Go Lift

A talk that's secretly about Category Theory, delivered at dotGo 2017

Transcript

1. GO LIFT John Cinnamond dotGo 2017
2. PART 1 Constant interruptions
3. Connect to the server
4. Connect to the server Send first command
5. Connect to the server Send first command Wait for ok
6. Connect to the server Send first command Wait for ok Send second command
7. conn := net.Dial(”tcp”, ”server:9876”)
9. What if this goes wrong?
10. conn, err := net.Dial(”tcp”, ”server:9876”)
11. conn, err := net.Dial(”tcp”, ”server:9876”) if err != nil { panic(err) }
12. conn, err := net.Dial(”tcp”, ”server:9876”) if err != nil { panic(err) } conn.Write(command1)
14. What if this goes wrong?
15. conn, err := net.Dial(”tcp”, ”server:9876”) if err != nil { panic(err) } , err := conn.Write(command1)
16. conn, err := net.Dial(”tcp”, ”server:9876”) if err != nil { panic(err) } , err := conn.Write(command1) if err != nil { panic(err) }
17. conn, err := net.Dial(”tcp”, ”server:9876”) if err != nil { panic(err) } , err := conn.Write(command1) if err != nil { panic(err) } r := bufio.NewReader(conn) status := r.ReadString(’\n’)
19. . . . collaborate and listen
20. conn, err := net.Dial(”tcp”, ”server:9876”) if err != nil { panic(err) } , err := conn.Write(command1) if err != nil { panic(err) } r := bufio.NewReader(conn) status, err := r.ReadString(’\n’)
21. conn, err := net.Dial(”tcp”, ”server:9876”) if err != nil { panic(err) } , err := conn.Write(command1) if err != nil { panic(err) } r := bufio.NewReader(conn) status, err := r.ReadString(’\n’) if err != nil { panic(err) }
22. conn, err := net.Dial(”tcp”, ”server:9876”) if err != nil { panic(err) } , err := conn.Write(command1) if err != nil { panic(err) } r := bufio.NewReader(conn) status, err := r.ReadString(’\n’) if err != nil { panic(err) } if status == ”ok” { conn.Write(command2) }
24. . . . Hammer time
25. conn, err := net.Dial(”tcp”, ”server:9876”) if err != nil { panic(err) } , err := conn.Write(command1) if err != nil { panic(err) } r := bufio.NewReader(conn) status, err := r.ReadString(’\n’) if err != nil { panic(err) } if status == ”ok” { , err := conn.Write(command2) if err != nil { panic(err) } }
26. Handling errors is a good thing
27. Errors shouldn’t get in the way
28. conn := net.Dial(”tcp”, ”server:9876”) conn.Write(command1) r := bufio.NewReader(conn) status := r.ReadString(’\n’) if status == ”ok” { conn.Write(command2) }
29. conn := net.Dial(”tcp”, ”server:9876”) conn.Write(command1) r := bufio.NewReader(conn) status := r.ReadString(’\n’) if status == ”ok” { conn.Write(command2) } if err != nil if err != nil if err != nil if err != nil if err != nil if err != nil if err != nil if err != nil if err != nil if err != nil if err != nil
30. Just use Haskell
31. PART 2 if err != nil
33. Errors are values http://blog.golang.org/errors-are-values
34. conn.Write(command1)
35. conn.Write(command1) Not really the network
36. conn.Write(command1) Not really the network An abstraction
37. conn.Write(command1) Not really the network An abstraction A value
38. conn.Write(command1) Not really the network An abstraction A value . . . containing data about the connection
39. conn.Write(command1) Not really the network An abstraction A value . . . containing data about the connection . . . and behaviour to use it
40. A value . . . containing data about the connection . . . and behaviour to use it Determined by the type
41. Let’s add to the behaviour of net.Conn to handle errors
42. Let’s add to the behaviour of net.Conn to handle errors ⇒ create a new type
43. type SafeConn struct { }
44. type SafeConn struct { conn net.Conn }
45. type SafeConn struct { conn net.Conn err error }
46. func (c ∗SafeConn) write(b []byte) { }
47. func (c ∗SafeConn) write(b []byte) { c.conn.Write(b) }
48. Add error handling
49. func (c ∗SafeConn) write(b []byte) { c.conn.Write(b) }
50. func (c ∗SafeConn) write(b []byte) { , c.err := c.conn.Write(b) }
51. func (c ∗SafeConn) write(b []byte) { if c.err != nil { return } , c.err := c.Write(b) }
52. c := SafeConn{conn, nil}
53. c := SafeConn{conn, nil} c.write(command1) c.write(command2)
54. c := SafeConn{conn, nil} c.write(command1) // if this has an error c.write(command2)
55. c := SafeConn{conn, nil} c.write(command1) // if this has an error c.write(command2) // then this does nothing
56. c := SafeConn{conn, nil} c.write(command1) // if this has an error c.write(command2) // then this does nothing if c.err != nil { panic(”omg”) }
57. Repeat for other errors
58. conn, err := net.Dial(”tcp”, ”server:9876”) if err != nil { panic(err) }
59. func safeDial(network, address string) SafeConn { }
60. func safeDial(network, address string) SafeConn { conn, err := net.Dial(network, address) }
61. func safeDial(network, address string) SafeConn { conn, err := net.Dial(network, address) return SafeConn{conn, err} }
62. c := safeDial(”tcp”, ”server:9876”)
63. c := safeDial(”tcp”, ”server:9876”) c.write(command1) c.write(command2)
64. c := safeDial(”tcp”, ”server:9876”) c.write(command1) c.write(command2) if err != nil { panic(”never”) }
65. c := safeDial(”tcp”, ”server:9876”) // if this fails c.write(command1) c.write(command2) if err != nil { panic(”gonna”) }
66. c := safeDial(”tcp”, ”server:9876”) // if this fails c.write(command1) // then this does nothing c.write(command2) if err != nil { panic(”give”) }
67. c := safeDial(”tcp”, ”server:9876”) // if this fails c.write(command1) // then this does nothing c.write(command2) // same for this if err != nil { panic(”you”) }
68. We still handle the error
69. . . . but the error handling doesn’t get in the way
70. c := safeDial(”tcp”, ”server:9876”) c.write(command1) c.write(command2) if err != nil { panic(”up”) }
71. But wait!
72. We introduced a new abstraction
73. Abstractions have costs
74. c := safeDial(”tcp”, ”server:9876”) c.write(command1) // what happens if this fails c.write(command2) if err != nil { panic(err) }
75. Some details are hidden
76. . . . but this is nothing new
77. We use abstractions all the time
78. Is this abstraction appropriate?
79. PART 3 Division
80. Given 3 numbers (a, b, c) ⇒ a/b/c
81. func divide(a, b, c int ) { answer := a / b / c fmt. Println (answer) }
82. func divide(a, b, c int ) { answer := a / b / c fmt. Println (answer) } ... divide(100, 10, 2) // 5
83. func divide(a, b, c int ) { answer := a / b / c fmt. Println (answer) } ... divide(100, 10, 0)
84. func divide(a, b, c int ) { answer := a / b / c fmt. Println (answer) } ... divide(100, 10, 0) // panic: runtime error: integer divide by zero
85. func divide(a, b, c int ) { answer := a / b / c fmt. Println (answer) } ... divide(100, 10, 0) // Can’t divide by zero
86. Let’s solve this badly
87. func divide(a, b, c int ) { if b == 0 || c == 0 { fmt. Printf (”Can’t divide by zero”) return } answer = a / b / c fmt. Println (answer) }
88. func divide(a, b, c int ) { answer := a / b / c fmt. Println (answer) }
89. func divide(a, b, c int ) { answer := a / b / c fmt. Println (answer) } if b == 0 if b == 0 return if c == 0 if c == 0 if b == 0 if c == 0 if b == 0 if b == 0 if c == 0 if c == 0 if b == 0 return return
90. Is this like err != nil ?
91. Create a new type
92. Create a new type Wrap the initial value
93. Create a new type Wrap the initial value Wrap the behaviour
94. Create a new type Wrap the initial value Wrap the behaviour Wrap the conditionals
95. Create a new type
96. type Divideinator struct { answer int }
97. Wrap the initial value
98. d := Divideinator{a}
99. Wrap the behaviour
100. func (d ∗Divideinator) divide(x int ) { d.answer = d.answer / x }
101. Wrap the conditional
102. func (d ∗Divideinator) divide(x int ) { if x == 0 { d.isZero = true return } d.answer = d.answer / x }
103. func (d Divideinator) String () string { if d.isZero { return fmt. Sprintf (”Can’t divide by zero”) } return fmt. Sprintf (”%d”, d.answer) }
104. Put it all together
105. func divide(a, b, c int ) { d := Divideinator{a} }
106. func divide(a, b, c int ) { d := Divideinator{a} d.divide(b) }
107. func divide(a, b, c int ) { d := Divideinator{a} d.divide(b) d.divide(c) }
108. func divide(a, b, c int ) { d := Divideinator{a} d.divide(b) d.divide(c) fmt. Println (d) }
109. func divide(a, b, c int ) { d := Divideinator{a} d.divide(b) d.divide(c) fmt. Println (d) } ... divide(100, 10, 2) // 5
110. func divide(a, b, c int ) { d := Divideinator{a} d.divide(b) d.divide(c) fmt. Println (d) } ... divide(100, 0, 2) // Can’t divide by zero
111. func divide(a, b, c int ) { d := Divideinator{a} d.divide(b) d.divide(c) fmt. Println (d) } ... divide(100, 10, 0) // Can’t divide by zero
112. This is the same approach as error handling
113. Create a new type
114. Create a new type Wrap the initial value
115. Create a new type Wrap the initial value Wrap the behaviour
116. Create a new type Wrap the initial value Wrap the behaviour Wrap the conditional
117. I want to really understand this
118. Let’s look at the shape of the code
119. D(100) D(10) D(5) 100 10 5 divide(10) divide(2) /10 /2
120. D(100) D(10) D(5) 100 10 5 divide(10) divide(2) /10 /10 ; /2 /2
121. D(100) D(10) D(5) 100 10 5 divide(x) ; divide(y) divide(10) divide(2) /10 /0 /2 /0
122. Lift the initial value into a new type
123. D(100) D(10) D(5) 100 10 5 divide(x) ; divide(y) divide(10) divide(2) /10 /0 Divideinator /2 /0
124. Lift the behaviour
125. D(100) D(10) D(5) 100 10 5 divide(x) ; divide(y) divide(10) divide(2) /10 /0 Divideinator /2 /0
126. D(100) D(10) D(5) 100 10 5 divide(x) ; divide(y) divide(10) divide(2) /10 /0 Divideinator /2 /0
127. Lift the conditionals
128. D(100) D( ) D( ) 100 10 5 divide(x) ; divide(y) divide(0) divide(0) /10 /0 Divideinator /2 /0
129. D(100) D D 100 10 5 divide(x) ; divide(y) divide(x) divide(y) /10 /0 /2 /0
130. D(100) D D 100 10 5 divide(x) ; divide(y) divide(x) divide(y) /10 /0 /2 /0
131. SafeConn SafeConn SafeConn conn conn conn write() ; write() Write error Write error
132. SafeConn SafeConn SafeConn conn conn conn write() ; write() Write error Write error
133. SafeConn SafeConn SafeConn conn conn conn write() ; write() Write error Write error
134. SafeConn SafeConn SafeConn conn conn conn write() ; write() Write error Write error
135. SafeConn SafeConn SafeConn conn conn conn write() ; write()
136. Ta Tb Tc a b c Tf Tf() ; Tg() Tg f g
137. Ta Tb Tc a b c Tf Tf() ; Tg() Tg f g
138. Ta Tb Tc a b c Tf Tf() ; Tg() Tg f T g
139. Ta Tb Tc a b c Tf Tf() ; Tg() Tg f T g
140. Ta Tb Tc a b c Tf Tf() ; Tg() Tg f T g T
141. Ta Tb Tc a b c Tf Tf() ; Tg() Tg f T g T T
142. Ta Tb Tc a b c Tf Tf() ; Tg() Tg f T g T T T
143. Ta Tb Tc a b c Tf Tf() ; Tg() Tg f g
144. Ta Tb Tc a b c Tf Tf() ; Tg() Tg f g
145. Less math. More code.
146. PART 4 Building a webapp
147. http://doesgohavegenericsyet.com
148. func signupHandler(w http.ResponseWriter, r ∗http.Request) { email := r.FormValue(”email”) validateEmail(email) }
149. func signupHandler(w http.ResponseWriter, r ∗http.Request) { email := r.FormValue(”email”) validateEmail(email) checkAlreadyRegistered(email) }
150. func signupHandler(w http.ResponseWriter, r ∗http.Request) { email := r.FormValue(”email”) validateEmail(email) checkAlreadyRegistered(email) register (email) }
151. func signupHandler(w http.ResponseWriter, r ∗http.Request) { email := r.FormValue(”email”) validateEmail(email) checkAlreadyRegistered(email) register (email) sellEmailToRecruiters(email) }
152. func signupHandler(w http.ResponseWriter, r ∗http.Request) { email := r.FormValue(”email”) validateEmail(email) checkAlreadyRegistered(email) register (email) sellEmailToRecruiters(email) logRequest(r) }
153. func signupHandler(w http.ResponseWriter, r ∗http.Request) { email := r.FormValue(”email”) validateEmail(email) checkAlreadyRegistered(email) register (email) sellEmailToRecruiters(email) logRequest(r) fmt. Fprintln (w, ...) }
154. func signupHandler(w http.ResponseWriter, r ∗http.Request) { email := r.FormValue(”email”) if !validateEmail(email) { http .Error(w, ...) return } checkAlreadyRegistered(email) register (email) sellEmailToRecruiters(email) logRequest(r) fmt. Fprintln (w, ...) }
155. func signupHandler(w http.ResponseWriter, r ∗http.Request) { email := r.FormValue(”email”) if !validateEmail(email) { logRequest(”invalid email”, r) http .Error(w, ...) return } checkAlreadyRegistered(email) register (email) sellEmailToRecruiters(email) logRequest(r) fmt. Fprintln (w, ...) }
156. func signupHandler(w http.ResponseWriter, r ∗http.Request) { email := r.FormValue(”email”) if !validateEmail(email) { logRequest(”invalid email”, r) http .Error(w, ...) return } if alreadyRegistered(email) { logRequest(”already registered”, r) http .Error(w, ...) return } register (email) sellEmailToRecruiters(email) logRequest(r) fmt. Fprintln (w, ...) }
157. func signupHandler(w http.ResponseWriter, r ∗http.Request) { email := r.FormValue(”email”) if !validateEmail(email) { logRequest(”invalid email”, r) http .Error(w, ...) return } if alreadyRegistered(email) { sellEmailToRecruiters(email) logRequest(”already registered”, r) http .Error(w, ...) return } register (email) sellEmailToRecruiters(email) logRequest(r) fmt. Fprintln (w, ...) }
158. func signupHandler(w http.ResponseWriter, r ∗http.Request) { email := r.FormValue(”email”) if !validateEmail(email) { logRequest(”invalid email”, r) http .Error(w, ...) return } if alreadyRegistered(email) { sellEmailToRecruiters(email) logRequest(”already registered”, r) http .Error(w, ...) return } if err := register (email); err != nil { sellEmailToRecruiters(email) logRequest(”registration failed ”, r) http .Error(w, ...) return }
159. func signupHandler(w http.ResponseWriter, r ∗http.Request) { email := r.FormValue(”email”) validateEmail(email) checkAlreadyRegistered(email) register (email) sellEmailToRecruiters(email) logRequest(r) fmt. Fprintln (w, ...) }
160. func signupHandler(w http.ResponseWriter, r ∗http.Request) { email := r.FormValue(”email”) validateEmail(email) checkAlreadyRegistered(email) register (email) sellEmailToRecruiters(email) logRequest(r) fmt. Fprintln (w, ...) } if err != nil if alreadyRegistered logRequest if err != nil if !validateEmail logRequest if alreadyRegistered if err != nil sellEmail logRequest
161. Ta Tb Tc Td Te a b c d e λ Tf Tf() ; Tg() ; Th() ; Ti() Tg Th Ti f g h i
162. Ta Tb Tc Td Te a b c d e λ Tf Tf() ; Tg() ; Th() ; Ti() Tg Th Ti f g h i
163. Ta Tb Tc Td Te a b c d e λ Tf Tf() ; Tg() ; Th() ; Ti() Tg Th Ti f g h i
164. We know how to do this
165. Ta Tb Tc Td Te a b c d e λ Tf Tf() ; Tg() ; Th() ; Ti() Tg Th Ti f T g h i
166. Ta Tb Tc Td Te a b c d e λ Tf Tf() ; Tg() ; Th() ; Ti() Tg Th Ti f T T g T h T i T
167. Ta Tb Tc Td Te a b c d e λ Tf Tf() ; Tg() ; Th() ; Ti() Tg Th Ti f T T g T h T i T T T T T
168. Create a new type Lift initial data Lift behaviour Lift control flow
169. Create a new type
170. type SignupRequest struct { }
171. Lift initial data
172. type SignupRequest struct { w http .ResponseWriter r ∗http.Request }
173. Lift behaviour
174. func (s ∗SignupRequest) validate() { if s.email == ”” || ... { s.err = ” invalid email” } }
175. func (s ∗SignupRequest) checkNewRegistration() { if existingEmails.Contain(s.email) { s.err = ”already registered” } }
176. Lift control flow
177. func (s ∗SignupRequest) checkNewRegistration() { if existingEmails.Contain(s.email) { s.err = ”already registered” } }
178. func (s ∗SignupRequest) checkNewRegistration() { if s.err != nil { return } if existingEmails.Contain(s.email) { s.err = ”already registered” } }
179. Compose the functions
180. func signupHandler(w http.ResponseWriter, r ∗http.Request) { s := newSignupRequest(w, r) }
181. func signupHandler(w http.ResponseWriter, r ∗http.Request) { s := newSignupRequest(w, r) s.validate () }
182. func signupHandler(w http.ResponseWriter, r ∗http.Request) { s := newSignupRequest(w, r) s.validate () s.checkNewRegistration() }
183. func signupHandler(w http.ResponseWriter, r ∗http.Request) { s := newSignupRequest(w, r) s.validate () s.checkNewRegistration() s. register () }
184. func signupHandler(w http.ResponseWriter, r ∗http.Request) { s := newSignupRequest(w, r) s.validate () s.checkNewRegistration() s. register () s.sellEmailToRecruiter() }
185. func signupHandler(w http.ResponseWriter, r ∗http.Request) { s := newSignupRequest(w, r) s.validate () s.checkNewRegistration() s. register () s.sellEmailToRecruiter() s.log() }
186. func signupHandler(w http.ResponseWriter, r ∗http.Request) { s := newSignupRequest(w, r) s.validate () s.checkNewRegistration() s. register () s.sellEmailToRecruiter() s.log() s.respond() }
187. Please don’t write your request handlers like this
188. I’m not trying to tell you how to write code
189. I’m trying to give you something new to think about
190. Think about the shape of the code
191. Think about using types
192. I want to give you new tools
193. . . . to cope with complex code
194. It’s up to you to decide how to use them
195. Epilogue
196. We solve problems by breaking them into smaller pieces
197. . . . but then we need to join the pieces back together again
198. We need to understand the forces at play
199. Mathematics gives us this understanding
200. Mathematics lets us achieve more
201. Mathematics is pretty useful
202. Thank you John Cinnamond dotGo 2017 Gophers adapted from The Go Gopher by Renee French https://blog.golang.org/gopher