Building Robust Software (Episode 1)

Presented at https://www.meetkt.org on 22 July 2021

Most of us can build software that works. But, building robust software that can withstand the test of time requires a shift in both mindset and skillset. Often, we have trouble finding out where to start.

This talk aims to shed some light on making that shift in both mindset and skillset. We'll begin by drawing some inspiration from the philosophy behind TCP and venture into functional programming principles to adopt ideas that can help us build expressive and robust programs.

We'll also look into an example for each idea and see how you can adopt it and implement it at work the very next day. We will leverage Kotlin's multi-paradigm abilities and its type system to bring most of these ideas to fruition.

Transcript

1. Ragunath Jawahar • @ragunathjawahar Building Robust Software Episode 1 Small ideas, big impact
2. Write code that communicates well.
3. Example 1 API Call / External Systems
4. interface AccountsApi { fun signup(email: String, password: String): Response? } AccountsApi.kt
5. interface AccountsApi { fun signup(email: String, password: String): Response? } AccountsApi.kt
6. interface AccountsApi { fun signup(email: String, password: String): Response? } AccountsApi.kt
7. interface AccountsApi { fun signup(email: String, password: String): Response? } AccountsApi.kt
8. interface AccountsApi { fun signup(email: String, password: String): Response? } AccountsApi.kt
9. interface AccountsApi { fun signup(email: String, password: String): Response? } AccountsApi.kt
10. interface AccountsApi { fun signup(email: String, password: String): Response? } AccountsApi.kt
11. Response? • Successfully creates a user account and returns a Response 🎉 • null • retrofit2.HttpException • java.net.SocketTimeoutException • i.ll.MakeYourLifeMoreInterestingException 🤷
12. Response? • Successfully creates a user account and returns a Response 🎉 • null • retrofit2.HttpException • java.net.SocketTimeoutException • i.ll.MakeYourLifeMoreInterestingException 🤷
13. Response? • Successfully creates a user account and returns a Response 🎉 • null • retrofit2.HttpException • java.net.SocketTimeoutException • i.ll.MakeYourLifeMoreInterestingException 🤷
14. Response? • Successfully creates a user account and returns a Response 🎉 • null • retrofit2.HttpException • java.net.SocketTimeoutException • i.ll.MakeYourLifeMoreInterestingException 🤷
15. Response? • Successfully creates a user account and returns a Response 🎉 • null • retrofit2.HttpException • java.net.SocketTimeoutException • i.ll.MakeYourLifeMoreInterestingException 🤷
16. 😊 😕 😨 Caller’s emotions
17. 😊 😕 😨 Caller’s emotions
18. 😊 😕 😨 Caller’s emotions
19. 😊 😕 😨 Caller’s emotions
20. Be conservative in what you send, be liberal in what you accept Robustness Principle
21. Be conservative in what you send, be liberal in what you accept Robustness Principle
22. Robustness Principle Be conservative in what you send, be liberal in what you accept
23. interface AccountsApi { fun signup(email: String, password: String): Response? } AccountsApi.kt
24. interface AccountsApi { fun signup(email: String, password: String): Response? } AccountsApi.kt
25. interface AccountsApi { fun signup(email: String, password: String): Response? } AccountsApi.kt
26. interface AccountsApi { fun signup(email: String, password: String): Response? } AccountsApi.kt
27. interface AccountsApi { fun signup(email: String, password: String): SignupResponse } AccountsApi.kt
28. SignupResponse • Successfully creates a user account • Email already registered • Validation errors • Connection errors • 5xx server errors • Unknown errors
29. SignupResponse • Successfully creates a user account • Email already registered • Validation errors • Connection errors • 5xx server errors • Unknown errors
30. SignupResponse • Successfully creates a user account • Email already registered • Validation errors • Connection errors • 5xx server errors • Unknown errors
31. SignupResponse • Successfully creates a user account • Email already registered • Validation errors • Connection errors • 5xx server errors • Unknown errors
32. SignupResponse • Successfully creates a user account • Email already registered • Validation errors • Connection errors • 5xx server errors • Unknown errors
33. SignupResponse • Successfully creates a user account • Email already registered • Validation errors • Connection errors • 5xx server errors • Unknown errors
34. SignupResponse • Successfully creates a user account • Email already registered • Validation errors • Connection errors • 5xx server errors • Unknown errors
35. SignupResponse • Successfully creates a user account • Email already registered • Validation errors • Connection errors • 5xx server errors • Unknown errors
36. Sealed Classes Sealed classes are used for representing restricted class hierarchies, when a value can have one of the types from a limited set, but cannot have any other type.
37. Sealed Classes Sealed classes are used for representing restricted class hierarchies, when a value can have one of the types from a limited set, but cannot have any other type.
38. Sealed Classes Sealed classes are used for representing restricted class hierarchies, when a value can have one of the types from a limited set, but cannot have any other type.
39. SignupResponse.kt sealed class SignupResponse { data class AccountCreated(val userId: String) : SignupResponse() data class EmailAlreadyRegistered(val registrationEmail: String) : SignupResponse() data class InputValidationFailed(val errors: List<ValidationError>) : SignupResponse() object ConnectionError : SignupResponse() data class ServerError(val statusCode: Int) : SignupResponse() data class UnknownError(val statusCode: Int, val responseBody: String?) : SignupResponse() }
40. SignupResponse.kt sealed class SignupResponse { data class AccountCreated(val userId: String) : SignupResponse() data class EmailAlreadyRegistered(val registrationEmail: String) : SignupResponse() data class InputValidationFailed(val errors: List<ValidationError>) : SignupResponse() object ConnectionError : SignupResponse() data class ServerError(val statusCode: Int) : SignupResponse() data class UnknownError(val statusCode: Int, val responseBody: String?) : SignupResponse() }
41. SignupResponse.kt sealed class SignupResponse { data class AccountCreated(val userId: String) : SignupResponse() data class EmailAlreadyRegistered(val registrationEmail: String) : SignupResponse() data class InputValidationFailed(val errors: List<ValidationError>) : SignupResponse() object ConnectionError : SignupResponse() data class ServerError(val statusCode: Int) : SignupResponse() data class UnknownError(val statusCode: Int, val responseBody: String?) : SignupResponse() }
42. SignupResponse.kt sealed class SignupResponse { data class AccountCreated(val userId: String) : SignupResponse() data class EmailAlreadyRegistered(val registrationEmail: String) : SignupResponse() data class InputValidationFailed(val errors: List<ValidationError>) : SignupResponse() object ConnectionError : SignupResponse() data class ServerError(val statusCode: Int) : SignupResponse() data class UnknownError(val statusCode: Int, val responseBody: String?) : SignupResponse() }
43. SignupResponse.kt sealed class SignupResponse { data class AccountCreated(val userId: String) : SignupResponse() data class EmailAlreadyRegistered(val registrationEmail: String) : SignupResponse() data class InputValidationFailed(val errors: List<ValidationError>) : SignupResponse() object ConnectionError : SignupResponse() data class ServerError(val statusCode: Int) : SignupResponse() data class UnknownError(val statusCode: Int, val responseBody: String?) : SignupResponse() }
44. SignupResponse.kt sealed class SignupResponse { data class AccountCreated(val userId: String) : SignupResponse() data class EmailAlreadyRegistered(val registrationEmail: String) : SignupResponse() data class InputValidationFailed(val errors: List<ValidationError>) : SignupResponse() object ConnectionError : SignupResponse() data class ServerError(val statusCode: Int) : SignupResponse() data class UnknownError(val statusCode: Int, val responseBody: String?) : SignupResponse() }
45. SignupResponse.kt sealed class SignupResponse { data class AccountCreated(val userId: String) : SignupResponse() data class EmailAlreadyRegistered(val registrationEmail: String) : SignupResponse() data class InputValidationFailed(val errors: List<ValidationError>) : SignupResponse() object ConnectionError : SignupResponse() data class ServerError(val statusCode: Int) : SignupResponse() data class UnknownError(val statusCode: Int, val responseBody: String?) : SignupResponse() }
46. SignupResponse.kt sealed class SignupResponse { data class AccountCreated(val userId: String) : SignupResponse() data class EmailAlreadyRegistered(val registrationEmail: String) : SignupResponse() data class InputValidationFailed(val errors: List<ValidationError>) : SignupResponse() object ConnectionError : SignupResponse() data class ServerError(val statusCode: Int) : SignupResponse() data class UnknownError(val statusCode: Int, val responseBody: String?) : SignupResponse() }
47. SignupResponse.kt sealed class SignupResponse { data class AccountCreated(val userId: String) : SignupResponse() data class EmailAlreadyRegistered(val registrationEmail: String) : SignupResponse() data class InputValidationFailed(val errors: List<ValidationError>) : SignupResponse() object ConnectionError : SignupResponse() data class ServerError(val statusCode: Int) : SignupResponse() data class UnknownError(val statusCode: Int, val responseBody: String?) : SignupResponse() }
48. Usage - Sealed Class Exhaustiveness val response = accountsApi.signup("John Doe", "john@example.org") when (response) { is AccountCreated -> saveUserId(response.userId) is EmailAlreadyRegistered -> showEmailAlreadyRegistered(response.registrationEmail) is InputValidationFailed -> showValidationErrors(response.errors) ConnectionError -> showCheckConnectionMessage() is ServerError, is UnknownError -> showTryAgainInSometimeMessage() }.exhaustive()
49. Usage - Sealed Class Exhaustiveness val response = accountsApi.signup("John Doe", "john@example.org") when (response) { is AccountCreated -> saveUserId(response.userId) is EmailAlreadyRegistered -> showEmailAlreadyRegistered(response.registrationEmail) is InputValidationFailed -> showValidationErrors(response.errors) ConnectionError -> showCheckConnectionMessage() is ServerError, is UnknownError -> showTryAgainInSometimeMessage() }.exhaustive()
50. Usage - Sealed Class Exhaustiveness val response = accountsApi.signup("John Doe", "john@example.org") when (response) { is AccountCreated -> saveUserId(response.userId) is EmailAlreadyRegistered -> showEmailAlreadyRegistered(response.registrationEmail) is InputValidationFailed -> showValidationErrors(response.errors) ConnectionError -> showCheckConnectionMessage() is ServerError, is UnknownError -> showTryAgainInSometimeMessage() }.exhaustive()
51. Usage - Sealed Class Exhaustiveness val response = accountsApi.signup("John Doe", "john@example.org") when (response) { is AccountCreated -> saveUserId(response.userId) is EmailAlreadyRegistered -> showEmailAlreadyRegistered(response.registrationEmail) is InputValidationFailed -> showValidationErrors(response.errors) ConnectionError -> showCheckConnectionMessage() is ServerError, is UnknownError -> showTryAgainInSometimeMessage() }.exhaustive()
52. Usage - Sealed Class Exhaustiveness val response = accountsApi.signup("John Doe", "john@example.org") when (response) { is AccountCreated -> saveUserId(response.userId) is EmailAlreadyRegistered -> showEmailAlreadyRegistered(response.registrationEmail) is InputValidationFailed -> showValidationErrors(response.errors) ConnectionError -> showCheckConnectionMessage() is ServerError, is UnknownError -> showTryAgainInSometimeMessage() }.exhaustive()
53. Usage - Sealed Class Exhaustiveness val response = accountsApi.signup("John Doe", "john@example.org") when (response) { is AccountCreated -> saveUserId(response.userId) is EmailAlreadyRegistered -> showEmailAlreadyRegistered(response.registrationEmail) is InputValidationFailed -> showValidationErrors(response.errors) ConnectionError -> showCheckConnectionMessage() is ServerError, is UnknownError -> showTryAgainInSometimeMessage() }.exhaustive()
54. Usage - Sealed Class Exhaustiveness val response = accountsApi.signup("John Doe", "john@example.org") when (response) { is AccountCreated -> saveUserId(response.userId) is EmailAlreadyRegistered -> showEmailAlreadyRegistered(response.registrationEmail) is InputValidationFailed -> showValidationErrors(response.errors) ConnectionError -> showCheckConnectionMessage() is ServerError, is UnknownError -> showTryAgainInSometimeMessage() }.exhaustive()
55. Usage - Sealed Class Exhaustiveness val response = accountsApi.signup("John Doe", "john@example.org") when (response) { is AccountCreated -> saveUserId(response.userId) is EmailAlreadyRegistered -> showEmailAlreadyRegistered(response.registrationEmail) is InputValidationFailed -> showValidationErrors(response.errors) ConnectionError -> showCheckConnectionMessage() is ServerError, is UnknownError -> showTryAgainInSometimeMessage() }.exhaustive()
56. Usage - Sealed Class Exhaustiveness val response = accountsApi.signup("John Doe", "john@example.org") when (response) { is AccountCreated -> saveUserId(response.userId) is EmailAlreadyRegistered -> showEmailAlreadyRegistered(response.registrationEmail) is InputValidationFailed -> showValidationErrors(response.errors) ConnectionError -> showCheckConnectionMessage() is ServerError, is UnknownError -> showTryAgainInSometimeMessage() }.exhaustive()
57. Enforcing exhaustive when statements https://github.com/cashapp/exhaustive (compiler plugin) https://youtrack.jetbrains.com/issue/KT-12380 (Kotlin language proposal)
58. Usage - Sealed Class Exhaustiveness val response = accountsApi.signup("John Doe", "john@example.org") when (response) { is AccountCreated -> saveUserId(response.userId) is EmailAlreadyRegistered -> showEmailAlreadyRegistered(response.registrationEmail) is InputValidationFailed -> showValidationErrors(response.errors) ConnectionError -> showCheckConnectionMessage() is ServerError, is UnknownError -> showTryAgainInSometimeMessage() }.exhaustive()
59. 😊 😌 Caller’s emotions
60. 😊 😌 Caller’s emotions
61. 😊 😌 Caller’s emotions
62. What have we achieved? • Certainty • Convey business ideas in code • Improved readability • Represent errors as data
63. What have we learnt? • Total functions • Algebraic data types • Robustness principle
64. What have we learnt? • Total functions • Algebraic data types • Robustness principle
65. Total Functions Total functions are functions that give you a valid return value for every combination of valid arguments.
66. Partial Functions A partial function is a function that is not defined for all possible input values; in some cases returns a value, it may never return at all, throw an exception or simply crash a system.
67. Partial Functions A partial function is a function that is not defined for all possible input values; in some cases returns a value, it may never return at all, throw an exception or simply crash a system.
68. Partial Functions A partial function is a function that is not defined for all possible input values; in some cases returns a value, it may never return at all, throw an exception or simply crash a system.
69. Partial Functions A partial function is a function that is not defined for all possible input values; in some cases returns a value, it may never return at all, throw an exception or simply crash a system.
70. Partial Functions A partial function is a function that is not defined for all possible input values; in some cases returns a value, it may never return at all, throw an exception or simply crash a system.
71. Partial Functions A partial function is a function that is not defined for all possible input values; in some cases returns a value, it may never return at all, throw an exception or simply crash a system.
72. Partial Functions A partial function is a function that is not defined for all possible input values; in some cases returns a value, it may never return at all, throw an exception or simply crash a system.
73. Partial Functions A partial function is a function that is not defined for all possible input values; in some cases returns a value, it may never return at all, throw an exception or simply crash a system.
74. // Partial function fun signup(email: String, password: String): Response? // Total function fun signup(email: String, password: String): SignupResponse Partial vs. Total Functions
75. // Partial function fun signup(email: String, password: String): Response? // Total function fun signup(email: String, password: String): SignupResponse Partial vs. Total Functions
76. // Partial function fun signup(email: String, password: String): Response? // Total function fun signup(email: String, password: String): SignupResponse Partial vs. Total Functions
77. // Partial function fun signup(email: String, password: String): Response? // Total function fun signup(email: String, password: String): SignupResponse Partial vs. Total Functions
78. // Partial function fun signup(email: String, password: String): Response? // Total function fun signup(email: String, password: String): SignupResponse Partial vs. Total Functions
79. // Partial function fun signup(email: String, password: String): Response? // Total function fun signup(email: String, password: String): SignupResponse Partial vs. Total Functions
80. // Partial function fun signup(email: String, password: String): Response? // Total function fun signup(email: String, password: String): SignupResponse Partial vs. Total Functions
81. What have we learnt? • Total functions • Algebraic data types • Robustness principle
82. What have we learnt? • Total functions • Algebraic data types • Robustness principle
83. Algebraic Data Types An algebraic data type is a kind of composite type, i.e., a type formed by combining other types.
84. SignupResponse.kt sealed class SignupResponse { data class AccountCreated(val userId: String) : SignupResponse() data class EmailAlreadyRegistered(val registrationEmail: String) : SignupResponse() data class InputValidationFailed(val errors: List<ValidationError>) : SignupResponse() object ConnectionError : SignupResponse() data class ServerError(val statusCode: Int) : SignupResponse() data class UnknownError(val statusCode: Int, val responseBody: String?) : SignupResponse() }
85. SignupResponse.kt sealed class SignupResponse { data class AccountCreated(val userId: String) : SignupResponse() data class EmailAlreadyRegistered(val registrationEmail: String) : SignupResponse() data class InputValidationFailed(val errors: List<ValidationError>) : SignupResponse() object ConnectionError : SignupResponse() data class ServerError(val statusCode: Int) : SignupResponse() data class UnknownError(val statusCode: Int, val responseBody: String?) : SignupResponse() }
86. SignupResponse.kt sealed class SignupResponse { data class AccountCreated(val userId: String) : SignupResponse() data class EmailAlreadyRegistered(val registrationEmail: String) : SignupResponse() data class InputValidationFailed(val errors: List<ValidationError>) : SignupResponse() object ConnectionError : SignupResponse() data class ServerError(val statusCode: Int) : SignupResponse() data class UnknownError(val statusCode: Int, val responseBody: String?) : SignupResponse() }
87. SignupResponse.kt sealed class SignupResponse { data class AccountCreated(val userId: String) : SignupResponse() data class EmailAlreadyRegistered(val registrationEmail: String) : SignupResponse() data class InputValidationFailed(val errors: List<ValidationError>) : SignupResponse() object ConnectionError : SignupResponse() data class ServerError(val statusCode: Int) : SignupResponse() data class UnknownError(val statusCode: Int, val responseBody: String?) : SignupResponse() }
88. What have we learnt? • Total functions • Algebraic data types • Robustness principle
89. What have we learnt? • Total functions • Algebraic data types • Robustness principle
90. Where to apply? • Use it heavily on integration points with external systems (code that lives outside your own application) • Anti-corruption layers (both internal and external) • Take failure frequency and domain context into account (databases and fi le systems may not be good candidates. Network layer is a good starting point.)
91. Example 2 Division / Exceptions
92. fun divide(dividend: Int, divisor: Int): Int { return dividend / divisor } Arithmetic.kt
93. fun divide(dividend: Int, divisor: Int): Int { return dividend / divisor } Arithmetic.kt
94. fun divide(dividend: Int, divisor: Int): Int { return dividend / divisor } Arithmetic.kt
95. fun divide(dividend: Int, divisor: Int): Int { return dividend / divisor } Arithmetic.kt
96. fun divide(dividend: Int, divisor: Int): Int { return dividend / divisor } Arithmetic.kt
97. fun divide(dividend: Int, divisor: Int): Int { return dividend / divisor } Arithmetic.kt
98. fun divide(dividend: Int, divisor: Int): Int { return dividend / divisor } Arithmetic.kt
99. divisor = 0
100. Exception in thread "main" java.lang.ArithmeticException: / by zero at org.example.ArithmeticKt.divide(Arithmetic.kt:3) at org.example.ArithmeticKt.main(Arithmetic.kt:7) at org.example.ArithmeticKt.main(Arithmetic.kt)
101. Be conservative in what you send, be liberal in what you accept
102. fun divide(dividend: Int, divisor: Int): Quotient { return if (divisor == 0) { Infinity } else { Result(dividend / divisor) } } Arithmetic.kt
103. fun divide(dividend: Int, divisor: Int): Quotient { return if (divisor == 0) { Infinity } else { Result(dividend / divisor) } } Arithmetic.kt
104. fun divide(dividend: Int, divisor: Int): Quotient { return if (divisor == 0) { Infinity } else { Result(dividend / divisor) } } Arithmetic.kt
105. fun divide(dividend: Int, divisor: Int): Quotient { return if (divisor == 0) { Infinity } else { Result(dividend / divisor) } } Arithmetic.kt
106. fun divide(dividend: Int, divisor: Int): Quotient { return if (divisor == 0) { Infinity } else { Result(dividend / divisor) } } Arithmetic.kt
107. fun divide(dividend: Int, divisor: Int): Quotient { return if (divisor == 0) { Infinity } else { Result(dividend / divisor) } } Arithmetic.kt
108. fun divide(dividend: Int, divisor: Int): Quotient { return if (divisor == 0) { Infinity } else { Result(dividend / divisor) } } Arithmetic.kt
109. fun divide(dividend: Int, divisor: Int): Quotient { return if (divisor == 0) { Infinity } else { Result(dividend / divisor) } } Arithmetic.kt
110. fun divide(dividend: Int, divisor: Int): Quotient { return if (divisor == 0) { Infinity } else { Result(dividend / divisor) } } Arithmetic.kt
111. fun divide(dividend: Int, divisor: Int): Quotient { return if (divisor == 0) { Infinity } else { Result(dividend / divisor) } } Arithmetic.kt
112. sealed class Quotient { object Infinity : Quotient() data class Result(val value: Double) : Quotient() } Quotient.kt
113. sealed class Quotient { object Infinity : Quotient() data class Result(val value: Double) : Quotient() } Quotient.kt
114. sealed class Quotient { object Infinity : Quotient() data class Result(val value: Double) : Quotient() } Quotient.kt
115. sealed class Quotient { object Infinity : Quotient() data class Result(val value: Double) : Quotient() } Quotient.kt
116. sealed class Quotient { object Infinity : Quotient() data class Result(val value: Double) : Quotient() } Quotient.kt
117. 🤔
118. Implications • Places burden on the consumer to handle both Infinity and Result types • The consumer may pass the Quotient type downstream. If that happens, downstream consumers should also deal with the added complexity
119. Be conservative in what you send, be liberal in what you accept
120. Be conservative in what you send, be liberal in what you accept conservative
121. fun divide(dividend: Int, divisor: Integer.NonZero): Int { return dividend / divisor.number } Arithmetic.kt
122. fun divide(dividend: Int, divisor: Integer.NonZero): Int { return dividend / divisor.number } Arithmetic.kt
123. fun divide(dividend: Int, divisor: Integer.NonZero): Int { return dividend / divisor.number } Arithmetic.kt
124. fun divide(dividend: Int, divisor: Integer.NonZero): Int { return dividend / divisor.number } Arithmetic.kt
125. fun divide(dividend: Int, divisor: Integer.NonZero): Int { return dividend / divisor.number } Arithmetic.kt
126. fun divide(dividend: Int, divisor: Integer.NonZero): Int { return dividend / divisor.number } Arithmetic.kt
127. fun divide(dividend: Int, divisor: Integer.NonZero): Int { return dividend / divisor.number } Arithmetic.kt
128. fun divide(dividend: Int, divisor: Integer.NonZero): Int { return dividend / divisor.number } Arithmetic.kt
129. fun divide(dividend: Int, divisor: Integer.NonZero): Int { return dividend / divisor.number } Arithmetic.kt
130. fun divide(dividend: Int, divisor: Integer.NonZero): Int { return dividend / divisor.number } Arithmetic.kt
131. sealed class Integer { object Zero : Integer() data class NonZero(val number: Int) : Integer() companion object { fun from(number: Int): Integer = if (number == 0) { Zero } else { NonZero(number) } } } Integer.kt
132. sealed class Integer { object Zero : Integer() data class NonZero(val number: Int) : Integer() companion object { fun from(number: Int): Integer = if (number == 0) { Zero } else { NonZero(number) } } } Integer.kt
133. sealed class Integer { object Zero : Integer() data class NonZero(val number: Int) : Integer() companion object { fun from(number: Int): Integer = if (number == 0) { Zero } else { NonZero(number) } } } Integer.kt
134. sealed class Integer { object Zero : Integer() data class NonZero(val number: Int) : Integer() companion object { fun from(number: Int): Integer = if (number == 0) { Zero } else { NonZero(number) } } } Integer.kt
135. sealed class Integer { object Zero : Integer() data class NonZero(val number: Int) : Integer() companion object { fun from(number: Int): Integer = if (number == 0) { Zero } else { NonZero(number) } } } Integer.kt
136. sealed class Integer { object Zero : Integer() data class NonZero(val number: Int) : Integer() companion object { fun from(number: Int): Integer = if (number == 0) { Zero } else { NonZero(number) } } } Integer.kt
137. sealed class Integer { object Zero : Integer() data class NonZero(val number: Int) : Integer() companion object { fun from(number: Int): Integer = if (number == 0) { Zero } else { NonZero(number) } } } Integer.kt
138. sealed class Integer { object Zero : Integer() data class NonZero(val number: Int) : Integer() companion object { fun from(number: Int): Integer = if (number == 0) { Zero } else { NonZero(number) } } } Integer.kt
139. sealed class Integer { object Zero : Integer() data class NonZero(val number: Int) : Integer() companion object { fun from(number: Int): Integer = if (number == 0) { Zero } else { NonZero(number) } } } Integer.kt
140. sealed class Integer { object Zero : Integer() data class NonZero(val number: Int) : Integer() companion object { fun from(number: Int): Integer = if (number == 0) { Zero } else { NonZero(number) } } } Integer.kt
141. val divisor = Integer.from(4) if (divisor is NonZero) { val quotient = divide(9, divisor) // Do fancy things! } else { // Fail fast… } Usage
142. val divisor = Integer.from(4) if (divisor is NonZero) { val quotient = divide(9, divisor) // Do fancy things! } else { // Fail fast… } Usage
143. val divisor = Integer.from(4) if (divisor is NonZero) { val quotient = divide(9, divisor) // Do fancy things! } else { // Fail fast… } Usage
144. val divisor = Integer.from(4) if (divisor is NonZero) { val quotient = divide(9, divisor) // Do fancy things! } else { // Fail fast… } Usage
145. val divisor = Integer.from(4) if (divisor is NonZero) { val quotient = divide(9, divisor) // Do fancy things! } else { // Fail fast… } Usage
146. val divisor = Integer.from(4) if (divisor is NonZero) { val quotient = divide(9, divisor) // Do fancy things! } else { // Fail fast… } Usage
147. val divisor = Integer.from(4) if (divisor is NonZero) { val quotient = divide(9, divisor) // Do fancy things! } else { // Fail fast… } Usage
148. val divisor = Integer.from(4) if (divisor is NonZero) { val quotient = divide(9, divisor) // Do fancy things! } else { // Fail fast… } Usage
149. val divisor = Integer.from(4) if (divisor is NonZero) { val quotient = divide(9, divisor) // Do fancy things! } else { // Fail fast… } Usage
150. // Liberal inputs, conservative output (original) fun divide(dividend: Int, divisor: Int): Quotient // Conservative inputs, conservative output (tweaked) fun divide(dividend: Int, divisor: Integer.NonZero): Int Original vs. Tweaked
151. // Liberal inputs, conservative output (original) fun divide(dividend: Int, divisor: Int): Quotient // Conservative inputs, conservative output (tweaked) fun divide(dividend: Int, divisor: Integer.NonZero): Int Original vs. Tweaked
152. // Liberal inputs, conservative output (original) fun divide(dividend: Int, divisor: Int): Quotient // Conservative inputs, conservative output (tweaked) fun divide(dividend: Int, divisor: Integer.NonZero): Int Original vs. Tweaked
153. // Liberal inputs, conservative output (original) fun divide(dividend: Int, divisor: Int): Quotient // Conservative inputs, conservative output (tweaked) fun divide(dividend: Int, divisor: Integer.NonZero): Int Original vs. Tweaked
154. // Liberal inputs, conservative output (original) fun divide(dividend: Int, divisor: Int): Quotient // Conservative inputs, conservative output (tweaked) fun divide(dividend: Int, divisor: Integer.NonZero): Int Original vs. Tweaked
155. // Liberal inputs, conservative output (original) fun divide(dividend: Int, divisor: Int): Quotient // Conservative inputs, conservative output (tweaked) fun divide(dividend: Int, divisor: Integer.NonZero): Int Original vs. Tweaked
156. // Liberal inputs, conservative output (original) fun divide(dividend: Int, divisor: Int): Quotient // Conservative inputs, conservative output (tweaked) fun divide(dividend: Int, divisor: Integer.NonZero): Int Original vs. Tweaked
157. // Liberal inputs, conservative output (original) fun divide(dividend: Int, divisor: Int): Quotient // Conservative inputs, conservative output (tweaked) fun divide(dividend: Int, divisor: Integer.NonZero): Int Original vs. Tweaked
158. // Liberal inputs, conservative output (original) fun divide(dividend: Int, divisor: Int): Quotient // Conservative inputs, conservative output (tweaked) fun divide(dividend: Int, divisor: Integer.NonZero): Int Original vs. Tweaked
159. // Liberal inputs, conservative output (original) fun divide(dividend: Int, divisor: Int): Quotient // Conservative inputs, conservative output (tweaked) fun divide(dividend: Int, divisor: Integer.NonZero): Int Original vs. Tweaked
160. // Liberal inputs, conservative output (original) fun divide(dividend: Int, divisor: Int): Quotient // Conservative inputs, conservative output (tweaked) fun divide(dividend: Int, divisor: Integer.NonZero): Int Original vs. Tweaked
161. // Liberal inputs, conservative output (original) fun divide(dividend: Int, divisor: Int): Quotient // Conservative inputs, conservative output (tweaked) fun divide(dividend: Int, divisor: Integer.NonZero): Int Original vs. Tweaked
162. // Liberal inputs, conservative output (original) fun divide(dividend: Int, divisor: Int): Quotient // Conservative inputs, conservative output (tweaked) fun divide(dividend: Int, divisor: Integer.NonZero): Int Original vs. Tweaked
163. What have we learnt? • Robustness principle (tweaked)
164. Where to apply? • All layers and components that are within your system boundary
165. Example 3 File / Defensive Programming
166. File
167. fun countFiles(directory: File, extension: String): Int { if (!directory.exists() || !directory.isDirectory) { return 0 } return directory .listFiles() ?. filter { it.name.endsWith(".$extension") } ?. count() ?: 0 } FileStats.kt
168. fun countFiles(directory: File, extension: String): Int { if (!directory.exists() || !directory.isDirectory) { return 0 } return directory .listFiles() ?. filter { it.name.endsWith(".$extension") } ?. count() ?: 0 } FileStats.kt
169. fun countFiles(directory: File, extension: String): Int { if (!directory.exists() || !directory.isDirectory) { return 0 } return directory .listFiles() ?. filter { it.name.endsWith(".$extension") } ?. count() ?: 0 } FileStats.kt
170. fun countFiles(directory: File, extension: String): Int { if (!directory.exists() || !directory.isDirectory) { return 0 } return directory .listFiles() ?. filter { it.name.endsWith(".$extension") } ?. count() ?: 0 } FileStats.kt
171. fun countFiles(directory: File, extension: String): Int { if (!directory.exists() || !directory.isDirectory) { return 0 } return directory .listFiles() ?. filter { it.name.endsWith(".$extension") } ?. count() ?: 0 } FileStats.kt
172. fun countFiles(directory: File, extension: String): Int { if (!directory.exists() || !directory.isDirectory) { return 0 } return directory .listFiles() ?. filter { it.name.endsWith(".$extension") } ?. count() ?: 0 } FileStats.kt
173. fun countFiles(directory: File, extension: String): Int { if (!directory.exists() || !directory.isDirectory) { return 0 } return directory .listFiles() ?. filter { it.name.endsWith(".$extension") } ?. count() ?: 0 } FileStats.kt
174. fun countFiles(directory: File, extension: String): Int { if (!directory.exists() || !directory.isDirectory) { return 0 } return directory .listFiles() ?. filter { it.name.endsWith(".$extension") } ?. count() ?: 0 } FileStats.kt
175. fun countFiles(directory: File, extension: String): Int { if (!directory.exists() || !directory.isDirectory) { return 0 } return directory .listFiles() ?. filter { it.name.endsWith(".$extension") } ?. count() ?: 0 } FileStats.kt
176. fun countFiles(directory: File, extension: String): Int { if (!directory.exists() || !directory.isDirectory) { return 0 } return directory .listFiles() ?. filter { it.name.endsWith(".$extension") } ?. count() ?: 0 } FileStats.kt
177. fun countFiles(directory: File, extension: String): Int { if (!directory.exists() || !directory.isDirectory) { return 0 } return directory .listFiles() ?. filter { it.name.endsWith(".$extension") } ?. count() ?: 0 } FileStats.kt
178. Be conservative in what you send, be liberal in what you accept conservative
179. fun countFiles(directory: Directory.Valid, extension: String): Int { return directory .location .listFiles() ?. filter { it.name.endsWith(".$extension") } ?. count() ?: 0 } FileStats.kt
180. fun countFiles(directory: Directory.Valid, extension: String): Int { return directory .location .listFiles() ?. filter { it.name.endsWith(".$extension") } ?. count() ?: 0 } FileStats.kt
181. fun countFiles(directory: Directory.Valid, extension: String): Int { return directory .location .listFiles() ?. filter { it.name.endsWith(".$extension") } ?. count() ?: 0 } FileStats.kt
182. fun countFiles(directory: Directory.Valid, extension: String): Int { return directory .location .listFiles() ?. filter { it.name.endsWith(".$extension") } ?. count() ?: 0 } FileStats.kt
183. fun countFiles(directory: Directory.Valid, extension: String): Int { return directory .location .listFiles() ?. filter { it.name.endsWith(".$extension") } ?. count() ?: 0 } FileStats.kt
184. fun countFiles(directory: Directory.Valid, extension: String): Int { return directory .location .listFiles() ?. filter { it.name.endsWith(".$extension") } ?. count() ?: 0 } FileStats.kt
185. fun countFiles(directory: Directory.Valid, extension: String): Int { return directory .location .listFiles() ?. filter { it.name.endsWith(".$extension") } ?. count() ?: 0 } FileStats.kt
186. fun countFiles(directory: Directory.Valid, extension: String): Int { return directory .location .listFiles() ?. filter { it.name.endsWith(".$extension") } ?. count() ?: 0 } FileStats.kt
187. fun countFiles(directory: Directory.Valid, extension: String): Int { return directory .location .listFiles() ?. filter { it.name.endsWith(".$extension") } ?. count() ?: 0 } FileStats.kt
188. fun countFiles(directory: Directory.Valid, extension: String): Int { return directory .location .listFiles() ?. filter { it.name.endsWith(".$extension") } ?. count() ?: 0 } FileStats.kt
189. sealed class Directory { data class Valid(val location: File) : Directory() data class Invalid(val rawPath: String) : Directory() companion object { fun from(rawPath: String): Directory { val possibleDirectory = File(rawPath) val isDirectory = possibleDirectory.exists() & & possibleDirectory.isDirectory return if (isDirectory) { Valid(File(rawPath)) } else { Invalid(rawPath) } } } } Directory.kt
190. sealed class Directory { data class Valid(val location: File) : Directory() data class Invalid(val rawPath: String) : Directory() companion object { fun from(rawPath: String): Directory { val possibleDirectory = File(rawPath) val isDirectory = possibleDirectory.exists() & & possibleDirectory.isDirectory return if (isDirectory) { Valid(File(rawPath)) } else { Invalid(rawPath) } } } } Directory.kt
191. sealed class Directory { data class Valid(val location: File) : Directory() data class Invalid(val rawPath: String) : Directory() companion object { fun from(rawPath: String): Directory { val possibleDirectory = File(rawPath) val isDirectory = possibleDirectory.exists() & & possibleDirectory.isDirectory return if (isDirectory) { Valid(File(rawPath)) } else { Invalid(rawPath) } } } } Directory.kt
192. sealed class Directory { data class Valid(val location: File) : Directory() data class Invalid(val rawPath: String) : Directory() companion object { fun from(rawPath: String): Directory { val possibleDirectory = File(rawPath) val isDirectory = possibleDirectory.exists() & & possibleDirectory.isDirectory return if (isDirectory) { Valid(File(rawPath)) } else { Invalid(rawPath) } } } } Directory.kt
193. sealed class Directory { data class Valid(val location: File) : Directory() data class Invalid(val rawPath: String) : Directory() companion object { fun from(rawPath: String): Directory { val possibleDirectory = File(rawPath) val isDirectory = possibleDirectory.exists() & & possibleDirectory.isDirectory return if (isDirectory) { Valid(File(rawPath)) } else { Invalid(rawPath) } } } } Directory.kt
194. sealed class Directory { data class Valid(val location: File) : Directory() data class Invalid(val rawPath: String) : Directory() companion object { fun from(rawPath: String): Directory { val possibleDirectory = File(rawPath) val isDirectory = possibleDirectory.exists() & & possibleDirectory.isDirectory return if (isDirectory) { Valid(File(rawPath)) } else { Invalid(rawPath) } } } } Directory.kt
195. sealed class Directory { data class Valid(val location: File) : Directory() data class Invalid(val rawPath: String) : Directory() companion object { fun from(rawPath: String): Directory { val possibleDirectory = File(rawPath) val isDirectory = possibleDirectory.exists() & & possibleDirectory.isDirectory return if (isDirectory) { Valid(File(rawPath)) } else { Invalid(rawPath) } } } } Directory.kt
196. sealed class Directory { data class Valid(val location: File) : Directory() data class Invalid(val rawPath: String) : Directory() companion object { fun from(rawPath: String): Directory { val possibleDirectory = File(rawPath) val isDirectory = possibleDirectory.exists() & & possibleDirectory.isDirectory return if (isDirectory) { Valid(File(rawPath)) } else { Invalid(rawPath) } } } } Directory.kt
197. sealed class Directory { data class Valid(val location: File) : Directory() data class Invalid(val rawPath: String) : Directory() companion object { fun from(rawPath: String): Directory { val possibleDirectory = File(rawPath) val isDirectory = possibleDirectory.exists() & & possibleDirectory.isDirectory return if (isDirectory) { Valid(File(rawPath)) } else { Invalid(rawPath) } } } } Directory.kt
198. sealed class Directory { data class Valid(val location: File) : Directory() data class Invalid(val rawPath: String) : Directory() companion object { fun from(rawPath: String): Directory { val possibleDirectory = File(rawPath) val isDirectory = possibleDirectory.exists() & & possibleDirectory.isDirectory return if (isDirectory) { Valid(File(rawPath)) } else { Invalid(rawPath) } } } } Directory.kt
199. sealed class Directory { data class Valid(val location: File) : Directory() data class Invalid(val rawPath: String) : Directory() companion object { fun from(rawPath: String): Directory { val possibleDirectory = File(rawPath) val isDirectory = possibleDirectory.exists() & & possibleDirectory.isDirectory return if (isDirectory) { Valid(File(rawPath)) } else { Invalid(rawPath) } } } } Directory.kt
200. val directory = Directory.from("~/Documents/Finance") if (directory is Directory.Valid) { val spreadSheetsCount = countFiles(directory, "xlsx") // Do fancy things! } else { // Fail fast… } Usage
201. val directory = Directory.from("~/Documents/Finance") if (directory is Directory.Valid) { val spreadSheetsCount = countFiles(directory, "xlsx") // Do fancy things! } else { // Fail fast… } Usage
202. val directory = Directory.from("~/Documents/Finance") if (directory is Directory.Valid) { val spreadSheetsCount = countFiles(directory, "xlsx") // Do fancy things! } else { // Fail fast… } Usage
203. val directory = Directory.from("~/Documents/Finance") if (directory is Directory.Valid) { val spreadSheetsCount = countFiles(directory, "xlsx") // Do fancy things! } else { // Fail fast… } Usage
204. val directory = Directory.from("~/Documents/Finance") if (directory is Directory.Valid) { val spreadSheetsCount = countFiles(directory, "xlsx") // Do fancy things! } else { // Fail fast… } Usage
205. val directory = Directory.from("~/Documents/Finance") if (directory is Directory.Valid) { val spreadSheetsCount = countFiles(directory, "xlsx") // Do fancy things! } else { // Fail fast… } Usage
206. val directory = Directory.from("~/Documents/Finance") if (directory is Directory.Valid) { val spreadSheetsCount = countFiles(directory, "xlsx") // Do fancy things! } else { // Fail fast… } Usage
207. val directory = Directory.from("~/Documents/Finance") if (directory is Directory.Valid) { val spreadSheetsCount = countFiles(directory, "xlsx") // Do fancy things! } else { // Fail fast… } Usage
208. val directory = Directory.from("~/Documents/Finance") if (directory is Directory.Valid) { val spreadSheetsCount = countFiles(directory, "xlsx") // Do fancy things! } else { // Fail fast… } Usage
209. What have we learnt? • Protecting users of a type from making unnecessary defensive checks
210. Where to apply? • All layers and components that are within your system boundary
211. Example 4 Username / Primitive Obsession
212. / / Failing fast…
213. View Presenter Model Model - View - Presenter
214. View Presenter Model Model - View - Presenter
215. View Presenter Model Model - View - Presenter
216. View Presenter Model Model - View - Presenter
217. View Presenter Model Model - View - Presenter
218. View Presenter Model Model - View - Presenter
219. View Presenter Model Model - View - Presenter
220. View Presenter Model Model - View - Presenter
221. View Presenter Model Model - View - Presenter
222. View Presenter Model Model - View - Presenter
223. View Presenter Model Model - View - Presenter
224. Failure 1 username: String View Presenter Model username username failure failure
225. Failure 1 username: String View Presenter Model username username failure failure
226. Failure 1 username: String View Presenter Model username username failure failure
227. Failure 1 username: String View Presenter Model username username failure failure
228. Failure 1 username: String View Presenter Model username username failure failure
229. Failure 1 username: String View Presenter Model username username failure failure
230. Failure 1 username: String View Presenter Model username username failure failure
231. Data Behavior
232. Data Behavior
233. Data Behavior
234. Data Behavior
235. Data Behavior
236. sealed class Username { data class Valid(val text: String) : Username() data class Invalid(val reasons: Set<Reason>) : Username() companion object { fun from(possiblyUsername: String): Username { possiblyUsername.find { !it.isLetter() } ?: return Valid(possiblyUsername) return Invalid(setOf(INVALID_CHAR)) } } } Username.kt
237. sealed class Username { data class Valid(val text: String) : Username() data class Invalid(val reasons: Set<Reason>) : Username() companion object { fun from(possiblyUsername: String): Username { possiblyUsername.find { !it.isLetter() } ?: return Valid(possiblyUsername) return Invalid(setOf(INVALID_CHAR)) } } } Username.kt
238. sealed class Username { data class Valid(val text: String) : Username() data class Invalid(val reasons: Set<Reason>) : Username() companion object { fun from(possiblyUsername: String): Username { possiblyUsername.find { !it.isLetter() } ?: return Valid(possiblyUsername) return Invalid(setOf(INVALID_CHAR)) } } } Username.kt
239. sealed class Username { data class Valid(val text: String) : Username() data class Invalid(val reasons: Set<Reason>) : Username() companion object { fun from(possiblyUsername: String): Username { possiblyUsername.find { !it.isLetter() } ?: return Valid(possiblyUsername) return Invalid(setOf(INVALID_CHAR)) } } } Username.kt
240. sealed class Username { data class Valid(val text: String) : Username() data class Invalid(val reasons: Set<Reason>) : Username() companion object { fun from(possiblyUsername: String): Username { possiblyUsername.find { !it.isLetter() } ?: return Valid(possiblyUsername) return Invalid(setOf(INVALID_CHAR)) } } } Username.kt
241. sealed class Username { data class Valid(val text: String) : Username() data class Invalid(val reasons: Set<Reason>) : Username() companion object { fun from(possiblyUsername: String): Username { possiblyUsername.find { !it.isLetter() } ?: return Valid(possiblyUsername) return Invalid(setOf(INVALID_CHAR)) } } } Username.kt
242. Failure 2 username: Username.Invalid View Presenter Model username failure
243. Failure 2 username: Username.Invalid View Presenter Model username failure
244. Failure 2 username: Username.Invalid View Presenter Model username failure
245. Failure 2 username: Username.Invalid View Presenter Model username failure
246. View Presenter Model username username failure failure View Presenter Model username failure
247. View Presenter Model username username failure failure View Presenter Model username failure
248. View Presenter Model username username failure failure View Presenter Model username failure
249. Username Algebraic Data Type View Presenter Model username Invalid Valid
250. Username Algebraic Data Type View Presenter Model username Invalid Valid
251. Username Algebraic Data Type View Presenter Model username Invalid Valid
252. Username Algebraic Data Type View Presenter Model username Invalid Valid
253. What have we learnt? • Enforce control fl ow for data between components • Failing fast by reducing data travel between components • Reduce coupling between components
254. Where to apply? • All layers and components that are within your system boundary, unless you have a strong reason not to
255. Impact on Overall System Design • Enhances certainty on micro and macroscopic levels • Errors are treated as fi rst-class citizens • Reduced range of motion for your data from its point of origin • Easy to move towards a functional core and an imperative shell style of architectural pattern • Focus moves towards data over actors • Promotes value boundaries over call boundaries • E ff ective cure against primitive obsession • Reduces defensive programming • Promotes value testing over behaviour testing
256. Summary • Robustness Principle • Total Functions • Algebraic Data Types • Robustness Principle (Tweaked) • Preventing duplication of defensive checks • Shaping program’s control fl ow • Failing fast
257. Questions? @ragunathjawahar • https: / / ragunath.xyz