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
   

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2. Write code that communicates well.
   

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3. Example 1
   
   
   API Call / External Systems
   

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4. interface AccountsApi {
   
   
   fun signup(email: String, password: String): Response?
   
   
   }
   AccountsApi.kt
   

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5. interface AccountsApi {
   
   
   fun signup(email: String, password: String): Response?
   
   
   }
   AccountsApi.kt
   

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6. interface AccountsApi {
   
   
   fun signup(email: String, password: String): Response?
   
   
   }
   AccountsApi.kt
   

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7. interface AccountsApi {
   
   
   fun signup(email: String, password: String): Response?
   
   
   }
   AccountsApi.kt
   

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8. interface AccountsApi {
   
   
   fun signup(email: String, password: String): Response?
   
   
   }
   AccountsApi.kt
   

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9. interface AccountsApi {
   
   
   fun signup(email: String, password: String): Response?
   
   
   }
   AccountsApi.kt
   

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10. interface AccountsApi {
    
    
    fun signup(email: String, password: String): Response?
    
    
    }
    AccountsApi.kt
    

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11. Response?
    • Successfully creates a user account and returns a Response 🎉
    
    • null
    
    • retrofit2.HttpException
    
    
    • java.net.SocketTimeoutException
    
    
    • i.ll.MakeYourLifeMoreInterestingException 🤷
    

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12. Response?
    • Successfully creates a user account and returns a Response 🎉
    
    • null
    
    • retrofit2.HttpException
    
    
    • java.net.SocketTimeoutException
    
    
    • i.ll.MakeYourLifeMoreInterestingException 🤷
    

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13. Response?
    • Successfully creates a user account and returns a Response 🎉
    
    • null
    
    • retrofit2.HttpException
    
    
    • java.net.SocketTimeoutException
    
    
    • i.ll.MakeYourLifeMoreInterestingException 🤷
    

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14. Response?
    • Successfully creates a user account and returns a Response 🎉
    
    • null
    
    • retrofit2.HttpException
    
    
    • java.net.SocketTimeoutException
    
    
    • i.ll.MakeYourLifeMoreInterestingException 🤷
    

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15. Response?
    • Successfully creates a user account and returns a Response 🎉
    
    • null
    
    • retrofit2.HttpException
    
    
    • java.net.SocketTimeoutException
    
    
    • i.ll.MakeYourLifeMoreInterestingException 🤷
    

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16. 😊 😕 😨
    Caller’s emotions
    

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17. 😊 😕 😨
    Caller’s emotions
    

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18. 😊 😕 😨
    Caller’s emotions
    

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19. 😊 😕 😨
    Caller’s emotions
    

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20. Be conservative in what you send,
    be liberal in what you accept
    Robustness Principle
    

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21. Be conservative in what you send,
    be liberal in what you accept
    Robustness Principle
    

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22. Robustness Principle
    Be conservative in what you send,
    be liberal in what you accept
    

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23. interface AccountsApi {
    
    
    fun signup(email: String, password: String): Response?
    
    
    }
    AccountsApi.kt
    

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24. interface AccountsApi {
    
    
    fun signup(email: String, password: String): Response?
    
    
    }
    AccountsApi.kt
    

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25. interface AccountsApi {
    
    
    fun signup(email: String, password: String): Response?
    
    
    }
    AccountsApi.kt
    

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26. interface AccountsApi {
    
    
    fun signup(email: String, password: String): Response?
    
    
    }
    AccountsApi.kt
    

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27. interface AccountsApi {
    
    
    fun signup(email: String, password: String): SignupResponse
    
    
    }
    AccountsApi.kt
    

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28. SignupResponse
    • Successfully creates a user account
    
    • Email already registered
    
    • Validation errors
    
    • Connection errors
    
    • 5xx server errors
    
    • Unknown errors
    

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29. SignupResponse
    • Successfully creates a user account
    
    • Email already registered
    
    • Validation errors
    
    • Connection errors
    
    • 5xx server errors
    
    • Unknown errors
    

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30. SignupResponse
    • Successfully creates a user account
    
    • Email already registered
    
    • Validation errors
    
    • Connection errors
    
    • 5xx server errors
    
    • Unknown errors
    

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31. SignupResponse
    • Successfully creates a user account
    
    • Email already registered
    
    • Validation errors
    
    • Connection errors
    
    • 5xx server errors
    
    • Unknown errors
    

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32. SignupResponse
    • Successfully creates a user account
    
    • Email already registered
    
    • Validation errors
    
    • Connection errors
    
    • 5xx server errors
    
    • Unknown errors
    

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33. SignupResponse
    • Successfully creates a user account
    
    • Email already registered
    
    • Validation errors
    
    • Connection errors
    
    • 5xx server errors
    
    • Unknown errors
    

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34. SignupResponse
    • Successfully creates a user account
    
    • Email already registered
    
    • Validation errors
    
    • Connection errors
    
    • 5xx server errors
    
    • Unknown errors
    

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35. SignupResponse
    • Successfully creates a user account
    
    • Email already registered
    
    • Validation errors
    
    • Connection errors
    
    • 5xx server errors
    
    • Unknown errors
    

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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.
    

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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.
    

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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.
    

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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) : SignupResponse()
    
    
    object ConnectionError : SignupResponse()
    
    
    data class ServerError(val statusCode: Int) : SignupResponse()
    
    
    data class UnknownError(val statusCode: Int, val responseBody: String?) : SignupResponse()
    
    
    }
    

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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) : SignupResponse()
    
    
    object ConnectionError : SignupResponse()
    
    
    data class ServerError(val statusCode: Int) : SignupResponse()
    
    
    data class UnknownError(val statusCode: Int, val responseBody: String?) : SignupResponse()
    
    
    }
    

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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) : SignupResponse()
    
    
    object ConnectionError : SignupResponse()
    
    
    data class ServerError(val statusCode: Int) : SignupResponse()
    
    
    data class UnknownError(val statusCode: Int, val responseBody: String?) : SignupResponse()
    
    
    }
    

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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) : SignupResponse()
    
    
    object ConnectionError : SignupResponse()
    
    
    data class ServerError(val statusCode: Int) : SignupResponse()
    
    
    data class UnknownError(val statusCode: Int, val responseBody: String?) : SignupResponse()
    
    
    }
    

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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) : SignupResponse()
    
    
    object ConnectionError : SignupResponse()
    
    
    data class ServerError(val statusCode: Int) : SignupResponse()
    
    
    data class UnknownError(val statusCode: Int, val responseBody: String?) : SignupResponse()
    
    
    }
    

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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) : SignupResponse()
    
    
    object ConnectionError : SignupResponse()
    
    
    data class ServerError(val statusCode: Int) : SignupResponse()
    
    
    data class UnknownError(val statusCode: Int, val responseBody: String?) : SignupResponse()
    
    
    }
    

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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) : SignupResponse()
    
    
    object ConnectionError : SignupResponse()
    
    
    data class ServerError(val statusCode: Int) : SignupResponse()
    
    
    data class UnknownError(val statusCode: Int, val responseBody: String?) : SignupResponse()
    
    
    }
    

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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) : SignupResponse()
    
    
    object ConnectionError : SignupResponse()
    
    
    data class ServerError(val statusCode: Int) : SignupResponse()
    
    
    data class UnknownError(val statusCode: Int, val responseBody: String?) : SignupResponse()
    
    
    }
    

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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) : SignupResponse()
    
    
    object ConnectionError : SignupResponse()
    
    
    data class ServerError(val statusCode: Int) : SignupResponse()
    
    
    data class UnknownError(val statusCode: Int, val responseBody: String?) : SignupResponse()
    
    
    }
    

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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()
    

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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()
    

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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()
    

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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()
    

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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()
    

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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()
    

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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()
    

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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()
    

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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()
    

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57. Enforcing exhaustive when statements
    
    https://github.com/cashapp/exhaustive (compiler plugin)
    
    https://youtrack.jetbrains.com/issue/KT-12380 (Kotlin language proposal)
    

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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()
    

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59. 😊 😌
    Caller’s emotions
    

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60. 😊 😌
    Caller’s emotions
    

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61. 😊 😌
    Caller’s emotions
    

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62. What have we achieved?
    • Certainty
    
    • Convey business ideas in code
    
    • Improved readability
    
    • Represent errors as data
    

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63. What have we learnt?
    • Total functions
    
    • Algebraic data types
    
    • Robustness principle
    

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64. What have we learnt?
    • Total functions
    
    • Algebraic data types
    
    • Robustness principle
    

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65. Total Functions
    Total functions are functions that give you a valid return value for every
    combination of valid arguments.
    

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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.
    

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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.
    

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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.
    

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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.
    

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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.
    

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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.
    

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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.
    

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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.
    

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74. //
    Partial function
    
    
    fun signup(email: String, password: String): Response?
    
    
    //
    Total function
    
    
    fun signup(email: String, password: String): SignupResponse
    Partial vs. Total Functions
    

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75. //
    Partial function
    
    
    fun signup(email: String, password: String): Response?
    
    
    //
    Total function
    
    
    fun signup(email: String, password: String): SignupResponse
    Partial vs. Total Functions
    

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76. //
    Partial function
    
    
    fun signup(email: String, password: String): Response?
    
    
    //
    Total function
    
    
    fun signup(email: String, password: String): SignupResponse
    Partial vs. Total Functions
    

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77. //
    Partial function
    
    
    fun signup(email: String, password: String): Response?
    
    
    //
    Total function
    
    
    fun signup(email: String, password: String): SignupResponse
    Partial vs. Total Functions
    

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78. //
    Partial function
    
    
    fun signup(email: String, password: String): Response?
    
    
    //
    Total function
    
    
    fun signup(email: String, password: String): SignupResponse
    Partial vs. Total Functions
    

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79. //
    Partial function
    
    
    fun signup(email: String, password: String): Response?
    
    
    //
    Total function
    
    
    fun signup(email: String, password: String): SignupResponse
    Partial vs. Total Functions
    

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80. //
    Partial function
    
    
    fun signup(email: String, password: String): Response?
    
    
    //
    Total function
    
    
    fun signup(email: String, password: String): SignupResponse
    Partial vs. Total Functions
    

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81. What have we learnt?
    • Total functions
    
    • Algebraic data types
    
    • Robustness principle
    

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82. What have we learnt?
    • Total functions
    
    • Algebraic data types
    
    • Robustness principle
    

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83. Algebraic Data Types
    An algebraic data type is a kind of composite type, i.e., a type formed
    by combining other types.
    

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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) : SignupResponse()
    
    
    object ConnectionError : SignupResponse()
    
    
    data class ServerError(val statusCode: Int) : SignupResponse()
    
    
    data class UnknownError(val statusCode: Int, val responseBody: String?) : SignupResponse()
    
    
    }
    

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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) : SignupResponse()
    
    
    object ConnectionError : SignupResponse()
    
    
    data class ServerError(val statusCode: Int) : SignupResponse()
    
    
    data class UnknownError(val statusCode: Int, val responseBody: String?) : SignupResponse()
    
    
    }
    

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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) : SignupResponse()
    
    
    object ConnectionError : SignupResponse()
    
    
    data class ServerError(val statusCode: Int) : SignupResponse()
    
    
    data class UnknownError(val statusCode: Int, val responseBody: String?) : SignupResponse()
    
    
    }
    

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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) : SignupResponse()
    
    
    object ConnectionError : SignupResponse()
    
    
    data class ServerError(val statusCode: Int) : SignupResponse()
    
    
    data class UnknownError(val statusCode: Int, val responseBody: String?) : SignupResponse()
    
    
    }
    

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88. What have we learnt?
    • Total functions
    
    • Algebraic data types
    
    • Robustness principle
    

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89. What have we learnt?
    • Total functions
    
    • Algebraic data types
    
    • Robustness principle
    

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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.)
    

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91. Example 2
    
    
    Division / Exceptions
    

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92. fun divide(dividend: Int, divisor: Int): Int {
    
    
    return dividend / divisor
    
    
    }
    Arithmetic.kt
    

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93. fun divide(dividend: Int, divisor: Int): Int {
    
    
    return dividend / divisor
    
    
    }
    Arithmetic.kt
    

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94. fun divide(dividend: Int, divisor: Int): Int {
    
    
    return dividend / divisor
    
    
    }
    Arithmetic.kt
    

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95. fun divide(dividend: Int, divisor: Int): Int {
    
    
    return dividend / divisor
    
    
    }
    Arithmetic.kt
    

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96. fun divide(dividend: Int, divisor: Int): Int {
    
    
    return dividend / divisor
    
    
    }
    Arithmetic.kt
    

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97. fun divide(dividend: Int, divisor: Int): Int {
    
    
    return dividend / divisor
    
    
    }
    Arithmetic.kt
    

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98. fun divide(dividend: Int, divisor: Int): Int {
    
    
    return dividend / divisor
    
    
    }
    Arithmetic.kt
    

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99. divisor = 0
    

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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)
     

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101. Be conservative in what you send,
     be liberal in what you accept
     

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102. fun divide(dividend: Int, divisor: Int): Quotient {
     
     
     return if (divisor
     ==
     0) {
     
     
     Infinity
     
     
     } else {
     
     
     Result(dividend / divisor)
     
     
     }
     
     
     }
     Arithmetic.kt
     

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103. fun divide(dividend: Int, divisor: Int): Quotient {
     
     
     return if (divisor
     ==
     0) {
     
     
     Infinity
     
     
     } else {
     
     
     Result(dividend / divisor)
     
     
     }
     
     
     }
     Arithmetic.kt
     

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104. fun divide(dividend: Int, divisor: Int): Quotient {
     
     
     return if (divisor
     ==
     0) {
     
     
     Infinity
     
     
     } else {
     
     
     Result(dividend / divisor)
     
     
     }
     
     
     }
     Arithmetic.kt
     

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105. fun divide(dividend: Int, divisor: Int): Quotient {
     
     
     return if (divisor
     ==
     0) {
     
     
     Infinity
     
     
     } else {
     
     
     Result(dividend / divisor)
     
     
     }
     
     
     }
     Arithmetic.kt
     

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106. fun divide(dividend: Int, divisor: Int): Quotient {
     
     
     return if (divisor
     ==
     0) {
     
     
     Infinity
     
     
     } else {
     
     
     Result(dividend / divisor)
     
     
     }
     
     
     }
     Arithmetic.kt
     

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107. fun divide(dividend: Int, divisor: Int): Quotient {
     
     
     return if (divisor
     ==
     0) {
     
     
     Infinity
     
     
     } else {
     
     
     Result(dividend / divisor)
     
     
     }
     
     
     }
     Arithmetic.kt
     

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108. fun divide(dividend: Int, divisor: Int): Quotient {
     
     
     return if (divisor
     ==
     0) {
     
     
     Infinity
     
     
     } else {
     
     
     Result(dividend / divisor)
     
     
     }
     
     
     }
     Arithmetic.kt
     

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109. fun divide(dividend: Int, divisor: Int): Quotient {
     
     
     return if (divisor
     ==
     0) {
     
     
     Infinity
     
     
     } else {
     
     
     Result(dividend / divisor)
     
     
     }
     
     
     }
     Arithmetic.kt
     

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110. fun divide(dividend: Int, divisor: Int): Quotient {
     
     
     return if (divisor
     ==
     0) {
     
     
     Infinity
     
     
     } else {
     
     
     Result(dividend / divisor)
     
     
     }
     
     
     }
     Arithmetic.kt
     

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111. fun divide(dividend: Int, divisor: Int): Quotient {
     
     
     return if (divisor
     ==
     0) {
     
     
     Infinity
     
     
     } else {
     
     
     Result(dividend / divisor)
     
     
     }
     
     
     }
     Arithmetic.kt
     

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112. sealed class Quotient {
     
     
     object Infinity : Quotient()
     
     
     data class Result(val value: Double) : Quotient()
     
     
     }
     Quotient.kt
     

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113. sealed class Quotient {
     
     
     object Infinity : Quotient()
     
     
     data class Result(val value: Double) : Quotient()
     
     
     }
     Quotient.kt
     

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114. sealed class Quotient {
     
     
     object Infinity : Quotient()
     
     
     data class Result(val value: Double) : Quotient()
     
     
     }
     Quotient.kt
     

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115. sealed class Quotient {
     
     
     object Infinity : Quotient()
     
     
     data class Result(val value: Double) : Quotient()
     
     
     }
     Quotient.kt
     

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116. sealed class Quotient {
     
     
     object Infinity : Quotient()
     
     
     data class Result(val value: Double) : Quotient()
     
     
     }
     Quotient.kt
     

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117. 🤔
     

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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
     

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119. Be conservative in what you send,
     be liberal in what you accept
     

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120. Be conservative in what you send,
     be liberal in what you accept
     conservative
     

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121. fun divide(dividend: Int, divisor: Integer.NonZero): Int {
     
     
     return dividend / divisor.number
     
     
     }
     Arithmetic.kt
     

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122. fun divide(dividend: Int, divisor: Integer.NonZero): Int {
     
     
     return dividend / divisor.number
     
     
     }
     Arithmetic.kt
     

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123. fun divide(dividend: Int, divisor: Integer.NonZero): Int {
     
     
     return dividend / divisor.number
     
     
     }
     Arithmetic.kt
     

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124. fun divide(dividend: Int, divisor: Integer.NonZero): Int {
     
     
     return dividend / divisor.number
     
     
     }
     Arithmetic.kt
     

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125. fun divide(dividend: Int, divisor: Integer.NonZero): Int {
     
     
     return dividend / divisor.number
     
     
     }
     Arithmetic.kt
     

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126. fun divide(dividend: Int, divisor: Integer.NonZero): Int {
     
     
     return dividend / divisor.number
     
     
     }
     Arithmetic.kt
     

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127. fun divide(dividend: Int, divisor: Integer.NonZero): Int {
     
     
     return dividend / divisor.number
     
     
     }
     Arithmetic.kt
     

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128. fun divide(dividend: Int, divisor: Integer.NonZero): Int {
     
     
     return dividend / divisor.number
     
     
     }
     Arithmetic.kt
     

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129. fun divide(dividend: Int, divisor: Integer.NonZero): Int {
     
     
     return dividend / divisor.number
     
     
     }
     Arithmetic.kt
     

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130. fun divide(dividend: Int, divisor: Integer.NonZero): Int {
     
     
     return dividend / divisor.number
     
     
     }
     Arithmetic.kt
     

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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
     

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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
     

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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
     

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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
     

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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
     

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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
     

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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
     

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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
     

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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
     

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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
     

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141. val divisor = Integer.from(4)
     
     
     if (divisor is NonZero) {
     
     
     val quotient = divide(9, divisor)
     
     
     //
     Do fancy things!
     
     
     } else {
     
     
     //
     Fail fast…
     
     
     }
     Usage
     

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142. val divisor = Integer.from(4)
     
     
     if (divisor is NonZero) {
     
     
     val quotient = divide(9, divisor)
     
     
     //
     Do fancy things!
     
     
     } else {
     
     
     //
     Fail fast…
     
     
     }
     Usage
     

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143. val divisor = Integer.from(4)
     
     
     if (divisor is NonZero) {
     
     
     val quotient = divide(9, divisor)
     
     
     //
     Do fancy things!
     
     
     } else {
     
     
     //
     Fail fast…
     
     
     }
     Usage
     

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144. val divisor = Integer.from(4)
     
     
     if (divisor is NonZero) {
     
     
     val quotient = divide(9, divisor)
     
     
     //
     Do fancy things!
     
     
     } else {
     
     
     //
     Fail fast…
     
     
     }
     Usage
     

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145. val divisor = Integer.from(4)
     
     
     if (divisor is NonZero) {
     
     
     val quotient = divide(9, divisor)
     
     
     //
     Do fancy things!
     
     
     } else {
     
     
     //
     Fail fast…
     
     
     }
     Usage
     

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146. val divisor = Integer.from(4)
     
     
     if (divisor is NonZero) {
     
     
     val quotient = divide(9, divisor)
     
     
     //
     Do fancy things!
     
     
     } else {
     
     
     //
     Fail fast…
     
     
     }
     Usage
     

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147. val divisor = Integer.from(4)
     
     
     if (divisor is NonZero) {
     
     
     val quotient = divide(9, divisor)
     
     
     //
     Do fancy things!
     
     
     } else {
     
     
     //
     Fail fast…
     
     
     }
     Usage
     

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148. val divisor = Integer.from(4)
     
     
     if (divisor is NonZero) {
     
     
     val quotient = divide(9, divisor)
     
     
     //
     Do fancy things!
     
     
     } else {
     
     
     //
     Fail fast…
     
     
     }
     Usage
     

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149. val divisor = Integer.from(4)
     
     
     if (divisor is NonZero) {
     
     
     val quotient = divide(9, divisor)
     
     
     //
     Do fancy things!
     
     
     } else {
     
     
     //
     Fail fast…
     
     
     }
     Usage
     

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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
     

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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
     

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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
     

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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
     

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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
     

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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
     

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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
     

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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
     

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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
     

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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
     

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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
     

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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
     

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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
     

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163. What have we learnt?
     • Robustness principle (tweaked)
     

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164. Where to apply?
     • All layers and components that are within your system boundary
     

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165. Example 3
     
     
     File / Defensive Programming
     

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166. File
     

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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
     

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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
     

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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
     

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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
     

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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
     

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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
     

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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
     

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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
     

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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
     

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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
     

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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
     

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178. Be conservative in what you send,
     be liberal in what you accept
     conservative
     

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179. fun countFiles(directory: Directory.Valid, extension: String): Int {
     
     
     return directory
     
     
     .location
     
     
     .listFiles()
     
     
     ?.
     filter { it.name.endsWith(".$extension") }
     
     
     ?.
     count()
     ?:
     0
     
     
     }
     FileStats.kt
     

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180. fun countFiles(directory: Directory.Valid, extension: String): Int {
     
     
     return directory
     
     
     .location
     
     
     .listFiles()
     
     
     ?.
     filter { it.name.endsWith(".$extension") }
     
     
     ?.
     count()
     ?:
     0
     
     
     }
     FileStats.kt
     

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181. fun countFiles(directory: Directory.Valid, extension: String): Int {
     
     
     return directory
     
     
     .location
     
     
     .listFiles()
     
     
     ?.
     filter { it.name.endsWith(".$extension") }
     
     
     ?.
     count()
     ?:
     0
     
     
     }
     FileStats.kt
     

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182. fun countFiles(directory: Directory.Valid, extension: String): Int {
     
     
     return directory
     
     
     .location
     
     
     .listFiles()
     
     
     ?.
     filter { it.name.endsWith(".$extension") }
     
     
     ?.
     count()
     ?:
     0
     
     
     }
     FileStats.kt
     

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183. fun countFiles(directory: Directory.Valid, extension: String): Int {
     
     
     return directory
     
     
     .location
     
     
     .listFiles()
     
     
     ?.
     filter { it.name.endsWith(".$extension") }
     
     
     ?.
     count()
     ?:
     0
     
     
     }
     FileStats.kt
     

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184. fun countFiles(directory: Directory.Valid, extension: String): Int {
     
     
     return directory
     
     
     .location
     
     
     .listFiles()
     
     
     ?.
     filter { it.name.endsWith(".$extension") }
     
     
     ?.
     count()
     ?:
     0
     
     
     }
     FileStats.kt
     

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185. fun countFiles(directory: Directory.Valid, extension: String): Int {
     
     
     return directory
     
     
     .location
     
     
     .listFiles()
     
     
     ?.
     filter { it.name.endsWith(".$extension") }
     
     
     ?.
     count()
     ?:
     0
     
     
     }
     FileStats.kt
     

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186. fun countFiles(directory: Directory.Valid, extension: String): Int {
     
     
     return directory
     
     
     .location
     
     
     .listFiles()
     
     
     ?.
     filter { it.name.endsWith(".$extension") }
     
     
     ?.
     count()
     ?:
     0
     
     
     }
     FileStats.kt
     

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187. fun countFiles(directory: Directory.Valid, extension: String): Int {
     
     
     return directory
     
     
     .location
     
     
     .listFiles()
     
     
     ?.
     filter { it.name.endsWith(".$extension") }
     
     
     ?.
     count()
     ?:
     0
     
     
     }
     FileStats.kt
     

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188. fun countFiles(directory: Directory.Valid, extension: String): Int {
     
     
     return directory
     
     
     .location
     
     
     .listFiles()
     
     
     ?.
     filter { it.name.endsWith(".$extension") }
     
     
     ?.
     count()
     ?:
     0
     
     
     }
     FileStats.kt
     

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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
     

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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
     

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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
     

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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
     

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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
     

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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
     

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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
     

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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
     

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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
     

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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
     

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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
     

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200. val directory = Directory.from("~/Documents/Finance")
     
     
     if (directory is Directory.Valid) {
     
     
     val spreadSheetsCount = countFiles(directory, "xlsx")
     
     
     //
     Do fancy things!
     
     
     } else {
     
     
     //
     Fail fast…
     
     
     }
     Usage
     

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201. val directory = Directory.from("~/Documents/Finance")
     
     
     if (directory is Directory.Valid) {
     
     
     val spreadSheetsCount = countFiles(directory, "xlsx")
     
     
     //
     Do fancy things!
     
     
     } else {
     
     
     //
     Fail fast…
     
     
     }
     Usage
     

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202. val directory = Directory.from("~/Documents/Finance")
     
     
     if (directory is Directory.Valid) {
     
     
     val spreadSheetsCount = countFiles(directory, "xlsx")
     
     
     //
     Do fancy things!
     
     
     } else {
     
     
     //
     Fail fast…
     
     
     }
     Usage
     

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203. val directory = Directory.from("~/Documents/Finance")
     
     
     if (directory is Directory.Valid) {
     
     
     val spreadSheetsCount = countFiles(directory, "xlsx")
     
     
     //
     Do fancy things!
     
     
     } else {
     
     
     //
     Fail fast…
     
     
     }
     Usage
     

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204. val directory = Directory.from("~/Documents/Finance")
     
     
     if (directory is Directory.Valid) {
     
     
     val spreadSheetsCount = countFiles(directory, "xlsx")
     
     
     //
     Do fancy things!
     
     
     } else {
     
     
     //
     Fail fast…
     
     
     }
     Usage
     

     View Slide

205. val directory = Directory.from("~/Documents/Finance")
     
     
     if (directory is Directory.Valid) {
     
     
     val spreadSheetsCount = countFiles(directory, "xlsx")
     
     
     //
     Do fancy things!
     
     
     } else {
     
     
     //
     Fail fast…
     
     
     }
     Usage
     

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206. val directory = Directory.from("~/Documents/Finance")
     
     
     if (directory is Directory.Valid) {
     
     
     val spreadSheetsCount = countFiles(directory, "xlsx")
     
     
     //
     Do fancy things!
     
     
     } else {
     
     
     //
     Fail fast…
     
     
     }
     Usage
     

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207. val directory = Directory.from("~/Documents/Finance")
     
     
     if (directory is Directory.Valid) {
     
     
     val spreadSheetsCount = countFiles(directory, "xlsx")
     
     
     //
     Do fancy things!
     
     
     } else {
     
     
     //
     Fail fast…
     
     
     }
     Usage
     

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208. val directory = Directory.from("~/Documents/Finance")
     
     
     if (directory is Directory.Valid) {
     
     
     val spreadSheetsCount = countFiles(directory, "xlsx")
     
     
     //
     Do fancy things!
     
     
     } else {
     
     
     //
     Fail fast…
     
     
     }
     Usage
     

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209. What have we learnt?
     • Protecting users of a type from making unnecessary defensive checks
     

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210. Where to apply?
     • All layers and components that are within your system boundary
     

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211. Example 4
     
     
     Username / Primitive Obsession
     

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212. /
     /
     Failing fast…
     

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213. View Presenter Model
     Model - View - Presenter
     

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214. View Presenter Model
     Model - View - Presenter
     

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215. View Presenter Model
     Model - View - Presenter
     

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216. View Presenter Model
     Model - View - Presenter
     

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217. View Presenter Model
     Model - View - Presenter
     

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218. View Presenter Model
     Model - View - Presenter
     

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219. View Presenter Model
     Model - View - Presenter
     

     View Slide

220. View Presenter Model
     Model - View - Presenter
     

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221. View Presenter Model
     Model - View - Presenter
     

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222. View Presenter Model
     Model - View - Presenter
     

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223. View Presenter Model
     Model - View - Presenter
     

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224. Failure 1 username: String
     View Presenter Model
     username username
     failure
     failure
     

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225. Failure 1 username: String
     View Presenter Model
     username username
     failure
     failure
     

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226. Failure 1 username: String
     View Presenter Model
     username username
     failure
     failure
     

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227. Failure 1 username: String
     View Presenter Model
     username username
     failure
     failure
     

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228. Failure 1 username: String
     View Presenter Model
     username username
     failure
     failure
     

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229. Failure 1 username: String
     View Presenter Model
     username username
     failure
     failure
     

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230. Failure 1 username: String
     View Presenter Model
     username username
     failure
     failure
     

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231. Data Behavior
     

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232. Data Behavior
     

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233. Data Behavior
     

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234. Data Behavior
     

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235. Data Behavior
     

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236. sealed class Username {
     
     
     data class Valid(val text: String) : Username()
     
     
     data class Invalid(val reasons: Set) : Username()
     
     
     companion object {
     
     
     fun from(possiblyUsername: String): Username {
     
     
     possiblyUsername.find { !it.isLetter() }
     ?:
     return Valid(possiblyUsername)
     
     
     return Invalid(setOf(INVALID_CHAR))
     
     
     }
     
     
     }
     
     
     }
     Username.kt
     

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237. sealed class Username {
     
     
     data class Valid(val text: String) : Username()
     
     
     data class Invalid(val reasons: Set) : Username()
     
     
     companion object {
     
     
     fun from(possiblyUsername: String): Username {
     
     
     possiblyUsername.find { !it.isLetter() }
     ?:
     return Valid(possiblyUsername)
     
     
     return Invalid(setOf(INVALID_CHAR))
     
     
     }
     
     
     }
     
     
     }
     Username.kt
     

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238. sealed class Username {
     
     
     data class Valid(val text: String) : Username()
     
     
     data class Invalid(val reasons: Set) : Username()
     
     
     companion object {
     
     
     fun from(possiblyUsername: String): Username {
     
     
     possiblyUsername.find { !it.isLetter() }
     ?:
     return Valid(possiblyUsername)
     
     
     return Invalid(setOf(INVALID_CHAR))
     
     
     }
     
     
     }
     
     
     }
     Username.kt
     

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239. sealed class Username {
     
     
     data class Valid(val text: String) : Username()
     
     
     data class Invalid(val reasons: Set) : Username()
     
     
     companion object {
     
     
     fun from(possiblyUsername: String): Username {
     
     
     possiblyUsername.find { !it.isLetter() }
     ?:
     return Valid(possiblyUsername)
     
     
     return Invalid(setOf(INVALID_CHAR))
     
     
     }
     
     
     }
     
     
     }
     Username.kt
     

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240. sealed class Username {
     
     
     data class Valid(val text: String) : Username()
     
     
     data class Invalid(val reasons: Set) : Username()
     
     
     companion object {
     
     
     fun from(possiblyUsername: String): Username {
     
     
     possiblyUsername.find { !it.isLetter() }
     ?:
     return Valid(possiblyUsername)
     
     
     return Invalid(setOf(INVALID_CHAR))
     
     
     }
     
     
     }
     
     
     }
     Username.kt
     

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241. sealed class Username {
     
     
     data class Valid(val text: String) : Username()
     
     
     data class Invalid(val reasons: Set) : Username()
     
     
     companion object {
     
     
     fun from(possiblyUsername: String): Username {
     
     
     possiblyUsername.find { !it.isLetter() }
     ?:
     return Valid(possiblyUsername)
     
     
     return Invalid(setOf(INVALID_CHAR))
     
     
     }
     
     
     }
     
     
     }
     Username.kt
     

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242. Failure 2 username: Username.Invalid
     View Presenter Model
     username
     failure
     

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243. Failure 2 username: Username.Invalid
     View Presenter Model
     username
     failure
     

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244. Failure 2 username: Username.Invalid
     View Presenter Model
     username
     failure
     

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245. Failure 2 username: Username.Invalid
     View Presenter Model
     username
     failure
     

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246. View Presenter Model
     username username
     failure
     failure
     View Presenter Model
     username
     failure
     

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247. View Presenter Model
     username username
     failure
     failure
     View Presenter Model
     username
     failure
     

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248. View Presenter Model
     username username
     failure
     failure
     View Presenter Model
     username
     failure
     

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249. Username Algebraic Data Type
     View Presenter Model
     username
     Invalid
     Valid
     

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250. Username Algebraic Data Type
     View Presenter Model
     username
     Invalid
     Valid
     

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251. Username Algebraic Data Type
     View Presenter Model
     username
     Invalid
     Valid
     

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252. Username Algebraic Data Type
     View Presenter Model
     username
     Invalid
     Valid
     

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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
     

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254. Where to apply?
     • All layers and components that are within your system boundary, unless you
     have a strong reason not to
     

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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
     

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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
     

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257. Questions?
     
     
     @ragunathjawahar • https:
     /
     /
     ragunath.xyz
     

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