Will contracts replace interfaces?

Based on a previous talk named "Understanding the interface", in this talk I:
- discuss the pros and cons of interfaces,
- introduce the concepts from the generics draft for Go2 type parameters and contracts, and
- compare the two approaches.

1. Will contracts replace interfaces? @francesc
2. • Interfaces • Generics draft: ◦ Type Parameters ◦ Contracts • Interfaces vs Contracts Agenda
3. what is an interface?
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
6. "In object-oriented programming, a protocol or interface is a common means for unrelated objects to communicate with each other" - wikipedia
10. what is a Go interface?
11. abstract types concrete types
12. 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 }
13. int *os.File *strings.Reader *gzip.Writer []bool
14. 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
15. type Reader interface { Read(b []byte) (int, error) } type Writer interface { Write(b []byte) (int, error) } two interfaces
16. int *os.File *strings.Reader *gzip.Writer []bool io.Reader io.Writer
17. type ReadWriter interface { Read(b []byte) (int, error) Write(b []byte) (int, error) } union of interfaces
18. type ReadWriter interface { Reader Writer } union of interfaces
19. int *os.File *strings.Reader *gzip.Writer []bool io.Reader io.Writer io.ReadWriter
20. int *os.File *strings.Reader *gzip.Writer []bool io.Reader io.Writer io.ReadWriter ?
21. interface{}
22. “interface{} says nothing” - Rob Pike in his Go Proverbs
24. why do we use interfaces?
25. - writing generic algorithms - hiding implementation details - providing interception points why do we use interfaces?
26. 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?
27. Cons: • how would you test it? • what if you want to write to memory? Pros: • ? a) func WriteTo(f *os.File) error
28. 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
29. Which ones does WriteTo really need? - Write - Read - Close b) func WriteTo(w io.ReadWriteCloser) error c) func WriteTo(w io.Writer) error
30. “The bigger the interface, the weaker the abstraction” - Rob Pike in his Go Proverbs
31. “Be conservative in what you do, be liberal in what you accept from others” - Robustness Principle
32. “Be conservative in what you send, be liberal in what you accept” - Robustness Principle
33. Abstract Data Types
34. 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
35. top(push( , ))= X S X
36. pop(push( , ))= S X S
37. empty(new()) not empty(push(S, X))
38. Axioms: top(push(S, X)) = X pop(push(S, X)) = S empty(new()) !empty(push(S, X)) Example: stack ADT
39. type Stack interface { Push(v interface{}) Stack Top() interface{} Pop() Stack Empty() bool } a Stack interface
40. func Size(s Stack) int { if s.Empty() { return 0 } return Size(s.Pop()) + 1 } algorithms on Stack
41. type Interface interface { Less(i, j int) bool Swap(i, j int) Len() int } a sortable interface
42. func Sort(s Interface) func Stable(s Interface) func IsSorted(s Interface) bool algorithms on sortable
43. type Reader interface { Read(b []byte) (int, error) } type Writer interface { Write(b []byte) (int, error) } remember Reader and Writer?
44. 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
45. is this enough?
47. write generic algorithms on interfaces
48. “Be conservative in what you send, be liberal in what you accept” - Robustness Principle
49. a) func New() *os.File b) func New() io.ReadWriteCloser c) func New() io.Writer d) func New() interface{} what function do you prefer?
50. func New() *os.File
51. “Be conservative in what you send, be liberal in what you accept” - Robustness Principle
52. “Return concrete types, receive interfaces as parameters” - Robustness Principle applied to Go (me)
53. unless
54. Use interfaces to hide implementation details: - decouple implementation from API - easily switch between implementations / or provide multiple ones Hiding implementation details
55. context.Context
56. satisfying the Context interface context Context
57. satisfying the Context interface emptyCtx cancelCtx timerCtx valueCtx Context
58. interfaces hide implementation details
59. call dispatch
60. f.Do()
61. call dispatch Concrete types: static - known at compilation - very efficient - can’t intercept Abstract types: dynamic - unknown at compilation - less efficient - easy to intercept
62. type Client struct { Transport RoundTripper … } type RoundTripper interface { RoundTrip(*Request) (*Response, error) } interfaces: dynamic dispatch of calls
63. http.DefaultTransport http.Client
64. 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) }
65. 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”)
66. headers http.DefaultTransport http.Client
67. chaining interfaces
68. const input = `H4sIAAAAAAAA/7q+8vqc61Ovz1W4Pun61OtLrq+4vpALEAAA//+1jQx/FAAAAA==` 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
69. H4sIAAAAAAAA/7q+8vqc61Ovz1W4Pun61OtLrq+4vp ALEAAA//+1jQx/FAAAAA== io.Copy *gzip.Reader *base64.Decoder *strings.Reader *os.File סרפוג םולש
70. interfaces are interception points
71. - writing generic algorithms - hiding implementation details - providing interception points why do we use interfaces?
72. so … what’s new?
73. implicit interface satisfaction
74. no “implements”
75. funcdraw
76. package parse func Parse(s string) *Func type Func struct { … } func (f *Func) Eval(x float64) float64 Two packages: parse and draw
77. 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)) } … }
78. funcdraw package draw package parse
79. funcdraw with explicit satisfaction package draw package common package parse
80. funcdraw with implicit satisfaction package draw package parse
81. 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)) } … }
82. 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)) } … }
83. interfaces can break dependencies
84. define interfaces where you use them
85. But, how do I know what satisfies what, then?
86. guru a tool for answering questions about Go source code.
87. http://golang.org/s/using-guru
88. the super power of Go interfaces
89. type assertions
90. 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
91. func do(v interface{}) { switch v.(type) { case int: fmt.Println(“got int %d”, v) default: } } type assertions from interface to concrete type
92. func do(v interface{}) { switch 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
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{}) { switch v.(type) { case fmt.Stringer: fmt.Println(“got Stringer %v”, v) default: } } type assertions from interface to interface
95. func do(v interface{}) { select s := v.(type) { case fmt.Stringer: // s is of type fmt.Stringer fmt.Println(s.String()) default: // s is of type interface{} fmt.Println(“not sure what type”) } } type assertions from interface to interface
96. Many packages check whether a type satisfies an interface: - fmt.Stringer : implement String() string - json.Marshaler : implement MarshalJSON() ([]byte, error) - json.Unmarshaler : implement UnmarshalJSON([]byte) error - ... and adapt their behavior accordingly. Tip: Always look for exported interfaces in the standard library. 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 backwards 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. what about generics?
117. Type Parameters A new draft was proposed to support generic programming in Go. It adds type parameters to: - functions - types
118. Type Parameters on functions func Print(type T)(vs []T) { for _, v := range vs { fmt.Print(v) } }
119. Type Parameters on types Interfaces are not always enough for container types: • container/heap ◦ Provides only high level functions (Push, Pop, etc) ◦ Requires the user to implement the Heap interface • container/ring ◦ Provides a type Element containing a interface{} ◦ Requires constant type conversions, loses compile time checks.
120. type Stack(type T) []T func (s Stack(T)) Push(v T) Stack(T) { … } func (s Stack(T)) Top() T { … } func (s Stack(T)) Pop() Stack(T) { … } func (s Stack(T)) Empty() bool { … } Type Parameters on types
121. is this enough?
122. func Max(type T)(vs []T) T { var max T for _, v := range vs { if v > max { max = v } } return max } A generic Max function
123. A generic Max function Max(int)([]int{1, 3, 2}) // 3 Max(float64)([]float64{1.0, 3.0, 2.0}) // 3.0 Max(string)([]string{"a", "c", "b"}) // "c" Max(bool)([]bool{true, false}) // ? Max([]byte)([][]byte{{1, 2}, {2, 1}}) // ?
124. type contracts
125. Type contracts provides a set of constraints on types. Sounds familiar? They look like functions: where each operation becomes a constraint. Type Contracts
126. contract comparable(v T) { var b bool = v < v } func Max(type T comparable)(vs []T) T { … } Max([]int)([]int{1, 3, 2}) // 3 Max([]string)([]string{"a", "c", "b"}) // "c" A comparable type
127. contract length(v T) { var n int = len(v) } func Length(type T length)(x T) int { return len(x) } Length([]int)([]int{1, 2, 3}) // 3 Length(map[int]int)(map[int]int{1: 1}) // 1 A type with length
128. int *os.File *strings.Reader *gzip.Writer []bool contract Reader contract Writer contract length contract comparable contract anything() {}
129. so … interfaces or contracts?
130. Interfaces and contracts define sets of types. Interfaces constrain on methods, contracts on anything. → all interfaces can be represented as contracts contract Stringer(x T) { type Stringer interface { var s string = x.String() String() string } } Interfaces vs Contracts
131. Interfaces constrain on methods, contracts on anything. → not all contracts can be represented as interfaces contract comparable(v T) { var b bool = v < v } contract comparable(v T) { var b bool = v.Equal(v) } Interfaces vs Contracts
132. putting the con back in contract
133. Contracts are great, but Once instantiated they are concrete types, so unlike interfaces: • They hide no implementation • They don’t provide dynamic dispatching This is has some good effects: • the type system once compiled doesn’t change. • there’s no change to the reflect package.
134. contracts are* too smart *probably
136. Remember the comparable contract? contract comparable(v T) { var b bool = v.Equal(v) } What is the type of the Equal method? • func (v T) Equal(x T) bool • func (v *T) Equal(x T) bool • func (v T) Equal(x interface{}) bool contracts are probably too smart
137. Interfaces provide: - generic algorithms - hidden implementation - interception points Implicit satisfaction: - break dependencies Type assertions: - to extend behaviors - to classify errors - to maintain compatibility Contracts would provide: - generic algorithms - generic types Libraries for: - Concurrency patterns - Functional programming Remove duplication: - int, int32, int64 … float32, float64 … - bytes vs strings contracts vs interfaces
138. Conclusion Interfaces are amazing and I still love them <3 Type parameters are great! • A library of concurrency patterns? • Functional programming in Go? Contracts are interesting, but probably *too smart* for Go. I hope to see new iterations of the draft simplifying the concepts.
139. Go is about simplicity I’d argue contracts are not
140. !הדות Thanks, @francesc