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
2. PABLO GUARDIOLA Building a DSL… In Kotlin
3. Building a DSL… In Kotlin With KΛTEGORY PABLO GUARDIOLA
4. key concepts @Guardiola31337
5. key concepts @Guardiola31337 Composable
6. key concepts @Guardiola31337 Composable Extensible
7. key concepts @Guardiola31337 Composable Extensible Testable
8. Composable Extensible Testable Integration w/ current frameworks key concepts @Guardiola31337
9. dsl @Guardiola31337
10. dsl (noun) “A computer programming language of limited expressiveness focused on a particular domain.” @Guardiola31337
11. dsl @Guardiola31337 Programming language
12. dsl @Guardiola31337 Programming language Language nature
13. dsl @Guardiola31337 Programming language Language nature Limited expressiveness
14. Programming language Language nature Limited expressiveness Domain focus dsl @Guardiola31337
15. uiagesturegen @Guardiola31337
16. uiagesturegen KΛTEGORY @Guardiola31337
17. usage @Guardiola31337
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
19. @Guardiola31337
20. free dsl @Guardiola31337
21. free dsl @Guardiola31337 Define your algebra (a.k.a. operations)
22. algebra @higherkind sealed class GesturesDSL<A> : GesturesDSLKind<A> { data class WithView<A>(val f: (UiObject) -> A) : GesturesDSL<A>() object Click : GesturesDSL<Unit>() object DoubleTap : GesturesDSL<Unit>() data class PinchIn(val percent: Int, val steps: Int) : GesturesDSL<Unit>() // ... companion object : FreeApplicativeApplicativeInstance<GesturesDSLHK> } @Guardiola31337
23. algebra @higherkind sealed class GesturesDSL<A> : GesturesDSLKind<A> { data class WithView<A>(val f: (UiObject) -> A) : GesturesDSL<A>() object Click : GesturesDSL<Unit>() object DoubleTap : GesturesDSL<Unit>() data class PinchIn(val percent: Int, val steps: Int) : GesturesDSL<Unit>() // ... companion object : FreeApplicativeApplicativeInstance<GesturesDSLHK> } @Guardiola31337
24. algebra @higherkind sealed class GesturesDSL<A> : GesturesDSLKind<A> { data class WithView<A>(val f: (UiObject) -> A) : GesturesDSL<A>() object Click : GesturesDSL<Unit>() object DoubleTap : GesturesDSL<Unit>() data class PinchIn(val percent: Int, val steps: Int) : GesturesDSL<Unit>() // ... companion object : FreeApplicativeApplicativeInstance<GesturesDSLHK> } @Guardiola31337
25. algebra @higherkind sealed class GesturesDSL<A> : GesturesDSLKind<A> { data class WithView<A>(val f: (UiObject) -> A) : GesturesDSL<A>() object Click : GesturesDSL<Unit>() object DoubleTap : GesturesDSL<Unit>() data class PinchIn(val percent: Int, val steps: Int) : GesturesDSL<Unit>() // ... companion object : FreeApplicativeApplicativeInstance<GesturesDSLHK> } @Guardiola31337
26. free dsl @Guardiola31337 Define your algebra (a.k.a. operations) Lift everything to the context of free
27. lift fun <A> withView(f: (UiObject) -> A): ActionDSL<A> = FreeApplicative.liftF(GesturesDSL.WithView(f)) // ... fun ActionDSL<Unit>.click(): ActionDSL<Unit> = combine(this, com.pguardiola.uigesturegen.dsl.click()) fun click(): ActionDSL<Unit> = FreeApplicative.liftF(GesturesDSL.Click) // ... @Guardiola31337
28. lift fun <A> withView(f: (UiObject) -> A): ActionDSL<A> = FreeApplicative.liftF(GesturesDSL.WithView(f)) // ... fun ActionDSL<Unit>.click(): ActionDSL<Unit> = combine(this, com.pguardiola.uigesturegen.dsl.click()) fun click(): ActionDSL<Unit> = FreeApplicative.liftF(GesturesDSL.Click) // ... @Guardiola31337
29. lift fun ActionDSL<Unit>.doubleTap(): ActionDSL<Unit> = combine(this, com.pguardiola.uigesturegen.dsl.doubleTap()) fun doubleTap(): ActionDSL<Unit> = FreeApplicative.liftF(GesturesDSL.DoubleTap) fun ActionDSL<Unit>.pinchIn(percent: Int, steps: Int): ActionDSL<Unit> = combine(this, com.pguardiola.uigesturegen.dsl.pinchIn(percent, steps)) fun pinchIn(percent: Int, steps: Int): ActionDSL<Unit> = FreeApplicative.liftF(GesturesDSL.PinchOut(percent, steps)) // ... @Guardiola31337
30. lift fun ActionDSL<Unit>.doubleTap(): ActionDSL<Unit> = combine(this, com.pguardiola.uigesturegen.dsl.doubleTap()) fun doubleTap(): ActionDSL<Unit> = FreeApplicative.liftF(GesturesDSL.DoubleTap) fun ActionDSL<Unit>.pinchIn(percent: Int, steps: Int): ActionDSL<Unit> = combine(this, com.pguardiola.uigesturegen.dsl.pinchIn(percent, steps)) fun pinchIn(percent: Int, steps: Int): ActionDSL<Unit> = FreeApplicative.liftF(GesturesDSL.PinchOut(percent, steps)) // ... @Guardiola31337
31. free dsl @Guardiola31337 Define your algebra (a.k.a. operations) Lift everything to the context of free Provide semantics with the interpreter
32. interpreter fun safeInterpreter(view: UiObject): FunctionK<GesturesDSLHK, ValidatedKindPartial<NonEmptyList<GestureError>>> = object : FunctionK<GesturesDSLHK, ValidatedKindPartial<NonEmptyList<GestureError>>> { override fun <A> invoke(fa: GesturesDSLKind<A>): ValidatedKind<NonEmptyList<GestureError>, A> { // ... return when (g) { is GesturesDSL.Click -> view.click().validate({ GestureError.ClickError(view) }) // ... } as ValidatedKind<NonEmptyList<GestureError>, A> } } @Guardiola31337
33. interpreter fun loggingInterpreter(view: UiObject): FunctionK<GesturesDSLHK, EitherKindPartial<GestureError>> = object : FunctionK<GesturesDSLHK, EitherKindPartial<GestureError>> { override fun <A> invoke(fa: GesturesDSLKind<A>): EitherKind<GestureError, A> { println(fa) return safeInterpreterEither(view) as EitherKind<GestureError, A> } } @Guardiola31337
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
35. unfold fun <A> ActionDSL<A>.validate(view: UiObject): ValidatedNel<GestureError, A> = Try { this.foldMap( safeInterpreter(view), Validated.applicative<NonEmptyList<GestureError>>()).ev() }.fold({ GestureError.UnknownError(it).invalidNel() }, { it }) @Guardiola31337
36. unfold fun <A> ValidatedNel<GestureError, A>.hasCompletedCorrectly(): Boolean = this.isValid fun <A> ValidatedNel<GestureError, A>.errors(): List<GestureError> = this.fold({ it.all }, { emptyList() }) @Guardiola31337
37. unfold fun <A> ValidatedNel<GestureError, A>.hasCompletedCorrectly(): Boolean = this.isValid fun <A> ValidatedNel<GestureError, A>.errors(): List<GestureError> = this.fold({ it.all }, { emptyList() }) @Guardiola31337
38. usage private fun composingWithPlus(element: UiObject, pointers: List<Array<MotionEvent.PointerCoords>>): Boolean { val actions = pinchOut(75, 30) + swipeDown(30) + multiTouch(pointers) return actions.failFast(element).hasCompletedCorrectly() } @Guardiola31337
39. @Guardiola31337
40. takeaways @Guardiola31337
41. takeaways @Guardiola31337 Composable
42. takeaways @Guardiola31337 Composable Extensible
43. takeaways @Guardiola31337 Composable Extensible Reusable using different interpreters
44. Composable Extensible Reusable using different interpreters Integration w/ your current stack takeaways @Guardiola31337
45. thank you! PABLO GUARDIOLA
46. questions? PABLO GUARDIOLA
47. we’re hiring! mapbox.com/jobs PABLO GUARDIOLA
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