Building a DSL... In Kotlin

Kotlin is a multipurpose language that makes building domain-specific languages (DSLs) a breeze. But, what is a DSL? A domain-specific language is a small programming dictionary designed specifically to express solutions to problems focused on a particular aspect of a software system. There are a lot of them around, like CSS for styling, HTML for websites, and SQL for querying databases. A well-designed DSL is much easier to program with than a traditional library. This is just one of the nice things about DSLs. Let me show you how I built one in Kotlin.

In this session we will learn how to design a DSL and understand why we should use Functional Programming (FP) and Kotlin to build one. We will analyze Do’s and Don’ts from my experience creating a UI Automator-gesture generation DSL (https://github.com/Guardiola31337/uiagesturegen). Along with Kotlin's power and DSL building facilities, we will explore how we can make this DSL entirely stack safe, composable and reusable under different runtime requirements thanks to KΛTEGORY (https://github.com/kategory/kategory), a library which provides abstractions for FP in Kotlin.

By the end of the talk, you will be itching to write your own!

Transcript

1. PABLO GUARDIOLA
   Building a DSL…
   In Kotlin
   

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2. PABLO GUARDIOLA
   Building a DSL…
   In Kotlin
   

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3. Building a DSL…
   In Kotlin
   With
   KΛTEGORY
   PABLO GUARDIOLA
   

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4. key concepts
   @Guardiola31337
   

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5. key concepts
   @Guardiola31337
   Composable
   

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6. key concepts
   @Guardiola31337
   Composable
   Extensible
   

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7. key concepts
   @Guardiola31337
   Composable
   Extensible
   Testable
   

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8. Composable
   Extensible
   Testable
   Integration w/ current frameworks
   key concepts
   @Guardiola31337
   

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9. dsl
   @Guardiola31337
   

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10. dsl (noun)
    “A computer programming language of limited
    expressiveness focused on a particular domain.”
    @Guardiola31337
    

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11. dsl
    @Guardiola31337
    Programming language
    

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12. dsl
    @Guardiola31337
    Programming language
    Language nature
    

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13. dsl
    @Guardiola31337
    Programming language
    Language nature
    Limited expressiveness
    

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14. Programming language
    Language nature
    Limited expressiveness
    Domain focus
    dsl
    @Guardiola31337
    

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15. uiagesturegen
    @Guardiola31337
    

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16. uiagesturegen
    KΛTEGORY
    @Guardiola31337
    

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17. usage
    @Guardiola31337
    

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18. usage
    private fun composing(element: UiObject): Boolean {
    val actions = pinchOut(100, 50)
    .swipeDown(30)
    .swipeRight(30)
    .pinchOut(100, 50)
    .swipeUp(100)
    return actions.validate(element).hasCompletedCorrectly()
    }
    @Guardiola31337
    

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19. @Guardiola31337
    

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20. free dsl
    @Guardiola31337
    

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21. free dsl
    @Guardiola31337
    Define your algebra (a.k.a. operations)
    

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22. algebra
    @higherkind sealed class GesturesDSL : GesturesDSLKind {
    data class WithView(val f: (UiObject) -> A) : GesturesDSL()
    object Click : GesturesDSL()
    object DoubleTap : GesturesDSL()
    data class PinchIn(val percent: Int, val steps: Int) : GesturesDSL()
    // ...
    companion object : FreeApplicativeApplicativeInstance
    }
    @Guardiola31337
    

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23. algebra
    @higherkind sealed class GesturesDSL : GesturesDSLKind {
    data class WithView(val f: (UiObject) -> A) : GesturesDSL()
    object Click : GesturesDSL()
    object DoubleTap : GesturesDSL()
    data class PinchIn(val percent: Int, val steps: Int) : GesturesDSL()
    // ...
    companion object : FreeApplicativeApplicativeInstance
    }
    @Guardiola31337
    

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24. algebra
    @higherkind sealed class GesturesDSL : GesturesDSLKind {
    data class WithView(val f: (UiObject) -> A) : GesturesDSL()
    object Click : GesturesDSL()
    object DoubleTap : GesturesDSL()
    data class PinchIn(val percent: Int, val steps: Int) : GesturesDSL()
    // ...
    companion object : FreeApplicativeApplicativeInstance
    }
    @Guardiola31337
    

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25. algebra
    @higherkind sealed class GesturesDSL : GesturesDSLKind {
    data class WithView(val f: (UiObject) -> A) : GesturesDSL()
    object Click : GesturesDSL()
    object DoubleTap : GesturesDSL()
    data class PinchIn(val percent: Int, val steps: Int) : GesturesDSL()
    // ...
    companion object : FreeApplicativeApplicativeInstance
    }
    @Guardiola31337
    

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26. free dsl
    @Guardiola31337
    Define your algebra (a.k.a. operations)
    Lift everything to the context of free
    

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27. lift
    fun withView(f: (UiObject) -> A): ActionDSL =
    FreeApplicative.liftF(GesturesDSL.WithView(f))
    // ...
    fun ActionDSL.click(): ActionDSL =
    combine(this, com.pguardiola.uigesturegen.dsl.click())
    fun click(): ActionDSL = FreeApplicative.liftF(GesturesDSL.Click)
    // ...
    @Guardiola31337
    

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28. lift
    fun withView(f: (UiObject) -> A): ActionDSL =
    FreeApplicative.liftF(GesturesDSL.WithView(f))
    // ...
    fun ActionDSL.click(): ActionDSL =
    combine(this, com.pguardiola.uigesturegen.dsl.click())
    fun click(): ActionDSL = FreeApplicative.liftF(GesturesDSL.Click)
    // ...
    @Guardiola31337
    

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29. lift
    fun ActionDSL.doubleTap(): ActionDSL =
    combine(this, com.pguardiola.uigesturegen.dsl.doubleTap())
    fun doubleTap(): ActionDSL = FreeApplicative.liftF(GesturesDSL.DoubleTap)
    fun ActionDSL.pinchIn(percent: Int, steps: Int): ActionDSL =
    combine(this, com.pguardiola.uigesturegen.dsl.pinchIn(percent, steps))
    fun pinchIn(percent: Int, steps: Int): ActionDSL =
    FreeApplicative.liftF(GesturesDSL.PinchOut(percent, steps))
    // ...
    @Guardiola31337
    

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30. lift
    fun ActionDSL.doubleTap(): ActionDSL =
    combine(this, com.pguardiola.uigesturegen.dsl.doubleTap())
    fun doubleTap(): ActionDSL = FreeApplicative.liftF(GesturesDSL.DoubleTap)
    fun ActionDSL.pinchIn(percent: Int, steps: Int): ActionDSL =
    combine(this, com.pguardiola.uigesturegen.dsl.pinchIn(percent, steps))
    fun pinchIn(percent: Int, steps: Int): ActionDSL =
    FreeApplicative.liftF(GesturesDSL.PinchOut(percent, steps))
    // ...
    @Guardiola31337
    

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31. free dsl
    @Guardiola31337
    Define your algebra (a.k.a. operations)
    Lift everything to the context of free
    Provide semantics with the interpreter
    

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32. interpreter
    fun safeInterpreter(view: UiObject): FunctionK>> =
    object : FunctionK>> {
    override fun invoke(fa: GesturesDSLKind): ValidatedKind, A> {
    // ...
    return when (g) {
    is GesturesDSL.Click -> view.click().validate({ GestureError.ClickError(view) })
    // ...
    } as ValidatedKind, A>
    }
    }
    @Guardiola31337
    

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33. interpreter
    fun loggingInterpreter(view: UiObject): FunctionK> =
    object : FunctionK> {
    override fun invoke(fa: GesturesDSLKind): EitherKind {
    println(fa)
    return safeInterpreterEither(view) as EitherKind
    }
    }
    @Guardiola31337
    

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34. Define your algebra (a.k.a. operations)
    Lift everything to the context of free
    Provide semantics with the interpreter
    Execute the AST using the interpreter
    free dsl
    @Guardiola31337
    

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35. unfold
    fun ActionDSL.validate(view: UiObject): ValidatedNel =
    Try {
    this.foldMap(
    safeInterpreter(view),
    Validated.applicative>()).ev()
    }.fold({ GestureError.UnknownError(it).invalidNel() }, { it })
    @Guardiola31337
    

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36. unfold
    fun ValidatedNel.hasCompletedCorrectly(): Boolean =
    this.isValid
    fun ValidatedNel.errors(): List =
    this.fold({ it.all }, { emptyList() })
    @Guardiola31337
    

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37. unfold
    fun ValidatedNel.hasCompletedCorrectly(): Boolean =
    this.isValid
    fun ValidatedNel.errors(): List =
    this.fold({ it.all }, { emptyList() })
    @Guardiola31337
    

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38. usage
    private fun composingWithPlus(element: UiObject,
    pointers: List>): Boolean {
    val actions = pinchOut(75, 30) + swipeDown(30) + multiTouch(pointers)
    return actions.failFast(element).hasCompletedCorrectly()
    }
    @Guardiola31337
    

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39. @Guardiola31337
    

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40. takeaways
    @Guardiola31337
    

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41. takeaways
    @Guardiola31337
    Composable
    

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42. takeaways
    @Guardiola31337
    Composable
    Extensible
    

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43. takeaways
    @Guardiola31337
    Composable
    Extensible
    Reusable using different interpreters
    

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44. Composable
    Extensible
    Reusable using different interpreters
    Integration w/ your current stack
    takeaways
    @Guardiola31337
    

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45. thank you!
    PABLO GUARDIOLA
    

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46. questions?
    PABLO GUARDIOLA
    

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47. we’re hiring!
    mapbox.com/jobs
    PABLO GUARDIOLA
    

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48. kategory.io
    https://github.com/Guardiola31337/uiagesturegen
    https://github.com/Kotlin/KEEP/pull/87
    https://github.com/JorgeCastilloPrz/KotlinAndroidFunctional
    Domain-Specific Languages. Martin Fowler.
    Special shout out to Raúl Raja, Paco Estévez and Jorge Castillo
    references
    @Guardiola31337
    

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