Container types and how to use them

What does Optional, Array, Result and Observable have in common? Why was Array’s ‘flatMap’ changed to ’compactMap’? Can you zip Results? What is ‘sequence’ function? This talks will answer these questions and explain the mental model of containers that’s behind various widely-used Swift types. Once you see how they relate to each other, we’ll explore what you can do with them and how to handle the issues that arive when you model your code in a container fashion.

Transcript

1. Container types
   and how to use them
   Krzysztof Siejkowski
   

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2. Questions
   are welcome!
   

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3. Container
   Container
   A Container holds 

   the instance 

   of type A
   while adding

   its context
   instance of type A
   context
   

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5. Almost
   5 years
   ago…
   

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6. Optional
   Optional
   A ∅
   instance of type A
   lack of
   instance of type A
   context:
   possible
   lack of value
   

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7. Result
   Result
   A E
   context:
   possible error
   instance of type A
   failure
   (usually error)
   

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8. Array
   Array
   context:
   possible
   multiple values
   some more possible
   instances of type A
   A
   A
   A
   A
   A
   A
   A
   A
   A
   A
   A
   A
   possible instance of type A
   

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10. Generator AKA Lazy
    Lazy
    context:
    deferred
    evaluation
    of value
    block that
    evaluates value
    when needed
    () -> A
    

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11. Future
    Future
    context:
    asynchronous
    computation
    of value
    block that provides value
    once it becomes available
    block that
    kicks off
    computation
    -> Void
    -> Void
    A
    

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12. Either
    Either
    A B
    context:
    one of
    two values
    instance of type A
    instance of type B
    

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13. Observable
    Observable
    context:
    sequence
    of events
    in time
    block that
    generates events
    (might be called
    multiple times)
    () -> Event
    

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14. Mental model
    Container
    A Container holds 

    the instance 

    of type A
    while adding

    its context
    instance of type A
    context
    

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16. flatMap
    A
    (A) ->
    Container
    B
    A
    Container
    + (A) -> Container
    = Container
    

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17. map
    A
    (A) -> B
    B
    A B
    Container
    + (A) -> B
    = Container
    

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18. !"
    

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19. Container
    + (A) -> Void
    = Container
    A
    (A) -> Void
    A
    do
    A
    

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20. reduce
    A A
    Context related
    side-effect
    Container => A or side-effect
    

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21. Container
    + (A) -> Void
    = Void
    A
    (A) -> Void
    A
    forEach
    

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22. zip
    C
    A
    B
    A
    B (A, B) -> C
    (Container, Container)
    + (A, B) -> C
    => Container
    C
    

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23. unzip
    C
    A
    B
    A
    B
    C -> (A, B)
    Container
    + (C) -> (A, B)
    => (Container,
    Container)
    C
    

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24. !"
    

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25. flatMap compactMap
    A
    (A) ->
    Optional
    B
    A
    Container
    + (A) -> Optional
    = Container
    B
    B
    

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26. flatMap
    A
    Container
    + (A) -> Container
    = Container
    A
    (A) ->
    Container
    B
    A
    

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28. What is a context?
    Container
    A Container holds 

    the instance 

    of type A
    while adding

    its context
    instance of type A
    context
    

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29. Context describes
    the constraints of
    your logic
    

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30. • nil / null
    • failure / error / exception
    • lazy evaluation
    • unknown number of values
    • asynchronous operation
    • … many more!
    

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34. let interestingData = networkClient
    .fetchData()
    .flatMap { validateStatusCode($0) }
    .flatMap { deserialize($0) }
    .map { getSubsetOfData($0) }
    .reduce { handleSideEffect($0) }
    

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36. Scott Wlaschin, “Railway Oriented Programming”
    https://www.slideshare.net/ScottWlaschin/railway-oriented-programming
    

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37. Scott Wlaschin, “Railway Oriented Programming”
    https://www.slideshare.net/ScottWlaschin/railway-oriented-programming
    

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38. Many contexts
    One API
    

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39. optionalValue
    .map { ... }
    .flatMap { ... }
    asynchronousValue
    .map { ... }
    .flatMap { ... }
    multipleValues
    .map { ... }
    .flatMap { ... }
    possibleErrorValue
    .map { ... }
    .flatMap { ... }
    

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42. As a user, I want to see
    all my Github repos
    written in Swift language
    

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43. 1. Fetch data from Github
    2. Deserialize
    3. Filter Swift repos
    

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44. githubClient
    .fetch(.repos, for: user)
    .map { deserialize($0) }
    .filter { $0.isSwiftRepo }
    

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45. 1. Fetch data from Github
    extension URLSession {
    func dataTask(
    with url: URL,
    completionHandler:
    @escaping (Data?, URLResponse?, Error?) -> Void
    ) -> URLSessionDataTask
    }
    

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46. 1. Fetch data from Github
    completionHandler
    => Future
    (Data?, URLResponse?, Error?)
    => Result
    Future>
    

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47. 2. Deserialize
    Response is a JSON object in form of an array of repos:
    [
    {
    “name”: “repo_name”,
    ...
    },
    ...
    deserialize: (Data) throws -> [Data]
    throws -> [Repo]
    

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48. 2. Deserialize
    deserialize: (Data) throws -> [Data]
    throws -> [Repo]
    => (Data) -> Result<[Data], DeserializationError>
    -> Result<[Result],
    DeserializationError>
    FutureResult<
    [Result],
    DeserializationError
    >,
    NetworkError >>
    

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49. 3. Filter Swift repos
    filter: (Repo) -> Bool
    repos.filter { (repo: Repo) -> Bool in
    repo.language == "Swift"
    }
    

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50. 3. Filter Swift repos
    let repos: FutureResult<
    [Result],
    DeserializationError
    >,
    NetworkError
    >> = githubClient.fetch(.repos, for: user)
    .map { deserialize($0) }
    repos.filter { }
    

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51. Future<
    Result<
    Result<
    Array<
    Result<
    Repo,
    DeserializationError
    >
    >,
    DeserializationError
    >,
    NetworkError
    >
    >
    
    
    ❌
    
    

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53. What’s the difference?
    Optional
    A ∅
    Either
    A Void
    

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54. You can write a transform!
    Optional
    A ∅
    Either
    A Void
    Optional
    A ∅
    Either
    A Void
    

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55. Validated
    A
    E
    E E
    E
    E
    E
    Result
    A E
    

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56. extension Result {
    func asValidated() -> Validated { ... }
    }
    Future<
    Validated<
    Validated<
    Array>,
    DeserializationError
    >,
    NetworkError
    >
    >
    

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57. sequence
    A
    A
    Container1> => Container2>
    

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58. Given: 

    Array>
    When:
    If any element of array is nil, then return nil
    Else, return array of unwrapped, non-nil elements
    Then:
    Optional>
    

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59. func sequence(_ vs: [Validated]) -> Validated<[A], E>
    Future<
    Validated<
    Validated<
    Validated<
    Array,
    DeserializationError
    >,
    DeserializationError
    >,
    NetworkError
    >
    >
    

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60. flatten
    A
    A
    Container> => Container
    

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61. extension Validated {
    func flatten() -> Validated
    where A == Validated { ... }
    }
    Future<
    Validated<
    Validated<
    Array,
    DeserializationError
    >,
    NetworkError
    >>
    

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62. extension Validated {
    func mapErrors(_ f: (E) -> F) -> Validated { ... }
    }
    // put errors into Either container
    Future<
    Validated<
    Validated<
    Array,
    Either
    >,
    Either
    >>
    

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63. We’re left with 3 containers-deep nesting
    // after flattening again
    Future< // 1st level
    Validated< // 2nd level
    [Repo], // 3rd level
    Either
    >
    >
    

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64. Filter Swift repos
    let repos: Future<
    Validated<
    Array,
    Either
    >
    > = githubClient.fetch(.repos, for: user)
    .map { deserialize($0) }
    .{ // transforms described before }
    repos.filter { }
    

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66. Container1> + (A) -> B
    = Container1>
    innerMap
    B
    A (A) -> B
    

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67. Two ways of
    implementing:
    • protocol-based
    • method-based
    

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68. Inverse of type erasure
    protocol Protocol
    { associatedtype A }
    Type erasure
    struct AnyProtocol 

    : Protocol
    struct Container
    “Type rasure”
    protocol ContainerType
    { associatedtype A }
    

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69. 1. Create protocol
    protocol ValidatedType {
    associatedtype PA
    associatedtype PE
    }
    extension Validated: ValidatedType {
    typealias PA = A
    typealias PE = E
    }
    

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70. 2. Duplicate API in protocol (return concrete type)
    protocol ValidatedType {
    associatedtype PA
    associatedtype PE
    func map(_ f: (PA) -> B) -> Validated
    }
    extension Validated: ValidatedType {}
    

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71. 3. Implement the nested method in a constrained extension
    extension Future where A: ValidatedType {
    func map(
    _ f: @escaping (A.PA) -> B
    ) -> Future> {
    return map { $0.map(f) }
    }
    }
    

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72. Method-based
    1. Generalize the method,

    not extension
    2. Add constraints to 

    the method, not extension
    

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73. 1. Generalize the method, not extension
    extension Future {
    func map(
    _ f: @escaping (VA) -> B
    ) -> Future> {
    // work in progress
    }
    }
    

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74. 2. Add constraints to the method, not extension
    extension Future {
    func map(
    _ f: @escaping (VA) -> B
    ) -> Future>
    where A == Validated {
    return map { $0.map(f) }
    }
    }
    

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75. !"
    

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76. Now we can filter Swift repos!
    let repos: Future<
    Validated<
    Array,
    Either
    >> = githubClient.fetch(.repos, for: user)
    .map { deserialize($0) }
    .{ // transforms described before }
    repos.filter { (repo: Repo) -> Bool in
    repo.language == “Swift”
    } // it works!
    

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78. Use code generation
    Please see this great talk for details:

    Elviro Rocca — Protocol-Oriented Monad Transformers
    https://www.youtube.com/watch?v=Zmb86zblcto
    

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79. Use “wide”
    containers,
    like Observable
    

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80. Future< <== observable is async
    Validated< <== observable can return error
    Array< <== observable is sequence
    Repo
    >,
    Either
    >
    >
    Observable <- but some information will be lost!
    

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82. Thanks!
    Questions?
    

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