteTo(w interface{}) error what function do you prefer?
26. Cons: • how would you test it? • what if you want to write to memory? Pros: • ? a) func WriteTo(f *os.File) error
27. Cons: • how do you even write to interface{}? • probably requires runtime checks Pros: • you can write really bad code d) func WriteTo(w interface{}) error
28. Which ones does WriteTo really need? - Write - Read - Close b) func WriteTo(w io.ReadWriteCloser) error c) func WriteTo(w io.Writer) error
29. “The bigger the interface, the weaker the abstraction” - Rob Pike in his Go Proverbs
30. “Be conservative in what you do, be liberal in what you accept from others” - Robustness Principle
31. “Be conservative in what you send, be liberal in what you accept” - Robustness Principle
32. Abstract Data Types
33. Abstract Data Types Mathematical model for data types Defined by its behavior in terms of: - possible values, - possible operations on data of this type, - and the behavior of these operations
34. top(push( , ))= X S X
35. pop(push( , ))= S X S
36. empty(new()) not empty(push(S, X))
37. Axioms: top(push(S, X)) = X pop(push(S, X)) = S empty(new()) !empty(push(S, X)) Example: stack ADT
38. type Stack interface { Push(v interface{}) Stack Pop() Stack Empty() bool } a Stack interface
39. func Size(s Stack) int { if s.Empty() { return 0 } return Size(s.Pop()) + 1 } algorithms on Stack
40. type Interface interface { Less(i, j int) bool Swap(i, j int) Len() int } a sortable interface
41. func Sort(s Interface) func Stable(s Interface) func IsSorted(s Interface) bool algorithms on sortable
42. type Reader interface { Read(b []byte) (int, error) } type Writer interface { Write(b []byte) (int, error) } remember Reader and Writer?
43. func Fprintln(w Writer, ar ...interface{}) (int, error) func Fscan(r Reader, a ...interface{}) (int, error) func Copy(w Writer, r Reader) (int, error) algorithms on Reader and Writer
44. is this enough?
46. write generic algorithms on interfaces
47. “Be conservative in what you send, be liberal in what you accept” - Robustness Principle
48. a) func New() *os.File b) func New() io.ReadWriteCloser c) func New() io.Writer d) func New() interface{} what function do you prefer?
49. func New() *os.File
50. “Be conservative in what you send, be liberal in what you accept” - Robustness Principle
51. “Return concrete types, receive interfaces as parameters” - Robustness Principle applied to Go (me)
52. unless
53. Use interfaces to hide implementation details: - decouple implementation from API - easily switch between implementations / or provide multiple ones Hiding implementation details
54. context.Context
55. satisfying the Context interface context Context
56. satisfying the Context interface emptyCtx cancelCtx timerCtx valueCtx Context
57. interfaces hide implementation details
58. call dispatch
59. f.Do()
60. call dispatch Concrete types: static - known at compilation - very efficient - can’t intercept Abstract types: dynamic - unknown at compilation - less efficient - easy to intercept
61. type Client struct { Transport RoundTripper … } type RoundTripper interface { RoundTrip(*Request) (*Response, error) } interfaces: dynamic dispatch of calls
62. http.DefaultTransport http.Client
63. interfaces: dynamic dispatch of calls type headers struct { rt http.RoundTripper v map[string]string } func (h headers) RoundTrip(r *http.Request) *http.Response { for k, v := range h.v { r.Header.Set(k, v) } return h.rt.RoundTrip(r) }
64. interfaces: dynamic dispatch of calls c := &http.Client{ Transport: headers{ rt: http.DefaultTransport, v: map[string]string{“foo”: “bar”}, }, } res, err := c.Get(“http://golang.org”)
65. headers http.DefaultTransport http.Client
66. chaining interfaces
67. const input = `H4sIAAAAAAAA/3qyd8GT3WueLt37fk/Ps46JT/d1vFw942nrlqezFzzZsQskMmfFi/0zX7b3cAECA AD//0G6Zq8rAAAA` var r io.Reader = strings.NewReader(input) r = base64.NewDecoder(base64.StdEncoding, r) r, err := gzip.NewReader(r) if err != nil {log.Fatal(err) } io.Copy(os.Stdout, r) Chaining interfaces
68. *strings.Reader H4sIAAAAAAAA/3qyd8GT3WueLt37fk/Ps46JT/d1vF w942nrlqezFzzZsQskMmfFi/0zX7b3cAECAAD//0G6 Zq8rAAAA *base64.Decoder *gzip.Reader *os.File io.Copy 你们好，我很高兴 因为我在这里
69. interfaces are interception points
70. - writing generic algorithms - hiding implementation details - providing interception points why do we use interfaces?
71. so … what’s new?
72. implicit interface satisfaction
73. no “implements”
74. funcdraw
75. package parse func Parse(s string) *Func type Func struct { … } func (f *Func) Eval(x float64) float64 Two packages: parse and draw
76. Two packages: parse and draw package draw import “.../parse” func Draw(f *parse.Func) image.Image { for x := minX; x < maxX; x += incX { paint(x, f.Eval(y)) } … }
77. funcdraw package draw package parse
78. funcdraw with explicit satisfaction package draw package common package parse
79. funcdraw with implicit satisfaction package draw package parse
80. Two packages: parse and draw package draw import “.../parse” func Draw(f *parse.Func) image.Image { for x := minX; x < maxX; x += incX { paint(x, f.Eval(y)) } … }
81. Two packages: parse and draw package draw type Evaler interface { Eval(float64) float64 } func Draw(e Evaler) image.Image { for x := minX; x < maxX; x += incX { paint(x, e.Eval(y)) } … }
82. interfaces can break dependencies
83. define interfaces where you use them
84. But, how do I know what satisfies what, then?
85. guru a tool for answering questions about Go source code.
86. http://golang.org/s/using-guru
87. the super power of Go interfaces
88. type assertions
89. func do(v interface{}) { i := v.(int) // will panic if v is not int i, ok := v.(int) // will return false } type assertions from interface to concrete type
90. func do(v interface{}) { select v.(type) { case int: fmt.Println(“got int %d”, v) Default: } } type assertions from interface to concrete type
91. func do(v interface{}) { select t := v.(type) { case int: // t is of type int fmt.Println(“got int %d”, t) default: // t is of type interface{} fmt.Println(“not sure what type”) } } type assertions from interface to concrete type
92. avoid abstract to concrete assertions
93. func do(v interface{}) { s := v.(fmt.Stringer) // might panic s, ok := v.(fmt.Stringer) // might return false } type assertions from interface to interface
94. func do(v interface{}) { select v.(type) { case fmt.Stringer(): fmt.Println(“got Stringer %d”, v) Default: } } runtime checks interface to concrete type
95. func do(v interface{}) { select s := v.(type) { case fmt.Stringer: // s is of type int fmt.Println(s.String()) default: // t is of type interface{} fmt.Println(“not sure what type”) } } runtime checks interface to concrete type
96. Many packages check whether a type satisfies an interface: - fmt.Stringer - json.Marshaler/Unmarhsaler - ... and adapt their behavior accordingly. type assertions as extension mechanism
97. use type assertions to extend behaviors
98. Go Proverb Dave Cheney - GopherCon 2016 Don’t just check errors, handle them gracefully
99. type Context interface { Done() <-chan struct{} Err() error Deadline() (deadline time.Time, ok bool) Value(key interface{}) interface{} } var Canceled, DeadlineExceeded error the Context interface
100. var Canceled = errors.New("context canceled") errors in context
101. var Canceled = errors.New("context canceled") var DeadlineExceeded error = deadlineExceededError{} errors in context
102. var Canceled = errors.New("context canceled") var DeadlineExceeded error = deadlineExceededError{} errors in context
103. var Canceled = errors.New("context canceled") var DeadlineExceeded error = deadlineExceededError{} type deadlineExceededError struct{} func (deadlineExceededError) Error() string { return "..." } func (deadlineExceededError) Timeout() bool { return true } func (deadlineExceededError) Temporary() bool { return true } errors in context
104. var Canceled = errors.New("context canceled") var DeadlineExceeded error = deadlineExceededError{} type deadlineExceededError struct{} func (deadlineExceededError) Error() string { return "..." } func (deadlineExceededError) Timeout() bool { return true } func (deadlineExceededError) Temporary() bool { return true } errors in context
105. var Canceled = errors.New("context canceled") var DeadlineExceeded error = deadlineExceededError{} type deadlineExceededError struct{} func (deadlineExceededError) Error() string { return "..." } func (deadlineExceededError) Timeout() bool { return true } func (deadlineExceededError) Temporary() bool { return true } errors in context
106. if tmp, ok := err.(interface { Temporary() bool }); ok { if tmp.Temporary() { // retry } else { // report } } errors in context
107. use type assertions to classify errors
108. Adding methods to an interface breaks backwards compatibility. type ResponseWriter interface { Header() Header Write([]byte) (int, error) WriteHeader(int) } How could you add one more method without breaking anyone’s code? type assertions as evolution mechanism
109. Step 1: add the method to your concrete type implementations Step 2: define an interface containing the new method Step 3: document it type assertions as evolution mechanism
110. type Pusher interface { Push(target string, opts *PushOptions) error } func handler(w http.ResponseWriter, r *http.Request) { if p, ok := w.(http.Pusher); ok { p.Push(“style.css”, nil) } } http.Pusher
111. use type assertions to maintain compatibility
112. In conclusion
113. Interfaces provide: - generic algorithms - hidden implementation - interception points In conclusion
114. Interfaces provide: - generic algorithms - hidden implementation - interception points Implicit satisfaction: - break dependencies In conclusion
115. Interfaces provide: - generic algorithms - hidden implementation - interception points implicit satisfaction: - break dependencies Type assertions: - to extend behaviors - to classify errors - to maintain compatibility In conclusion
116. 谢谢 Thanks, @francesc