Kotlin for the Pragmatic Functionalist

Many people are migrating their backends to Kotlin from other JVM languages, lured by the simplicity and expressivity of the language. The Android community embraced it even before release because it healed many longstanding wounds.

So, what can a functional programmer find when migrating to Kotlin? In this talk we'll cover several of the common constructs that are part of any functional stack and how they have already been introduced into the language.

For the people interested in infrastructure, we will also go through some design choices and limitations that JetBrains provided to keep Kotlin within a sane range of pragmatism.

Kategory: http://kategory.io

Transcript

1. @pacoworks
   Paco Estevez
   Kotlin for the
   Pragmatic Functionalist
   

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2. @pacoworks
   Kotlin logo: © 2015-present - JetBrains
   KotlinConf logo: © 2017 - JetBrains
   Kategory logo: © 2017–present - The Kategory maintainers
   ReasonML logo: © 2015–present - Facebook Inc.
   Flow logo: © 2014–present - Facebook Inc.
   2
   Special thanks to Facebook for allowing me to
   work on this on my spare time. Kategory is not
   an internal Facebook project, nor it is an
   incubator. It is an external project of community
   contributors.
   
   In any case facebookers are bunch of devs
   excited about Kotlin too!
   

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3. @pacoworks
   Kotlin is ready for Functional Programming
   TODAY
   We’re growing
   We’re improving
   We want all of you
   3
   

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4. @pacoworks
   Our community goal
   Use of Kotlin in JS, Native, JVM
   One codebase for mobile, backend, frontend
   Same idioms across languages and ecosystems
   4
   The objective of this talk is to give you a
   peek bleeding edge technologies for
   Kotlin, and what future language
   features will look like.
   

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5. @pacoworks
   Where is Kotlin today
   What are we bringing into the ecosystem
   Where would we like to be
   5
   

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6. @pacoworks
   Kotlin today
   6
   

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7. @pacoworks
   Key cornerstones
   Data classes, sealed classes
   Inliner
   Extension functions
   Type aliases
   Enhanced resolution of generic parameters
   7
   

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8. @pacoworks
   Data & sealed classes
   Plain information
   Stores specifics
   Closed and immutable by
   default
   8
   Pure behavior
   Indicates branching in
   execution
   Checked at compile time
   Data Sealed
   

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9. @pacoworks
   Inliner
   Copies a function in its call site
   Inline functions’ body is copied at compile time
   All call sites with static dispatch are known at
   compile time
   Enables runtime generics
   9
   Reified
   

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10. @pacoworks
    Extension functions
    Converts static functions into object methods
    All call sites with static dispatch are known at
    compile time
    Can extend just for specific generic parameters
    10
    

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11. @pacoworks
    Kotlin today
    with Kategory
    11
    

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12. @pacoworks
    Key features
    Extension Interfaces
    Ad-hoc polymorphism for generic type constructors
    Imperative async code
    Smarter boilerplate removal
    12
    

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13. @pacoworks
    Extension Interfaces
    Implement any interface for already existing types
    Use the extension interface as a parameter
    WIP: automatic extension interface lookup
    13
    Extension protocols
    

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14. @pacoworks
    Extension Interface
    Jsonable
    interface Jsonable {
    fun toJson(T element): String
    fun fromJson(String json): T
    }
    fun jsonable(): Jsonable =
    // lookup code
    14
    

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15. @pacoworks
    Extension Interface
    Jsonable
    object EmployeeJsonable: Jsonable {
    val gson = Gson()
    fun toJson(Employee element): String =
    gson.toJsonString(element)
    fun fromJson(String json): Employee =
    gson.fromJsonString(json)
    }
    15
    

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16. @pacoworks
    Extension Interface
    Jsonable
    fun parser(
    company: Company,
    parser: Jsonable = jsonable()
    ): String =
    parser.toJson(company)
    16
    Note the lookup code
    And if this seems like a lot of work, we
    have a KEEP to integrate a simpler
    version in the language
    

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17. @pacoworks
    KEEP-87
    17
    Introduce Extension Interfaces
    into the language under the name
    “Type Classes”
    Compile time lookup & injection
    Aim for parity with Swift’s
    Extension Protocols
    

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18. @pacoworks
    Ad-hoc polymorphism for
    generic type constructors
    Functions that support any generic type constructor
    Express agnosticity to implementation
    Safe downcast from generic to implementation
    18
    

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19. @pacoworks
    Generic type constructor
    interface Hk
    F is the type of the
    container
    A is the type of the
    content
    19
    We’re going to replace the F
    

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20. @pacoworks
    Generic constructor
    implementations
    sealed class Option: Hk
    data class ListWrap(…): Hk
    sealed class Try : Hk
    F becomes a “tag” type
    A remains as the type of
    the content
    20
    

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21. @pacoworks
    Returning generic type
    constructors
    fun getUserById(
    id: String
    ): Hk =
    ???
    21
    

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22. @pacoworks
    Extension Interface
    Factory
    interface Factory {
    fun create(element: A): Hk
    }
    fun factory(): Factory =
    // lookup code
    22
    

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23. @pacoworks
    Polymorphism for generic
    type constructors
    fun getUserById(
    id: String,
    factory: Factory = factory()
    ): Hk =
    factory.create(getUser(id))
    23
    How do I convert that F back to a real
    value?
    

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24. @pacoworks
    Safely downcasting
    generic containers
    24
    Extension functions can be defined to require just
    some of the generic parameters
    Extension functions are resolved at compile time
    

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25. @pacoworks
    Safely downcasting
    generic containers
    25
    fun Hk.ev() =
    this as Option
    fun Hk.ev() =
    this as Try
    

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26. @pacoworks
    Ad-hoc polymorphism for
    generic constructors
    26
    val a: Option = getUserById(“123”).ev()
    val b: Try = getUserById(“123”).ev()
    Lookup overhead: O(1) after first
    lookup. Ideally it should be 0 because
    the compiler does it for us.
    

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27. @pacoworks
    Ad-hoc polymorphism for
    generic constructors
    27
    object OptionFactory: Factory {
    fun create(element: A): Hk =
    Option.Some(a)
    }
    object TryFactory: Factory {
    fun create(element: A): Hk =
    Try { a }
    }
    <--
    global
    lookup!
    

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28. @pacoworks
    KindedJ
    28
    Initiative to share generic type
    constructors under the name
    “Higher Kinded Types”
    We’re speaking with other FP
    libraries for Java & eta-lang
    

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29. @pacoworks
    Imperative async code
    Express asynchronous sequential and parallel
    execution as if they were synchronous
    For any existing framework and abstraction
    29
    

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30. @pacoworks
    Regular async code
    30
    fun getUserFriends(
    id: String
    ): Observable> =
    getUserById(id).flatMap { user ->
    Observable.merge(
    user.friends.map { friend ->
    getUserById(friend.id)
    }
    ).toList()
    }
    

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31. @pacoworks
    Imperative async code
    31
    fun getUserFriends(
    id: String,
    coroutinable: Coroutinable = coroutinable()
    ): Hk> =
    coroutinable.bindingE {
    val user = getUserById(id).bind()
    val friendProfiles = user.friends.map { getUserById(it.id).bind() }
    yields(friendProfiles)
    }
    

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32. @pacoworks
    Ad-hoc polymorphism for
    imperative async code
    32
    val a: Either> = getUserFriends(“123”).ev()
    val b: Try> = getUserFriends(“123”).ev()
    val c: ObservableKW> = getUserFriends(“123”).ev()
    val d: IO> = getUserFriends(“123”).ev()
    

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33. @pacoworks
    Coroutinable?
    33
    typealias Coroutinable = MonadError
    

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34. @pacoworks
    Effects with MonadError
    Thin Extension Interface that can be implemented by
    any existing framework & abstraction
    With error handling, stack safety, threading &
    cancellation out of the box
    Current integrations: RxJava, kotlin.coroutines, IO
    Potential integrations: CompletableFuture,
    kotlinx.coroutines, AsyncTask, ArchComponents,
    Kovenant…
    34
    

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35. @pacoworks
    Implementations in Kategory
    Error handling: Option, Try, Validated, Either, Ior
    Collections: List, Sequence, Map, Set
    RWS: Reader, Writer, State
    Transformers: ReaderT, WriterT, OptionT, StateT,
    EitherT
    Evaluation: Eval, Trampoline, Free, Function0
    Others: Coproduct, Coreader, Const...
    35
    What if I want my own?
    

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36. @pacoworks
    Smarter boilerplate removal
    36
    Eugenio Marletti’s collaboration
    & mentorship
    Generate boilerplate for all the
    features show using annotations
    

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37. @pacoworks
    Bye bye boring code
    37
    Generic constructors
    @higherkind
    Extension Interfaces
    @derives
    @instance
    Manipulation of immutable data & sealed classes
    kategory-optics by Simon Vergauwen So how did we reach this point?
    

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38. @pacoworks
    Challenges & Pitfalls
    IDE resolution of deeply nested generics
    Scopes with ambiguity for `it` and `this`
    Reified generics are “contagious”
    Unexpected behavior caused by inlining
    Generics and interoperating with Java
    Error recovery in coroutines
    38
    And many more, come talk to us!
    

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39. @pacoworks
    Where would we
    like to be
    39
    2018
    

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40. @pacoworks
    40
    scala_logo.svg
    Swift5
    _0.png
    ELM

    LANG.
    svg
    haskell_lambda
    .png
    rustPro
    mo.png
    EtaLang
    .png
    F-sharp-
    main.png
    

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41. @pacoworks
    Bring your tools in
    Threading
    Databases
    Network
    Error Handling
    Reactive Programming
    41
    

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42. @pacoworks
    Bring your tools in
    Threading
    Databases
    Network
    Error Handling
    Reactive Programming
    42
    Parsers
    Compilers
    Static analyzers
    UI frameworks
    Math & statistics
    

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43. @pacoworks
    Bring your tools in
    Threading
    Databases
    Network
    Error Handling
    Reactive Programming
    43
    Parsers
    Compilers
    Static analyzers
    UI frameworks
    Math & statistics
    Data science
    Machine learning
    Videogames
    Computer vision
    Serialization
    

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44. @pacoworks
    Bring your tools in
    Threading
    Databases
    Network
    Error Handling
    Reactive Programming
    44
    Parsers
    Compilers
    Static analyzers
    UI frameworks
    Math & statistics
    Data science
    Machine learning
    Videogames
    Computer vision
    Serialization
    Category Theory
    Web frameworks
    New Architectures Package management
    Collection libraries
    Stream pipelines
    

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45. @pacoworks
    We’re growing our tools
    We’re improving our ecosystem
    We want all of you to be part of it
    45
    

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46. @pacoworks
    Kategory: kategory.io
    Introduction to Kategory:
    tinyurl.com/KatIntro
    KEEP-87:
    github.com/Kotlin/KEEP/pull/87
    Gitter: gitter.im/kategory
    #kategory in KotlinLang
    #kategory in ASG pacoworks.com
    @pacoworks
    github.com/pakoito
    Slides: tinyurl.com/FunKotlinSF17
    46
    Special thanks to all Kategory
    contributors & supporters!
    

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