Container types in Swift without 🤯

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Container types without ! Krzysztof Siejkowski / @_siejkowski

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No content

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Questions are fine Seriously They're fine

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Types

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Types?

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Type

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Optional

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Array

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Result

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Result !

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Result !

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

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Deferred

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Writer

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Observable

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Types? !

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Types!

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

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

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Types ❤

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Types !

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

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

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

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Deferred>

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

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

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

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

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

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

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

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

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3 solutions

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#1 Redesign your data model

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Errors are part of your data model

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

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

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

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

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

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

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#2 Discard information

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

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

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Deferred< Result< Result< Result<[String], RepoError>, RepoError >, NetworkAPIError > >

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Deferred< Result< Result<[String], RepoError>, NetworkAPIError > >

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#3 Provide transformers

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

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// 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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Generic ! type constraints

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// 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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// 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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// 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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2 ways of writing transformers

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Rasure (protocol-based)

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Rasure == Inversion of type erasure

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

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

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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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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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OptionalType, ResultType, ArrayType, DeferredType ...

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Constrained (method-based)

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

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

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

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Does it really prevent ! ?

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Too many transformers? 4 solutions

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

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

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3. Use wide types Observable == Future>

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

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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)