Matryoshka in Swift: handle nested containers with grace

Do you like container types? I mean, the Results, Optionals, Futures, Observables? Would you feel alright if your child married one?
If the answer is yes, then how would you like a container inside a container? And what about a container inside a container inside a container? Or a container inside a container inside a container inside a container? Oh boy, things are quickly getting out of hands!
In this talk I'll show some ideas on how to work with the real-world container nesting and the tradeoffs that are coming along.

Transcript

1. Matryoshka 

   in Swift
   Krzysztof Siejkowski, @_siejkowski
   

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3. What is container?
   Container
   A
   instance of type A
   Container holds 

   the instance 

   of type A
   while adding

   its context
   

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4. enum Optional {
   case some(A)
   case none
   }
   Optional
   Optional
   A ∅
   

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5. Result
   A E
   

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6. Either
   A B
   Result
   A E
   

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7. Either
   A B
   Result
   A E
   Future
   ((A) -> Void) -> Void
   

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8. Either
   A B
   Result
   A E
   Future
   ((A) -> Void) -> Void
   Observable
   () -> Event
   (A) -> Void
   

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

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12. forEach
    A (A) -> Void
    

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13. forEach
    A (A) -> Void
    A
    A
    reduce
    

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14. forEach
    A (A) -> Void
    A (A) ->
    Container
    B
    flatMap
    A
    A
    reduce
    

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

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

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

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

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

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22. 2. Deserialize
    Result is a JSON object in form of an array of repos:
    [
    {
    “name”: “repo_name”,
    ...
    },
    ...
    We want to know which ones of the repos couldn’t be deserialized
    deserialize: (Data) throws -> [Data]
    throws -> [Repo]
    

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

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

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

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

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27. Simplifying
    by
    transforming
    

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

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

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30. Validated
    A
    E
    E E
    E
    E
    E
    Result
    A E
    

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

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32. sequence
    A
    A
    Container1> => Container2>
    

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33. Given: 

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

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

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35. Container> => Container
    flatten
    A
    A
    

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

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

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

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40. Write “nested”
    methods for
    containers
    

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

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

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43. 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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44. 1. Create protocol
    protocol ValidatedType {
    associatedtype PA
    associatedtype PE
    }
    extension Validated: ValidatedType {
    typealias PA = A
    typealias PE = E
    }
    

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45. 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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46. 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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47. Method-based
    1. Generalize the method,

    not extension
    2. Add constraints to 

    the method, not extension
    

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

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

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51. A lot of 

    boilerplate?
    

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52. Use code generation
    (like Sourcery)
    Please see this great talk for details:

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

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53. Use “wide”
    container, like

    Observable
    

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54. Future< <== observable is async
    Validated< <== observable can return error
    Array< <== observable is sequence
    Repo
    >,
    Either
    >
    >
    

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56. Thanks!
    Questions?
    

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