Railway Oriented Programming with Kotlin

Many examples in Kotlin assume that you are always on the "happy path". But real-world applications must deal with a lot of types of errors: validation, network, and others.

So, how do you handle all of this?

Transcript

1. Marcello Galhardo
   marcello.galhardo@gmail.com
   A functional approach to error handling
   Railway Oriented
   Programming with Kotlin.
   

   View Slide

2. This talk is not about code…
   …it is about a paradigm.
   

   View Slide

3. “As a user I want to update my
   name and email address.”
   

   View Slide

4. Button clicks send Action to ViewModel
   

   View Slide

5. Button clicks send Action to ViewModel
   ViewModel invokes UseCase to validate
   and canonicalise Action data
   

   View Slide

6. Button clicks send Action to ViewModel
   ViewModel invokes UseCase to validate
   and canonicalise Action data
   UseCase ask Repository to update user data
   

   View Slide

7. Button clicks send Action to ViewModel
   ViewModel invokes UseCase to validate
   and canonicalise Action data
   UseCase ask Repository to update user data
   Returns result to UI
   

   View Slide

8. Action.kt
   sealed class Action {
   data class UpdateUser(
   val id: UserId,
   val name: String,
   val email: String
   ) : Action()
   }
   

   View Slide

9. Action.kt
   sealed class Action {
   data class UpdateUser(
   val id: UserId,
   val name: String,
   val email: String
   ) : Action()
   }
   

   View Slide

10. Action.kt
    sealed class Action {
    data class UpdateUser(
    val id: UserId,
    val name: String,
    val email: String
    ) : Action()
    }
    

    View Slide

11. View.kt
    // Action dispatcher in a View
    okButton.setOnClickListener {
    val name = nameTextField.value
    val email = emailTextField.value
    val action = Action.UpdateUser(userId, name, email)
    viewModel.dispatch(action)
    }
    

    View Slide

12. UpdateUser.kt
    fun updateUser(
    action: Action.UpdateUser
    ): Result {
    validate(action)
    val canonicalizedEmail = canonicalizeEmail(action)
    val user = repository.getUserById(id).copy(
    name = action.name,
    email = canonicalizedEmail
    )
    repository.update(user)
    return Result.success(user)
    }
    

    View Slide

13. UpdateUser.kt
    fun updateUser(
    action: Action.UpdateUser
    ): Result {
    validate(action)
    val canonicalizedEmail = canonicalizeEmail(action)
    val user = repository.getUserById(id).copy(
    name = action.name,
    email = canonicalizedEmail
    )
    repository.update(user)
    return Result.success(user)
    }
    

    View Slide

14. UpdateUser.kt
    fun updateUser(
    action: Action.UpdateUser
    ): Result {
    validate(action)
    val canonicalizedEmail = canonicalizeEmail(action)
    val user = repository.getUserById(id).copy(
    name = action.name,
    email = canonicalizedEmail
    )
    repository.update(user)
    return Result.success(user)
    }
    

    View Slide

15. UpdateUser.kt
    fun updateUser(
    action: Action.UpdateUser
    ): Result {
    validate(action)
    val canonicalizedEmail = canonicalizeEmail(action)
    val user = repository.getUserById(id).copy(
    name = action.name,
    email = canonicalizedEmail
    )
    repository.update(user)
    return Result.success(user)
    }
    

    View Slide

16. UpdateUser.kt
    fun updateUser(
    action: Action.UpdateUser
    ): Result {
    validate(action)
    val canonicalizedEmail = canonicalizeEmail(action)
    val user = repository.getUserById(id).copy(
    name = action.name,
    email = canonicalizedEmail
    )
    repository.update(user)
    return Result.success(user)
    }
    

    View Slide

17. UpdateUser.kt
    fun updateUser(
    action: Action.UpdateUser
    ): Result {
    validate(action)
    val canonicalizedEmail = canonicalizeEmail(action)
    val user = repository.getUserById(id).copy(
    name = action.name,
    email = canonicalizedEmail
    )
    repository.update(user)
    return Result.success(user)
    }
    

    View Slide

18. Functions
    // ValidateUser.kt
    fun validate(action: Action.UpdateUser): Boolean
    // CanonicalizeEmail.kt
    fun canonicalizeEmail(action: Action.UpdateUser): String
    // UserRepository.kt
    @Throws(Exception::class)
    fun Repository.getUserById(id: UserId): User?
    @Throws(Exception::class)
    fun Repository.update(user: User)
    

    View Slide

19. Functions
    // ValidateUser.kt
    fun validate(action: Action.UpdateUser): Boolean
    // CanonicalizeEmail.kt
    fun canonicalizeEmail(action: Action.UpdateUser): String
    // UserRepository.kt
    @Throws(Exception::class)
    fun Repository.getUserById(id: UserId): User?
    @Throws(Exception::class)
    fun Repository.update(user: User)
    

    View Slide

20. Functions
    // ValidateUser.kt
    fun validate(action: Action.UpdateUser): Boolean
    // CanonicalizeEmail.kt
    fun canonicalizeEmail(action: Action.UpdateUser): String
    // UserRepository.kt
    @Throws(Exception::class)
    fun Repository.getUserById(id: UserId): User?
    @Throws(Exception::class)
    fun Repository.update(user: User)
    

    View Slide

21. Functions
    // ValidateUser.kt
    fun validate(action: Action.UpdateUser): Boolean
    // CanonicalizeEmail.kt
    fun canonicalizeEmail(action: Action.UpdateUser): String
    // UserRepository.kt
    @Throws(Exception::class)
    fun Repository.getUserById(id: UserId): User?
    @Throws(Exception::class)
    fun Repository.update(user: User)
    

    View Slide

22. What do you do when
    something goes
    wrong?
    

    View Slide

23. “As a user I want to update my
    name and email address…
    …and see error messages
    when something goes wrong!"
    

    View Slide

24. “As a user I want to update my
    name and email address…
    …and see error messages
    when something goes wrong!"
    

    View Slide

25. Button clicks send Action to ViewModel
    ViewModel invokes UseCase to validate
    and canonicalise Action data
    UseCase ask Repository to update user data
    Returns result to UI
    

    View Slide

26. Button clicks send Action to ViewModel
    ViewModel invokes UseCase to validate
    and canonicalise Action data
    UseCase ask Repository to update user data
    Returns result to UI
    

    View Slide

27. Name is blank or email not valid
    Button clicks send Action to ViewModel
    ViewModel invokes UseCase to validate
    and canonicalise Action data
    UseCase ask Repository to update user data
    Returns result to UI
    

    View Slide

28. Name is blank or email not valid
    Button clicks send Action to ViewModel
    ViewModel invokes UseCase to validate
    and canonicalise Action data
    UseCase ask Repository to update user data
    Returns result to UI
    

    View Slide

29. Name is blank or email not valid
    User not found
    Button clicks send Action to ViewModel
    ViewModel invokes UseCase to validate
    and canonicalise Action data
    UseCase ask Repository to update user data
    Returns result to UI
    

    View Slide

30. UpdateUser.kt
    fun updateUser(
    action: Action.UpdateUser
    ): Result {
    validate(action)
    val canonicalizedEmail = canonicalizeEmail(action)
    val user = repository.getUserById(id).copy(
    name = action.name,
    email = canonicalizedEmail
    )
    repository.update(user)
    return Result.success(user)
    }
    

    View Slide

31. UpdateUser.kt
    fun updateUser(
    action: Action.UpdateUser
    ): Result {
    validate(action)
    val canonicalizedEmail = canonicalizeEmail(action)
    val user = repository.getUserById(id).copy(
    name = action.name,
    email = canonicalizedEmail
    )
    repository.update(user)
    return Result.success(user)
    }
    

    View Slide

32. UpdateUser.kt
    fun updateUser(
    action: Action.UpdateUser
    ): Result {
    val isValid = validate(action)
    if (!isValid) {
    return Result.error(UserNotValidException())
    }
    val canonicalizedEmail = canonicalizeEmail(action)
    val user = repository.getUserById(id).copy(
    name = action.name,
    email = canonicalizedEmail
    )
    repository.update(user)
    return Result.success(user)
    }
    

    View Slide

33. UpdateUser.kt
    fun updateUser(
    action: Action.UpdateUser
    ): Result {
    val isValid = validate(action)
    if (!isValid) {
    return Result.error(UserNotValidException())
    }
    val canonicalizedEmail = canonicalizeEmail(action)
    val user = repository.getUserById(id).copy(
    name = action.name,
    email = canonicalizedEmail
    )
    repository.update(user)
    return Result.success(user)
    }
    

    View Slide

34. UpdateUser.kt
    fun updateUser(
    action: Action.UpdateUser
    ): Result {
    val isValid = validate(action)
    if (!isValid) {
    return Result.error(UserNotValidException())
    }
    val canonicalizedEmail = canonicalizeEmail(action)
    val user = repository.getUserById(id).copy(
    name = action.name,
    email = canonicalizedEmail
    )
    repository.update(user)
    return Result.success(user)
    }
    

    View Slide

35. UpdateUser.kt
    fun updateUser(
    action: Action.UpdateUser
    ): Result {
    val isValid = validate(action)
    if (!isValid) {
    return Result.error(UserNotValidException())
    }
    val canonicalizedEmail = canonicalizeEmail(action)
    var user = null
    try {
    user = repository.getUserById(id).copy(
    name = action.name,
    email = canonicalizedEmail
    )
    repository.update(user)
    } catch (error: Exception) {
    return Result.error(error)
    }
    return Result.success(user)
    }
    

    View Slide

36. UpdateUser.kt
    fun updateUser(
    action: Action.UpdateUser
    ): Result {
    val isValid = validate(action)
    if (!isValid) {
    return Result.error(UserNotValidException())
    }
    val canonicalizedEmail = canonicalizeEmail(action)
    var user = null
    try {
    user = repository.getUserById(id).copy(
    name = action.name,
    email = canonicalizedEmail
    )
    repository.update(user)
    } catch (error: Exception) {
    return Result.error(error)
    }
    return Result.success(user)
    }
    

    View Slide

37. 7 lines added, more 140%
    

    View Slide

38. • Each use case will be equivalent to a single function
    • The function will return a sum type with two cases: Success or Failure
    • The use case function will be built from a series of smaller functions,
    each representing one step in a data flow
    • The errors from each step will be combined into a single failure
    

    View Slide

39. • Each use case will be equivalent to a single function
    • The function will return a sum type with two cases: Success, Failure
    • The use case function will be built from a series of smaller functions,
    each representing one step in a data flow
    • The errors from each step will be combined into a single failure
    

    View Slide

40. • Each use case will be equivalent to a single function
    • The function will return a sum type with two cases: Success, Failure
    • The use case function will be built from a series of smaller functions,
    each representing one step in a data flow
    • The errors from each step will be combined into a single failure
    

    View Slide

41. • Each use case will be equivalent to a single function
    • The function will return a sum type with two cases: Success, Failure
    • The use case function will be built from a series of smaller functions,
    each representing one step in a data flow
    • Each step will be combined into a single result
    

    View Slide

42. The following Result class and
    methods are simplified/custom
    versions for easy understanding
    

    View Slide

43. How do you return a sum type
    with two cases?
    

    View Slide

44. bit.do/kotlin-std-result
    sealed class Result {
    data class Success(
    val value: Value
    ): Result
    data class Failure(
    val error: Exception
    ): Result
    }
    

    View Slide

45. bit.do/kotlin-std-result
    sealed class Result {
    data class Success(
    val value: Value
    ): Result
    data class Failure(
    val error: Exception
    ): Result
    }
    

    View Slide

46. bit.do/kotlin-std-result
    sealed class Result {
    data class Success(
    val value: Value
    ): Result
    data class Failure(
    val error: Exception
    ): Result
    }
    

    View Slide

47. bit.do/kotlin-std-result
    sealed class Result {
    data class Success(
    val value: Value
    ): Result
    data class Failure(
    val error: String
    ): Result
    }
    

    View Slide

48. How to combine each step into
    a single result?
    

    View Slide

49. public inline fun Result.map(transform: (value: T) -> R): Result {
    return when {
    isSuccess -> Result.success(transform(value as T))
    else -> Result(value)
    }
    }
    bit.do/kotlin-std-result
    

    View Slide

50. public inline fun Result.map(transform: (value: T) -> R): Result {
    return when {
    isSuccess -> Result.success(transform(value as T))
    else -> Result(value)
    }
    }
    bit.do/kotlin-std-result
    

    View Slide

51. public inline fun Result.map(transform: (value: T) -> R): Result {
    return when {
    isSuccess -> Result.success(transform(value as T))
    else -> Result(value)
    }
    }
    bit.do/kotlin-std-result
    

    View Slide

52. public inline fun Result.map(transform: (value: T) -> R): Result {
    return when {
    isSuccess -> Result.success(transform(value as T))
    else -> Result(value)
    }
    }
    bit.do/kotlin-std-result
    

    View Slide

53. public inline fun Result.flatMap(
    transform: (value: T) -> Result
    ): Result {
    return when {
    isSuccess -> transform(value as T)
    else -> Result(value)
    }
    }
    bit.do/kotlin-std-result
    

    View Slide

54. public inline fun Result.flatMap(
    transform: (value: T) -> Result
    ): Result {
    return when {
    isSuccess -> transform(value as T)
    else -> Result(value)
    }
    }
    bit.do/kotlin-std-result
    

    View Slide

55. public inline fun Result.flatMap(
    transform: (value: T) -> Result
    ): Result {
    return when {
    isSuccess -> transform(value as T)
    else -> Result(value)
    }
    }
    bit.do/kotlin-std-result
    

    View Slide

56. public inline fun Result.flatMap(
    transform: (value: T) -> Result
    ): Result {
    return when {
    isSuccess -> transform(value as T)
    else -> Result(value)
    }
    }
    bit.do/kotlin-std-result
    

    View Slide

57. ViewModel invokes UseCase to validate
    and canonicalise Action data
    

    View Slide

58. ViewModel invokes UseCase to validate
    and canonicalise Action data
    Failure Success
    

    View Slide

59. ViewModel invokes UseCase to validate
    and canonicalise Action data
    Failure
    UseCase ask Repository to update user data
    ?
    Success
    

    View Slide

60. Success
    ViewModel invokes UseCase to validate
    and canonicalise Action data
    UseCase ask Repository to update user data
    Failure
    Success
    Failure
    ?
    

    View Slide

61. Success
    ViewModel invokes UseCase to validate
    and canonicalise Action data
    Failure
    UseCase ask Repository to update user data
    Failure
    Success
    

    View Slide

62. GetUserById.kt
    fun getUserById(input: Action.UpdateUser): Result {
    val user = repository.getUserByIdOrNull(input.id)
    return when {
    user == null -> Result.failure("User not found.")
    else -> Result.success(user)
    }
    }
    

    View Slide

63. NameNotBlank.kt
    fun nameNotBlank(user: User) = when {
    user.name == "" -> Result.failure("Name must not be blank.")
    else -> Result.success(user)
    }
    

    View Slide

64. NameIsBelowMaxLength.kt
    fun nameIsBelowMaxLength(user: User) = when {
    user.name.length > 50 -> Result.failure("Name must not be longer than 50 chars.")
    else -> Result.success(user)
    }
    

    View Slide

65. EmailNotBlank.kt
    fun emailNotBlank(user: User) = when {
    user.email == "" -> Result.failure("Email must not be blank.")
    else -> Result.success(user)
    }
    

    View Slide

66. UpdateUser.kt
    fun updateUser(
    action: Action.UpdateUser
    ): Result {
    return getUserById(action.id)
    .map { user -> user.copy(name = action.name) }
    .map { user -> user.copy(email = action.email) }
    .map(::nameNotBlank)
    .map(::nameIsBelowMaxLength)
    .map(::emailNotBlank)
    .map(::canonicalizeEmail)
    .map(::saveUser)
    }
    

    View Slide

67. UpdateUser.kt
    fun updateUser(
    action: Action.UpdateUser
    ): Result {
    return getUserById(action.id)
    .map { user -> user.copy(name = action.name) }
    .map { user -> user.copy(email = action.email) }
    .map(::nameNotBlank)
    .map(::nameIsBelowMaxLength)
    .map(::emailNotBlank)
    .map(::canonicalizeEmail)
    .map(::saveUser)
    }
    

    View Slide

68. UpdateUser.kt
    fun updateUser(
    action: Action.UpdateUser
    ): Result {
    return getUserById(action.id)
    .map { user -> user.copy(name = action.name) }
    .map { user -> user.copy(email = action.email) }
    .map(::nameNotBlank)
    .map(::nameIsBelowMaxLength)
    .map(::emailNotBlank)
    .map(::canonicalizeEmail)
    .map(::saveUser)
    }
    

    View Slide

69. UpdateUser.kt
    fun updateUser(
    action: Action.UpdateUser
    ): Result {
    return getUserById(action.id)
    .map { user -> user.copy(name = action.name) }
    .map { user -> user.copy(email = action.email) }
    .flatMap(::nameNotBlank)
    .flatMap(::nameIsBelowMaxLength)
    .flatMap(::emailNotBlank)
    .map(::canonicalizeEmail)
    .map(::saveUser)
    }
    

    View Slide

70. UpdateUser.kt
    fun updateUser(
    action: Action.UpdateUser
    ): Result {
    return getUserById(action.id)
    .map { user -> user.copy(name = action.name) }
    .map { user -> user.copy(email = action.email) }
    .flatMap(::nameNotBlank)
    .flatMap(::nameIsBelowMaxLength)
    .flatMap(::emailNotBlank)
    .flatMap(::canonicalizeEmail)
    .map(::saveUser)
    }
    

    View Slide

71. UpdateUser.kt
    fun updateUser(
    action: Action.UpdateUser
    ): Result {
    return getUserById(action.id)
    .map { user -> user.copy(name = action.name) }
    .map { user -> user.copy(email = action.email) }
    .flatMap(::nameNotBlank)
    .flatMap(::nameIsBelowMaxLength)
    .flatMap(::emailNotBlank)
    .flatMap(::canonicalizeEmail)
    .flatMap(::saveUser)
    }
    

    View Slide

72. UpdateUser.kt
    fun updateUser(
    action: Action.UpdateUser
    ): Result {
    return getUserById(action.id)
    .map { user -> user.copy(name = action.name) }
    .map { user -> user.copy(email = action.email) }
    .flatMap(::nameNotBlank)
    .flatMap(::nameIsBelowMaxLength)
    .flatMap(::emailNotBlank)
    .flatMap(::canonicalizeEmail)
    .flatMap(::saveUser)
    }
    

    View Slide

73. ViewModel.kt
    fun dispatch(action: Action) {
    useCase.invoke()
    .onSuccess {
    // Do something cool!
    }.onFailure {
    // Ops, something went wrong…
    }
    }
    

    View Slide

74. ViewModel.kt
    fun dispatch(action: Action) {
    updateUser(action)
    .onSuccess {
    // Do something cool!
    }.onFailure {
    // Ops, something went wrong…
    }
    }
    

    View Slide

75. ViewModel.kt
    fun dispatch(action: Action) {
    updateUser(action)
    .onSuccess { value ->
    // Do something cool!
    }.onFailure {
    // Ops, something went wrong…
    }
    }
    

    View Slide

76. ViewModel.kt
    fun dispatch(action: Action) {
    updateUser(action)
    .onSuccess { value ->
    // Do something cool!
    }.onFailure { error ->
    // Ops, something went wrong…
    }
    }
    

    View Slide

77. Recap
    

    View Slide

78. • A strategy for handling errors in a functional way
    • Design for errors: unhappy paths are requirements too
    • Do not use Exception handling for Control Flow.
    “Do or do not, there is no try”
    

    View Slide

79. • A strategy for handling errors in a functional way
    • Design for errors: unhappy paths are requirements too
    • Do not use Exception handling for Control Flow.
    “Do or do not, there is no try”
    

    View Slide

80. • A strategy for handling errors in a functional way
    • Design for errors: unhappy paths are requirements too
    • Do not use Exception handling for Control Flow
    “Do or do not, there is no try”
    

    View Slide

81. • A strategy for handling errors in a functional way
    • Design for errors: unhappy paths are requirements too
    • Do not use Exception handling for Control Flow
    “Do or do not, there is no try.”
    

    View Slide

82. Marcello Galhardo
    marcello.galhardo@gmail.com
    Any questions?
    Thank you.
    

    View Slide