Building Robust Apps (Swift Edition)

Transcript

1. Ragunath Jawahar
   Building Robust Software
   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. protocol AccountsApi {
   
   func signup(with email: String, and password: String) #-> Response?
   
   }
   AccountsApi.swift
   

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5. protocol AccountsApi {
   
   func signup(with email: String, and password: String) #-> Response?
   
   }
   AccountsApi.swift
   

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6. protocol AccountsApi {
   
   func signup(with email: String, and password: String) #-> Response?
   
   }
   AccountsApi.swift
   

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7. protocol AccountsApi {
   
   func signup(with email: String, and password: String) #-> Response?
   
   }
   AccountsApi.swift
   

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8. protocol AccountsApi {
   
   func signup(with email: String, and password: String) #-> Response?
   
   }
   AccountsApi.swift
   

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9. protocol AccountsApi {
   
   func signup(with email: String, and password: String) #-> Response?
   
   }
   AccountsApi.swift
   

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10. protocol AccountsApi {
    
    func signup(with email: String, and password: String) #-> Response?
    
    }
    AccountsApi.swift
    

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11. protocol AccountsApi {
    
    func signup(with email: String, and password: String) #-> Response?
    
    }
    AccountsApi.swift
    

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12. Response?
    • Successfully creates a user account and returns a Response 🎉
    
    • nil
    
    • NSURLErrorCannotConnectToHost
    
    • NSURLErrorCannotFindHost
    
    • NSURLError* "
    

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13. Response?
    • Successfully creates a user account and returns a Response 🎉
    
    • nil
    
    • NSURLErrorCannotConnectToHost
    
    • NSURLErrorCannotFindHost
    
    • NSURLError* "
    

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14. Response?
    • Successfully creates a user account and returns a Response 🎉
    
    • nil
    
    • NSURLErrorCannotConnectToHost
    
    • NSURLErrorCannotFindHost
    
    • NSURLError* "
    

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15. Response?
    • Successfully creates a user account and returns a Response 🎉
    
    • nil
    
    • NSURLErrorCannotConnectToHost
    
    • NSURLErrorCannotFindHost
    
    • NSURLError* "
    

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16. Response?
    • Successfully creates a user account and returns a Response 🎉
    
    • nil
    
    • NSURLErrorCannotConnectToHost
    
    • NSURLErrorCannotFindHost
    
    • NSURLError* "
    

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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. 😊 😕 😨
    Caller’s emotions
    

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21. Be conservative in what you send,
    be liberal in what you accept
    Robustness Principle / Postel’s Law
    

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22. Be conservative in what you send,
    be liberal in what you accept
    Robustness Principle / Postel’s Law
    

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23. Robustness Principle / Postel’s Law
    Be conservative in what you send,
    be liberal in what you accept
    

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24. protocol AccountsApi {
    
    func signup(with email: String, and password: String) #-> Response?
    
    }
    AccountsApi.swift
    

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25. protocol AccountsApi {
    
    func signup(with email: String, and password: String) #-> Response?
    
    }
    AccountsApi.swift
    

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26. protocol AccountsApi {
    
    func signup(with email: String, and password: String) #-> Response
    
    }
    AccountsApi.swift
    

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

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

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

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

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

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

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

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

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35. Enumerations
    An enumeration defines a common type for a group of related values
    and enables you to work with those values in a type-safe way within
    your code.
    
    Alternatively, enumeration cases can specify associated values of any
    type to be stored along with each different case value, much as unions
    or variants do in other languages.
    

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36. Enumerations
    An enumeration defines a common type for a group of related values
    and enables you to work with those values in a type-safe way within
    your code.
    
    Alternatively, enumeration cases can specify associated values of any
    type to be stored along with each different case value, much as unions
    or variants do in other languages.
    

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37. Enumerations
    An enumeration defines a common type for a group of related values
    and enables you to work with those values in a type-safe way within
    your code.
    
    Alternatively, enumeration cases can specify associated values of any
    type to be stored along with each different case value, much as unions
    or variants do in other languages.
    

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38. Enumerations
    An enumeration defines a common type for a group of related values
    and enables you to work with those values in a type-safe way within
    your code.
    
    Alternatively, enumeration cases can specify associated values of any
    type to be stored along with each different case value, much as unions
    or variants do in other languages.
    

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39. Response.swift
    enum Response {
    
    case accountCreated(String)
    
    case emailAlreadyRegistered(String)
    
    case InputValidationFailed([ValidationError])
    
    case connectionError
    
    case serverError(Int)
    
    case unknownError(Int, String?)
    
    }
    

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40. Response.swift
    enum Response {
    
    case accountCreated(String)
    
    case emailAlreadyRegistered(String)
    
    case InputValidationFailed([ValidationError])
    
    case connectionError
    
    case serverError(Int)
    
    case unknownError(Int, String?)
    
    }
    

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41. Response.swift
    enum Response {
    
    case accountCreated(String)
    
    case emailAlreadyRegistered(String)
    
    case InputValidationFailed([ValidationError])
    
    case connectionError
    
    case serverError(Int)
    
    case unknownError(Int, String?)
    
    }
    

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42. Response.swift
    enum Response {
    
    case accountCreated(String)
    
    case emailAlreadyRegistered(String)
    
    case InputValidationFailed([ValidationError])
    
    case connectionError
    
    case serverError(Int)
    
    case unknownError(Int, String?)
    
    }
    

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43. Response.swift
    enum Response {
    
    case accountCreated(String)
    
    case emailAlreadyRegistered(String)
    
    case InputValidationFailed([ValidationError])
    
    case connectionError
    
    case serverError(Int)
    
    case unknownError(Int, String?)
    
    }
    

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44. Response.swift
    enum Response {
    
    case accountCreated(String)
    
    case emailAlreadyRegistered(String)
    
    case InputValidationFailed([ValidationError])
    
    case connectionError
    
    case serverError(Int)
    
    case unknownError(Int, String?)
    
    }
    

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45. Response.swift
    enum Response {
    
    case accountCreated(String)
    
    case emailAlreadyRegistered(String)
    
    case InputValidationFailed([ValidationError])
    
    case connectionError
    
    case serverError(Int)
    
    case unknownError(Int, String?)
    
    }
    

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46. Response.swift
    enum Response {
    
    case accountCreated(String)
    
    case emailAlreadyRegistered(String)
    
    case InputValidationFailed([ValidationError])
    
    case connectionError
    
    case serverError(Int)
    
    case unknownError(Int, String?)
    
    }
    

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47. Usage - Enum Exhaustiveness
    val response = accountsApi.signup(with: "John Doe", and: "john@example.org")
    
    switch response {
    
    case .accountCreated(let userId):
    
    saveUserId(userId)
    
    case .emailAlreadyRegistered(let registrationEmail):
    
    showEmailAlreadyRegistered(registrationEmail)
    
    case .inputValidationFailed(let errors):
    
    showValidationErrors(errors)
    
    case .connectionError:
    
    showCheckConnectionMessage()
    
    case .serverError(_),
    
    .unknownError(_, _):
    
    showTryAgainInSometimeMessage()
    
    }
    

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48. Usage - Enum Exhaustiveness
    val response = accountsApi.signup(with: "John Doe", and: "john@example.org")
    
    switch response {
    
    case .accountCreated(let userId):
    
    saveUserId(userId)
    
    case .emailAlreadyRegistered(let registrationEmail):
    
    showEmailAlreadyRegistered(registrationEmail)
    
    case .inputValidationFailed(let errors):
    
    showValidationErrors(errors)
    
    case .connectionError:
    
    showCheckConnectionMessage()
    
    case .serverError(_),
    
    .unknownError(_, _):
    
    showTryAgainInSometimeMessage()
    
    }
    

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49. Usage - Enum Exhaustiveness
    val response = accountsApi.signup(with: "John Doe", and: "john@example.org")
    
    switch response {
    
    case .accountCreated(let userId):
    
    saveUserId(userId)
    
    case .emailAlreadyRegistered(let registrationEmail):
    
    showEmailAlreadyRegistered(registrationEmail)
    
    case .inputValidationFailed(let errors):
    
    showValidationErrors(errors)
    
    case .connectionError:
    
    showCheckConnectionMessage()
    
    case .serverError(_),
    
    .unknownError(_, _):
    
    showTryAgainInSometimeMessage()
    
    }
    

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50. Usage - Enum Exhaustiveness
    val response = accountsApi.signup(with: "John Doe", and: "john@example.org")
    
    switch response {
    
    case .accountCreated(let userId):
    
    saveUserId(userId)
    
    case .emailAlreadyRegistered(let registrationEmail):
    
    showEmailAlreadyRegistered(registrationEmail)
    
    case .inputValidationFailed(let errors):
    
    showValidationErrors(errors)
    
    case .connectionError:
    
    showCheckConnectionMessage()
    
    case .serverError(_),
    
    .unknownError(_, _):
    
    showTryAgainInSometimeMessage()
    
    }
    

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51. Usage - Enum Exhaustiveness
    val response = accountsApi.signup(with: "John Doe", and: "john@example.org")
    
    switch response {
    
    case .accountCreated(let userId):
    
    saveUserId(userId)
    
    case .emailAlreadyRegistered(let registrationEmail):
    
    showEmailAlreadyRegistered(registrationEmail)
    
    case .inputValidationFailed(let errors):
    
    showValidationErrors(errors)
    
    case .connectionError:
    
    showCheckConnectionMessage()
    
    case .serverError(_),
    
    .unknownError(_, _):
    
    showTryAgainInSometimeMessage()
    
    }
    

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52. Usage - Enum Exhaustiveness
    val response = accountsApi.signup(with: "John Doe", and: "john@example.org")
    
    switch response {
    
    case .accountCreated(let userId):
    
    saveUserId(userId)
    
    case .emailAlreadyRegistered(let registrationEmail):
    
    showEmailAlreadyRegistered(registrationEmail)
    
    case .inputValidationFailed(let errors):
    
    showValidationErrors(errors)
    
    case .connectionError:
    
    showCheckConnectionMessage()
    
    case .serverError(_),
    
    .unknownError(_, _):
    
    showTryAgainInSometimeMessage()
    
    }
    

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53. Usage - Enum Exhaustiveness
    val response = accountsApi.signup(with: "John Doe", and: "john@example.org")
    
    switch response {
    
    case .accountCreated(let userId):
    
    saveUserId(userId)
    
    case .emailAlreadyRegistered(let registrationEmail):
    
    showEmailAlreadyRegistered(registrationEmail)
    
    case .inputValidationFailed(let errors):
    
    showValidationErrors(errors)
    
    case .connectionError:
    
    showCheckConnectionMessage()
    
    case .serverError(_),
    
    .unknownError(_, _):
    
    showTryAgainInSometimeMessage()
    
    }
    

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54. Usage - Enum Exhaustiveness
    val response = accountsApi.signup(with: "John Doe", and: "john@example.org")
    
    switch response {
    
    case .accountCreated(let userId):
    
    saveUserId(userId)
    
    case .emailAlreadyRegistered(let registrationEmail):
    
    showEmailAlreadyRegistered(registrationEmail)
    
    case .inputValidationFailed(let errors):
    
    showValidationErrors(errors)
    
    case .connectionError:
    
    showCheckConnectionMessage()
    
    case .serverError(_),
    
    .unknownError(_, _):
    
    showTryAgainInSometimeMessage()
    
    }
    

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55. Usage - Enum Exhaustiveness
    val response = accountsApi.signup(with: "John Doe", and: "john@example.org")
    
    switch response {
    
    case .accountCreated(let userId):
    
    saveUserId(userId)
    
    case .emailAlreadyRegistered(let registrationEmail):
    
    showEmailAlreadyRegistered(registrationEmail)
    
    case .inputValidationFailed(let errors):
    
    showValidationErrors(errors)
    
    case .connectionError:
    
    showCheckConnectionMessage()
    
    case .serverError(_),
    
    .unknownError(_, _):
    
    showTryAgainInSometimeMessage()
    
    }
    

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56. 😊 😌
    Caller’s emotions
    

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57. 😊 😌
    Caller’s emotions
    

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58. 😊 😌
    Caller’s emotions
    

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

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

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

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

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63. 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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64. 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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65. 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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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 function
    
    func signup(with email: String, and password: String) #-> Response?
    
    !// Total function
    
    func signup(with email: String, and password: String) #-> Response
    Partial vs. Total Functions
    

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72. !// Partial function
    
    func signup(with email: String, and password: String) #-> Response?
    
    !// Total function
    
    func signup(with email: String, and password: String) #-> Response
    Partial vs. Total Functions
    

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73. !// Partial function
    
    func signup(with email: String, and password: String) #-> Response?
    
    !// Total function
    
    func signup(with email: String, and password: String) #-> Response
    Partial vs. Total Functions
    

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74. !// Partial function
    
    func signup(with email: String, and password: String) #-> Response?
    
    !// Total function
    
    func signup(with email: String, and password: String) #-> Response
    Partial vs. Total Functions
    

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75. !// Partial function
    
    func signup(with email: String, and password: String) #-> Response?
    
    !// Total function
    
    func signup(with email: String, and password: String) #-> Response
    Partial vs. Total Functions
    

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76. !// Partial function
    
    func signup(with email: String, and password: String) #-> Response?
    
    !// Total function
    
    func signup(with email: String, and password: String) #-> Response
    Partial vs. Total Functions
    

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77. !// Partial function
    
    func signup(with email: String, and password: String) #-> Response?
    
    !// Total function
    
    func signup(with email: String, and password: String) #-> Response
    Partial vs. Total Functions
    

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

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

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80. 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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81. Response.swift
    enum Response {
    
    case accountCreated(String)
    
    case emailAlreadyRegistered(String)
    
    case InputValidationFailed([ValidationError])
    
    case connectionError
    
    case serverError(Int)
    
    case unknownError(Int, String?)
    
    }
    

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82. Response.swift
    enum Response {
    
    case accountCreated(String)
    
    case emailAlreadyRegistered(String)
    
    case InputValidationFailed([ValidationError])
    
    case connectionError
    
    case serverError(Int)
    
    case unknownError(Int, String?)
    
    }
    

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83. Response.swift
    enum Response {
    
    case accountCreated(String)
    
    case emailAlreadyRegistered(String)
    
    case InputValidationFailed([ValidationError])
    
    case connectionError
    
    case serverError(Int)
    
    case unknownError(Int, String?)
    
    }
    

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84. Response.swift
    enum Response {
    
    case accountCreated(String)
    
    case emailAlreadyRegistered(String)
    
    case InputValidationFailed([ValidationError])
    
    case connectionError
    
    case serverError(Int)
    
    case unknownError(Int, String?)
    
    }
    

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

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

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87. 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 into account (databases and file systems may not be
    good candidates. Network layer is a good starting point.)
    

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88. Example 2
    Division / Exceptions
    

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89. func divide(dividend: Int, divisor: Int) #-> Int {
    
    return dividend / divisor
    
    }
    Arithmetic.swift
    

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90. func divide(dividend: Int, divisor: Int) #-> Int {
    
    return dividend / divisor
    
    }
    Arithmetic.swift
    

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91. func divide(dividend: Int, divisor: Int) #-> Int {
    
    return dividend / divisor
    
    }
    Arithmetic.swift
    

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92. func divide(dividend: Int, divisor: Int) #-> Int {
    
    return dividend / divisor
    
    }
    Arithmetic.swift
    

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93. func divide(dividend: Int, divisor: Int) #-> Int {
    
    return dividend / divisor
    
    }
    Arithmetic.swift
    

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94. func divide(dividend: Int, divisor: Int) #-> Int {
    
    return dividend / divisor
    
    }
    Arithmetic.swift
    

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95. func divide(dividend: Int, divisor: Int) #-> Int {
    
    return dividend / divisor
    
    }
    Arithmetic.swift
    

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96. divisor = 0
    

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97. ERROR : divide by zero
    
    error: Execution was interrupted, reason: EXC_BAD_INSTRUCTION
    (code=EXC_I386_INVOP, subcode=0x0).
    
    Fatal error: Division by zero: file Swift/x86_64-apple-ios-
    simulator.swiftinterface, line 32420
    

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

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99. func divide(dividend: Int, divisor: Int) #-> Quotient {
    
    return divisor #== 0
    
    ? .infinity
    
    : .result(dividend / divisor)
    
    }
    Arithmetic.swift
    

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100. func divide(dividend: Int, divisor: Int) #-> Quotient {
     
     return divisor #== 0
     
     ? .infinity
     
     : .result(dividend / divisor)
     
     }
     Arithmetic.swift
     

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101. func divide(dividend: Int, divisor: Int) #-> Quotient {
     
     return divisor #== 0
     
     ? .infinity
     
     : .result(dividend / divisor)
     
     }
     Arithmetic.swift
     

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102. func divide(dividend: Int, divisor: Int) #-> Quotient {
     
     return divisor #== 0
     
     ? .infinity
     
     : .result(dividend / divisor)
     
     }
     Arithmetic.swift
     

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103. func divide(dividend: Int, divisor: Int) #-> Quotient {
     
     return divisor #== 0
     
     ? .infinity
     
     : .result(dividend / divisor)
     
     }
     Arithmetic.swift
     

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104. func divide(dividend: Int, divisor: Int) #-> Quotient {
     
     return divisor #== 0
     
     ? .infinity
     
     : .result(dividend / divisor)
     
     }
     Arithmetic.swift
     

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105. func divide(dividend: Int, divisor: Int) #-> Quotient {
     
     return divisor #== 0
     
     ? .infinity
     
     : .result(dividend / divisor)
     
     }
     Arithmetic.swift
     

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106. func divide(dividend: Int, divisor: Int) #-> Quotient {
     
     return divisor #== 0
     
     ? .infinity
     
     : .result(dividend / divisor)
     
     }
     Arithmetic.swift
     

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107. func divide(dividend: Int, divisor: Int) #-> Quotient {
     
     return divisor #== 0
     
     ? .infinity
     
     : .result(dividend / divisor)
     
     }
     Arithmetic.swift
     

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108. func divide(dividend: Int, divisor: Int) #-> Quotient {
     
     return divisor #== 0
     
     ? .infinity
     
     : .result(dividend / divisor)
     
     }
     Arithmetic.swift
     

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109. func divide(dividend: Int, divisor: Int) #-> Quotient {
     
     return divisor #== 0
     
     ? .infinity
     
     : .result(dividend / divisor)
     
     }
     Arithmetic.swift
     

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110. enum Quotient {
     
     case .infinity
     
     case .result(Int)
     
     }
     Quotient.swift
     

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111. enum Quotient {
     
     case .infinity
     
     case .result(Int)
     
     }
     Quotient.swift
     

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112. enum Quotient {
     
     case .infinity
     
     case .result(Int)
     
     }
     Quotient.swift
     

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113. enum Quotient {
     
     case .infinity
     
     case .result(Int)
     
     }
     Quotient.swift
     

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114. enum Quotient {
     
     case .infinity
     
     case .result(Int)
     
     }
     Quotient.swift
     

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115. 🤔
     

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116. 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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117. Be conservative in what you send,
     be liberal in what you accept
     

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

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119. func divide(dividend: Int, divisor: NaturalNumber.NonZero) #-> Int {
     
     return dividend / divisor.value
     
     }
     Arithmetic.swift
     

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120. func divide(dividend: Int, divisor: NaturalNumber.NonZero) #-> Int {
     
     return dividend / divisor.value
     
     }
     Arithmetic.swift
     

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121. func divide(dividend: Int, divisor: NaturalNumber.NonZero) #-> Int {
     
     return dividend / divisor.value
     
     }
     Arithmetic.swift
     

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122. func divide(dividend: Int, divisor: NaturalNumber.NonZero) #-> Int {
     
     return dividend / divisor.value
     
     }
     Arithmetic.swift
     

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123. func divide(dividend: Int, divisor: NaturalNumber.NonZero) #-> Int {
     
     return dividend / divisor.value
     
     }
     Arithmetic.swift
     

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124. func divide(dividend: Int, divisor: NaturalNumber.NonZero) #-> Int {
     
     return dividend / divisor.value
     
     }
     Arithmetic.swift
     

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125. func divide(dividend: Int, divisor: NaturalNumber.NonZero) #-> Int {
     
     return dividend / divisor.value
     
     }
     Arithmetic.swift
     

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126. func divide(dividend: Int, divisor: NaturalNumber.NonZero) #-> Int {
     
     return dividend / divisor.value
     
     }
     Arithmetic.swift
     

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127. func divide(dividend: Int, divisor: NaturalNumber.NonZero) #-> Int {
     
     return dividend / divisor.value
     
     }
     Arithmetic.swift
     

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128. func divide(dividend: Int, divisor: NaturalNumber.NonZero) #-> Int {
     
     return dividend / divisor.value
     
     }
     Arithmetic.swift
     

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129. class NaturalNumber {
     
     class Zero: NaturalNumber { !/* empty !*/ }
     
     class NonZero: NaturalNumber {
     
     let value: Int
     
     fileprivate init(_ value: Int) {
     
     self.value = value
     }
     }
     
     !//!!...
     
     }
     NaturalNumber.swift
     

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130. class NaturalNumber {
     
     class Zero: NaturalNumber { !/* empty !*/ }
     
     class NonZero: NaturalNumber {
     
     let value: Int
     
     fileprivate init(_ value: Int) {
     
     self.value = value
     }
     }
     
     !//!!...
     
     }
     NaturalNumber.swift
     

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131. class NaturalNumber {
     
     class Zero: NaturalNumber { !/* empty !*/ }
     
     class NonZero: NaturalNumber {
     
     let value: Int
     
     fileprivate init(_ value: Int) {
     
     self.value = value
     }
     }
     
     !//!!...
     
     }
     NaturalNumber.swift
     

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132. class NaturalNumber {
     
     class Zero: NaturalNumber { !/* empty !*/ }
     
     class NonZero: NaturalNumber {
     
     let value: Int
     
     fileprivate init(_ value: Int) {
     
     self.value = value
     }
     }
     
     !//!!...
     
     }
     NaturalNumber.swift
     

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133. class NaturalNumber {
     
     class Zero: NaturalNumber { !/* empty !*/ }
     
     class NonZero: NaturalNumber {
     
     let value: Int
     
     fileprivate init(_ value: Int) {
     
     self.value = value
     }
     }
     
     !//!!...
     
     }
     NaturalNumber.swift
     

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134. class NaturalNumber {
     
     class Zero: NaturalNumber { !/* empty !*/ }
     
     class NonZero: NaturalNumber {
     
     let value: Int
     
     fileprivate init(_ value: Int) {
     
     self.value = value
     }
     }
     
     !//!!...
     
     }
     NaturalNumber.swift
     

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135. class NaturalNumber {
     
     class Zero: NaturalNumber { !/* empty !*/ }
     
     class NonZero: NaturalNumber {
     
     let value: Int
     
     fileprivate init(_ value: Int) {
     
     self.value = value
     }
     }
     
     !//!!...
     
     }
     NaturalNumber.swift
     

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136. class NaturalNumber {
     
     class Zero: NaturalNumber { !/* empty !*/ }
     
     class NonZero: NaturalNumber {
     
     let value: Int
     
     fileprivate init(_ value: Int) {
     
     self.value = value
     }
     }
     
     !//!!...
     
     }
     NaturalNumber.swift
     

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137. class NaturalNumber {
     
     class Zero: NaturalNumber { !/* empty !*/ }
     
     class NonZero: NaturalNumber {
     
     let value: Int
     
     fileprivate init(_ value: Int) {
     
     self.value = value
     }
     }
     
     !//!!...
     
     }
     NaturalNumber.swift
     

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138. class NaturalNumber {
     
     class Zero: NaturalNumber { !/* empty !*/ }
     
     class NonZero: NaturalNumber {
     
     let value: Int
     
     fileprivate init(_ value: Int) {
     
     self.value = value
     }
     }
     
     !//!!...
     
     }
     NaturalNumber.swift
     

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139. class NaturalNumber {
     
     !//!!...
     
     class func from(_ value: Int) #-> NaturalNumber {
     
     return value #== 0
     
     ? Zero()
     
     : NonZero(value)
     
     }
     
     }
     NaturalNumber.swift
     

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140. class NaturalNumber {
     
     !//!!...
     
     class func from(_ value: Int) #-> NaturalNumber {
     
     return value #== 0
     
     ? Zero()
     
     : NonZero(value)
     
     }
     
     }
     NaturalNumber.swift
     

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141. class NaturalNumber {
     
     !//!!...
     
     class func from(_ value: Int) #-> NaturalNumber {
     
     return value #== 0
     
     ? Zero()
     
     : NonZero(value)
     
     }
     
     }
     NaturalNumber.swift
     

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142. class NaturalNumber {
     
     !//!!...
     
     class func from(_ value: Int) #-> NaturalNumber {
     
     return value #== 0
     
     ? Zero()
     
     : NonZero(value)
     
     }
     
     }
     NaturalNumber.swift
     

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143. class NaturalNumber {
     
     !//!!...
     
     class func from(_ value: Int) #-> NaturalNumber {
     
     return value #== 0
     
     ? Zero()
     
     : NonZero(value)
     
     }
     
     }
     NaturalNumber.swift
     

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144. class NaturalNumber {
     
     !//!!...
     
     class func from(_ value: Int) #-> NaturalNumber {
     
     return value #== 0
     
     ? Zero()
     
     : NonZero(value)
     
     }
     
     }
     NaturalNumber.swift
     

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145. let divisor = NaturalNumber.from(4)
     
     if (divisor is NaturalNumber.NonZero) {
     
     val quotient = divide(divisor: 8, divisor: divisor)
     
     !// Do fancy things!
     
     } else {
     
     !// Fail fast…
     
     }
     Usage
     

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146. let divisor = NaturalNumber.from(4)
     
     if (divisor is NaturalNumber.NonZero) {
     
     val quotient = divide(divisor: 8, divisor: divisor)
     
     !// Do fancy things!
     
     } else {
     
     !// Fail fast…
     
     }
     Usage
     

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147. let divisor = NaturalNumber.from(4)
     
     if (divisor is NaturalNumber.NonZero) {
     
     val quotient = divide(divisor: 8, divisor: divisor)
     
     !// Do fancy things!
     
     } else {
     
     !// Fail fast…
     
     }
     Usage
     

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148. let divisor = NaturalNumber.from(4)
     
     if (divisor is NaturalNumber.NonZero) {
     
     val quotient = divide(divisor: 8, divisor: divisor)
     
     !// Do fancy things!
     
     } else {
     
     !// Fail fast…
     
     }
     Usage
     

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149. let divisor = NaturalNumber.from(4)
     
     if (divisor is NaturalNumber.NonZero) {
     
     val quotient = divide(divisor: 8, divisor: divisor)
     
     !// Do fancy things!
     
     } else {
     
     !// Fail fast…
     
     }
     Usage
     

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150. let divisor = NaturalNumber.from(4)
     
     if (divisor is NaturalNumber.NonZero) {
     
     val quotient = divide(divisor: 8, divisor: divisor)
     
     !// Do fancy things!
     
     } else {
     
     !// Fail fast…
     
     }
     Usage
     

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151. let divisor = NaturalNumber.from(4)
     
     if (divisor is NaturalNumber.NonZero) {
     
     val quotient = divide(divisor: 8, divisor: divisor)
     
     !// Do fancy things!
     
     } else {
     
     !// Fail fast…
     
     }
     Usage
     

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152. let divisor = NaturalNumber.from(4)
     
     if (divisor is NaturalNumber.NonZero) {
     
     val quotient = divide(divisor: 8, divisor: divisor)
     
     !// Do fancy things!
     
     } else {
     
     !// Fail fast…
     
     }
     Usage
     

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153. let divisor = NaturalNumber.from(4)
     
     if (divisor is NaturalNumber.NonZero) {
     
     val quotient = divide(divisor: 8, divisor: divisor)
     
     !// Do fancy things!
     
     } else {
     
     !// Fail fast…
     
     }
     Usage
     

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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: NaturalNumber.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: NaturalNumber.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: NaturalNumber.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: NaturalNumber.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: NaturalNumber.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: NaturalNumber.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: NaturalNumber.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: NaturalNumber.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: NaturalNumber.NonZero) #-> Int
     Original vs. Tweaked
     

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163. !// Liberal inputs, conservative output (original)
     
     fun divide(dividend: Int, divisor: Int) #-> Quotient
     
     !// Conservative inputs, conservative output (tweaked)
     
     fun divide(dividend: Int, divisor: NaturalNumber.NonZero) #-> Int
     Original vs. Tweaked
     

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164. !// Liberal inputs, conservative output (original)
     
     fun divide(dividend: Int, divisor: Int) #-> Quotient
     
     !// Conservative inputs, conservative output (tweaked)
     
     fun divide(dividend: Int, divisor: NaturalNumber.NonZero) #-> Int
     Original vs. Tweaked
     

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165. !// Liberal inputs, conservative output (original)
     
     fun divide(dividend: Int, divisor: Int) #-> Quotient
     
     !// Conservative inputs, conservative output (tweaked)
     
     fun divide(dividend: Int, divisor: NaturalNumber.NonZero) #-> Int
     Original vs. Tweaked
     

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166. !// Liberal inputs, conservative output (original)
     
     fun divide(dividend: Int, divisor: Int) #-> Quotient
     
     !// Conservative inputs, conservative output (tweaked)
     
     fun divide(dividend: Int, divisor: NaturalNumber.NonZero) #-> Int
     Original vs. Tweaked
     

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

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

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169. Impact on Overall System Design
     • Enhances certainty on micro and macroscopic levels
     
     • Errors are treated as first-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
     
     • Effective cure against primitive obsession
     
     • Reduces defensive programming
     
     • Promotes property testing over behaviour testing
     

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170. Summary
     • Robustness Principle
     
     • Total Functions
     
     • Algebraic Data Types
     
     • Robustness Principle (Tweaked)
     

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171. Questions?
     @ragunathjawahar
     

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