Container types in Swift without 🤯

Given at the swift.map #5 meetup in Poznań on 26.06.2018

Sample code: https://github.com/siejkowski/swift.map.5

References:

* Yasuhiro Inami's talk on Result vs Result: https://speakerdeck.com/inamiy/result-v-dot-s-result-t-e-english

* Elviro Rocca's talk on protocol based monad transformers: https://www.youtube.com/watch?v=Zmb86zblcto

* Elviro Rocca's library using the approach presented above: https://github.com/facile-it/FunctionalKit

* Yasuhiro Inami's talk on Higher-Kinded Types: https://speakerdeck.com/inamiy/swiftdegao-kaindoduo-xiang

* Yasuhiro Inami's library implementing HKT: https://github.com/inamiy/HigherKindSwift

Transcript

1. Container types without
   !
   Krzysztof Siejkowski / @_siejkowski
   

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3. Questions are fine
   Seriously
   They're fine
   

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4. Types
   

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5. Types?
   

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6. Type
   

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7. Optional
   

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8. Array
   

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9. Result
   

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10. Result
    !
    

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11. Result
    !
    

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12. Result V.S. Result
    Yasuhiro Inami
    https://speakerdeck.com/inamiy/result-v-dot-s-result-t-e-english
    

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13. Deferred
    

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14. Writer
    

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15. Observable
    

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16. Types?
    !
    

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17. Types!
    

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18. optional.map( )
    result.map( )
    array.map( )
    deferred.map( )
    

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19. type.map( ).filter( ).flatMap( ).reduce( )
    

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20. Types
    ❤
    

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21. Types
    !
    

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22. as a user, I want to see all
    my github repos written
    in Swift language
    

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23. extension URLSession {
    func dataTask(
    with url: URL,
    completionHandler: @escaping (Data?, URLResponse?, Error?) -> Void
    ) -> URLSessionDataTask
    }
    

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24. completionHandler:
    @escaping (Data?, URLResponse?, Error?) -> Void
    

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25. Deferred>
    

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26. // DESERIALIZATION
    (Data) -> [Repo]
    Deferred>
    

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27. // DESERIALIZATION
    (Data) -> Result
    Deferred<
    Result<
    Result?,
    NetworkError
    >
    >
    

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28. // DESERIALIZATION
    (Data) -> Result],
    DeserializationError>
    Deferred<
    Result<
    Result<
    [Result],
    DeserializationError
    >?,
    NetworkError
    >
    >
    

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29. // VALIDATION
    (Repo) -> Bool
    Deferred<
    Result<
    Result<
    [Result],
    DeserializationError
    >?,
    NetworkError
    >
    >
    

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30. // VALIDATION
    (Repo) -> Result
    Deferred<
    Result<
    Result<
    [Result<
    Result,
    DeserializationError
    >],
    DeserializationError
    >?,
    NetworkError
    >
    >
    

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31. // PRESENTATION
    (Repo) -> String
    Deferred<
    Result<
    Result<
    [Result<
    Result,
    DeserializationError
    >],
    DeserializationError
    >?,
    NetworkError
    >
    >
    

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32. Deferred<
    Result<
    Result<
    [Result<
    Result,
    DeserializationError
    >],
    DeserializationError
    >?,
    NetworkError
    >
    >
    !
    

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33. Deferred, DeserializationError>], DeserializationError>?, NetworkError>>
    THIS IS
    MADNESS
    !
    

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34. 3 solutions
    

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35. #1
    Redesign your
    data model
    

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36. Errors
    are part of your
    data model
    

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37. Deferred<
    Result<
    Result<
    [
    Result<
    Result,
    DeserializationError
    >
    ],
    DeserializationError
    >?,
    NetworkError
    >
    >
    

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38. Result, DeserializationError>
    enum RepoError: Error {
    // expressing both deserializaton and validation
    }
    Result
    

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39. Deferred<
    Result<
    Result<
    [
    Result
    ],
    RepoError
    >?,
    NetworkError
    >
    >
    

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40. Deferred<
    Result<
    Result<
    [
    Result
    ],
    RepoError
    >?,
    NetworkError
    >
    >
    

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41. Result<
    Result<
    [Result], RepoError
    >?,
    NetworkError
    >
    enum NetworkAPIError: Error {
    // previous cases
    case lackOfData // the (nil, nil) case
    }
    

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42. Deferred<
    Result<
    Result<
    [
    Result
    ],
    RepoError
    >,
    NetworkAPIError
    >
    >
    

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43. #2
    Discard
    information
    

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44. Deferred<
    Result<
    Result<
    [
    Result
    ],
    RepoError
    >,
    NetworkAPIError
    >
    >
    

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45. [Result]
    1. check if any is error
    2. if yes, return it as result
    3. if no, return the values
    Result
    

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46. Deferred<
    Result<
    Result<
    Result,
    RepoError
    >,
    NetworkAPIError
    >
    >
    

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47. Deferred<
    Result<
    Result,
    NetworkAPIError
    >
    >
    

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48. #3
    Provide
    transformers
    

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49. Transformers
    are APIs
    for working with
    nested containers
    They're providing functionality
    similar to Haskell types called Monad Transformers
    

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50. // WHAT WE WANT TO WRITE
    extension Deferred where DataType == Result {
    func mapInner(
    _ f: (DataType.T) -> OtherData
    ) -> Deferred> {
    return map { result in result.map(f) }
    }
    }
    

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51. Generic
    !
    type constraints
    

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52. // WHAT WE WANT TO WRITE
    extension Deferred where DataType == Result {
    func mapInner(
    _ f: (Result.T) -> OtherData
    ) -> Deferred> {
    return map { result in result.map(f) }
    }
    }
    // WHAT WE GET
    ERROR: Reference to generic type 'Result' requires arguments in
    

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53. // WHAT WE WANT TO WRITE
    extension Deferred where DataType == Result {
    func mapInner(
    _ f: (ResultData) -> OtherData
    ) -> Deferred> {
    return map { result in result.map(f) }
    }
    }
    // WHAT WE GET
    ERROR: Use of undeclared type 'ResultData'
    

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54. // WHAT WE WANT TO WRITE
    typealias DeferredResult =
    Deferred>
    where ResultError: Error
    extension DeferredResult {
    func mapInner(
    _ f: (ResultData) -> OtherData
    ) -> DeferredResult {
    return map { result in result.map(f) }
    }
    }
    // WHAT WE GET
    ERROR: Constrained extension must be declared on the unspecialized generic
    type 'DeferredResult' with constraints specified by a 'where' clause
    

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55. 2 ways
    of writing
    transformers
    

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56. Rasure
    (protocol-based)
    

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57. Rasure
    ==
    Inversion of
    type erasure
    

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58. 1. Create protocol
    2. Duplicate API (return concrete type)
    3. Constrain extensions with protocol
    

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59. 1. Create protocol
    protocol ResultType {
    associatedtype PT
    associatedtype PE: Error
    }
    extension Result: ResultType {
    typealias PT = T
    typealias PE = E
    }
    

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60. 2. Duplicate API
    protocol ResultType {
    associatedtype PT
    associatedtype PE: Error
    func map(_ f: (PT) -> OtherData)
    -> Result
    func flatMap(_ f: (PT) -> Result)
    -> Result
    }
    extension Result: ResultType {}
    

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61. 3. Constraint extensions with protocols
    extension Deferred where T: ResultType {
    func map(
    _ f: @escaping (T.PT) -> OtherData
    ) -> Deferred> {
    return map { result in result.map(f) }
    }
    }
    // WORKS!
    !
    

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62. OptionalType, ResultType,
    ArrayType, DeferredType ...
    

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63. Constrained
    (method-based)
    

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64. extension Deferred {
    func map(
    _ f: @escaping (ResultData) -> OtherData
    ) -> Deferred>
    where DataType == Result {
    return map { result in result.map(f) }
    }
    }
    

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65. extension Deferred {
    func map(
    _ f: @escaping (ResultData) -> OtherData
    ) -> Deferred>
    where DataType == Result {
    return map { result in result.map(f) }
    }
    }
    ! "
    

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66. func repoNames(forUser user: String)
    -> Deferred, NetworkAPIError>> {
    return Deferred {
    // (user: String) -> Result
    // fetching
    }.map {
    (data: Data) -> Result in
    // deserialization
    }.map {
    (repos: [Repo]) -> Result in
    // validation
    }.map {
    (repo: Repo) -> String in
    // presentation
    }
    

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67. Does it really prevent
    !
    ?
    

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68. Too many transformers?
    4 solutions
    

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69. 1. Stick to what you use
    You may then give them nice names like mapInternalResult
    

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70. 2. Generate them all
    Elviro Rocca
    Protocol-Oriented Monad Transformers
    https://www.youtube.com/watch?v=Zmb86zblcto
    Generation using Sourcery
    

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71. 3. Use wide types
    Observable
    ==
    Future>
    

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72. 4. Check out Higher-Kinded Types
    Yasuhiro Inami
    https://github.com/inamiy/HigherKindSwift
    

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73. 1. use types for fun and profit
    2. for nested types, do
    - transform data model
    - discard some information
    - provide transformers
    3. write transformers either/or
    - protocol-based ("rasure")
    - method-based (constraints on methods)
    4. when too many transformers
    - write only what you need
    - generate (Sourcery may help)
    - use wide types (like Observable)
    

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74. May the types be with you!
    

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75. Thanks
    Questions?
    

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