Kotlin DSLs and How to Build Them

Domain Specific Language (DSL) using Kotlin are increasingly becoming a prominent part of the impact of the language. They can be used to write type-safe, structured code while keeping it highly readable.

In this talk, I go through what a DSL is, why Kotlin makes it relevant and the important Koltin features that allow us to write great DSLs. I also go through the process of creating a custom DSL and things to keep in mind while making one. By the end of this talk, the attendees will have a good understanding of DSL in Kotlin and how to create one for their use case.

Transcript

1. Kotlin DSLs and How To Build
   Them
   Subhrajyoti Sen
   smallcase
   

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2. DSL??
   

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3. Domain Specific Language
   A DSL is a computer language specialized to a particular
   application domain. This is contrast to a general-purpose language
   (GPL), which is broadly applicable across domains.
   -Wikipedia
   

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4. Examples - HTML
   
   
   My First Heading
   My first paragraph.
   
   
   
   
   An Unordered List
   
   Coffee
   Tea
   
   
   
   

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5. Examples - CSS
   div {
   background-color: lightgrey;
   width: 300px;
   padding: 50px;
   }
   h1 {
   font-size: 40px;
   }
   h2 {
   font-size: 30px;
   }
   

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6. Examples - SQL
   SELECT * FROM Customers WHERE Last_Name=”Sen”
   SELECT First_Name, Nickname FROM Friends WHERE Nickname like ‘%brain%’
   

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7. Where’s my Kotlin?
   

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8. Examples - HTML
   val html = document.create.html {
   head {
   title("Hello world")
   }
   body {
   h1("h1Class"){
   +"My header1"
   }
   }
   }
   https://github.com/Kotlin/kotlinx.html
   

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9. Examples - CSS
   val css = Stylesheet {
   a {
   width = 10.px
   color = 0xffffff
   hover {
   color = 0xf2cacf
   }
   }
   }
   https://github.com/olegcherr/Aza-Kotlin-CSS
   

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10. Examples - SQL
    val row = from(Friends)
    .where { Nickname like "brian" }
    .select(Friends.First_name, Friends.Nickname)
    https://github.com/orangy/squash
    

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11. Examples - SQL
    https://github.com/arunkumar9t2/transition-x
    

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12. Advantages
    There must be a reason DSL
    exists and developers write
    customs DSLs
    

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13. Advantages
    There must be a reason DSL
    exists and developers write
    customs DSLs
    ● Expressive
    

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14. Advantages
    There must be a reason DSL
    exists and developers write
    customs DSLs
    ● Expressive
    ● Readable
    

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15. Advantages
    There must be a reason DSL
    exists and developers write
    customs DSLs
    ● Expressive
    ● Readable
    ● Concise - Less Boilerplate
    

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16. conference {
    name = "DevConfIN 19"
    location = "Bengaluru"
    room {
    roomNumber = 1
    floor = 2
    speaker {
    name = "Subhrajyoti Sen"
    talkTitle = "Intro to Kotlin DSL"
    }
    }
    room {
    roomNumber = 2
    floor = 3
    speaker {
    name = "Sayantika Banik"
    talkTitle = "Open Source"
    }
    }
    }
    

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17. Why Kotlin?
    Kotlin provides certain features
    that let us develop expressive
    and structured DSL
    ● Lambda with Receiver
    ● Lambda as method parameter
    ● Extension function
    

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18. val speaker = speaker {
    name = "Subhrajyoti Sen"
    talkTitle = "Intro to Kotlin DSL"
    }
    

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19. val speaker = speaker {
    name = "Subhrajyoti Sen"
    talkTitle = "Intro to Kotlin DSL"
    }
    Function
    

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20. data class SpeakerModel(
    var name: String = “”,
    var talkTitle: String = “”
    )
    

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21. fun speaker() {
    }
    

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22. fun speaker(): SpeakerModel {
    }
    

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23. fun speaker(): SpeakerModel {
    val speaker = SpeakerModel()
    }
    

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24. fun speaker(): SpeakerModel {
    val speaker = SpeakerModel()
    return speaker
    }
    

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25. fun speaker(block: (SpeakerModel) -> Unit): SpeakerModel {
    val speaker = SpeakerModel()
    return speaker
    }
    

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26. Lambda
    

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27. Lambda
    Lambdas are code blocks enclosed in curly braces.
    

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28. Lambda
    Lambdas are code blocks enclosed in curly braces.
    val multiply: (Int, Int) -> Int
    

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29. Lambda
    Lambdas are code blocks enclosed in curly braces.
    val multiply: (Int, Int) -> Int
    val multiply = { a: Int, b: Int -> a*b }
    

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30. Lambda
    Lambdas are code blocks enclosed in curly braces.
    val multiply: (Int, Int) -> Int
    val multiply = { a: Int, b: Int -> a*b }
    println(multiply(3,4))
    

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31. fun speaker(block: (SpeakerModel) -> Unit): SpeakerModel {
    val speaker = SpeakerModel()
    return speaker
    }
    

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32. fun speaker(block: (SpeakerModel) -> Unit): SpeakerModel {
    val speaker = SpeakerModel()
    block(speaker)
    return speaker
    }
    

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33. val speaker = speaker({
    it.name = "Subhrajyoti Sen"
    it.talkTitle = "Intro to Kotlin DSL"
    })
    

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34. fun speaker(block: (SpeakerModel) -> Unit): SpeakerModel {
    val speaker = SpeakerModel()
    block(speaker)
    return speaker
    }
    

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35. fun speaker(block: SpeakerModel.() -> Unit): SpeakerModel {
    val speaker = SpeakerModel()
    block(speaker)
    return speaker
    }
    

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36. fun speaker(block: SpeakerModel.() -> Unit): SpeakerModel {
    val speaker = SpeakerModel()
    block(speaker)
    return speaker
    }
    Receiver Lambda
    

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37. fun speaker(block: SpeakerModel.() -> Unit): SpeakerModel {
    val speaker = SpeakerModel()
    speaker.block()
    return speaker
    }
    

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38. val speaker = speaker({
    this.name = "Subhrajyoti Sen"
    this.talkTitle = "Intro to Kotlin DSL"
    })
    

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39. val speaker = speaker({
    name = "Subhrajyoti Sen"
    talkTitle = "Intro to Kotlin DSL"
    })
    

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40. val speaker = speaker({
    name = "Subhrajyoti Sen"
    talkTitle = "Intro to Kotlin DSL"
    })
    Extra bracket?
    

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41. Lambda as a Parameter
    fun modify(a: Int, lambda: (Int) -> Int)
    

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42. Lambda as a Parameter
    fun modify(a: Int, lambda: (Int) -> Int)
    modify(3, someLambda)
    

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43. Lambda as a Parameter
    fun modify(a: Int, lambda: (Int) -> Int)
    modify(3, someLambda)
    modify(3) {
    }
    

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44. val speaker = speaker({
    name = "Subhrajyoti Sen"
    talkTitle = "Intro to Kotlin DSL"
    })
    

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45. val speaker = speaker {
    name = "Subhrajyoti Sen"
    talkTitle = "Intro to Kotlin DSL"
    }
    

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46. room {
    roomNumber = 1
    floor = 2
    speaker {
    name = "Subhrajyoti Sen"
    talkTitle = "Intro to Kotlin DSL"
    }
    }
    

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47. data class RoomModel(
    var roomNumber: Int = 0,
    var floor: Int = 99,
    var speaker: SpeakerModel? = null
    )
    

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48. fun room(block: RoomModel.() -> Unit): RoomModel {
    val room = RoomModel()
    room.block()
    return room
    }
    

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49. room {
    roomNumber = 1
    floor = 2
    speaker {
    name = "Subhrajyoti Sen"
    talkTitle = "Intro to Kotlin DSL"
    }
    }
    

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50. But there’s a problem
    
    

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51. room {
    roomNumber = 1
    floor = 2
    speaker {
    name = "Subhrajyoti Sen"
    talkTitle = "Intro to Kotlin DSL"
    }
    }
    

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52. Extension Function
    A function that extends the functionality without modifying it
    fun Banner.isVisible(): Boolean {
    return this.visibility == Banner.VISIBLE
    }
    

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53. Extension Function
    A function that extends the functionality without modifying it
    fun Banner.isVisible(): Boolean {
    return this.visibility == Banner.VISIBLE
    }
    Base class
    

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54. Extension Function
    A function that extends the functionality without modifying it
    fun Banner.isVisible(): Boolean {
    return this.visibility == Banner.VISIBLE
    }
    Base class Extension function
    

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55. fun speaker(block: SpeakerModel.() -> Unit): SpeakerModel {
    val speaker = SpeakerModel()
    speaker.apply(block)
    return speaker
    }
    

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56. fun speaker(block: SpeakerModel.() -> Unit) {
    val speaker = SpeakerModel()
    speaker.apply(block)
    return speaker
    }
    

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57. fun speaker(block: SpeakerModel.() -> Unit) {
    val speaker = SpeakerModel()
    speaker.apply(block)
    }
    

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58. fun RoomModel.speaker(block: SpeakerModel.() -> Unit) {
    val speaker = SpeakerModel()
    speaker.apply(block)
    }
    

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59. fun RoomModel.speaker(block: SpeakerModel.() -> Unit) {
    val speaker = SpeakerModel()
    speaker.apply(block)
    this.speaker = speaker
    }
    

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60. room {
    roomNumber = 1
    floor = 2
    speaker {
    name = "Subhrajyoti Sen"
    talkTitle = "Intro to Kotlin DSL"
    }
    }
    

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61. val conference = conference {
    name = "DevConfIN 19"
    location = "Bengaluru"
    room {
    roomNumber = 1
    floor = 2
    speaker {
    name = "Subhrajyoti Sen"
    talkTitle = "Intro to Kotlin DSL"
    }
    }
    }
    

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62. data class ConferenceModel(
    var name: String = "",
    var location: String = "",
    var rooms: MutableList = mutableListOf()
    )
    

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63. data class ConferenceModel(
    var name: String = "",
    var location: String = "",
    var rooms: MutableList = mutableListOf()
    )
    Create an empty mutable list
    

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64. fun conference(block: ConferenceModel.() -> Unit):
    ConferenceModel {
    val conference = ConferenceModel()
    conference.apply(block)
    return conference
    }
    

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65. fun ConferenceModel.room(block: RoomModel.() -> Unit) {
    val room = RoomModel()
    room.apply(block)
    this.rooms.add(room)
    }
    

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66. conference {
    name = "DevConfIN 19"
    location = "Bengaluru"
    room {
    roomNumber = 1
    floor = 2
    speaker {
    name = "Subhrajyoti Sen"
    talkTitle = "Intro to Kotlin DSL"
    }
    }
    room {
    roomNumber = 2
    floor = 3
    speaker {
    name = "Sayantika Banik"
    talkTitle = "Open Source"
    }
    }
    }
    

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67. Not yet done
    

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68. conference {
    name = "DevConfIN 19"
    location = "Bengaluru"
    room {
    roomNumber = 1
    floor = 2
    speaker {
    name = "Subhrajyoti Sen"
    talkTitle = "Intro to Kotlin DSL"
    }
    }
    }
    

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69. conference {
    name = "DevConfIN 19"
    location = "Bengaluru"
    room {
    roomNumber = 1
    speaker {
    name = "Subhrajyoti Sen"
    talkTitle = "Intro to Kotlin DSL"
    floor = 2
    }
    }
    }
    

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70. This actually compiles
    and works
    
    

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71. conference {
    name = "DevConfIN 19"
    location = "Bengaluru"
    room {
    roomNumber = 1
    speaker {
    name = "Subhrajyoti Sen"
    talkTitle = "Intro to Kotlin DSL"
    floor = 2
    }
    }
    }
    

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72. conference {
    name = "DevConfIN 19"
    location = "Bengaluru"
    room {
    roomNumber = 1
    speaker {
    name = "Subhrajyoti Sen"
    talkTitle = "Intro to Kotlin DSL"
    floor = 2
    }
    }
    }
    Still inside the scope of room
    

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73. Scope Control - DSL Marker
    The lambda should be able to access only it’s nearest implicit receiver.
    

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74. Scope Control - DSL Marker
    The lambda should be able to access only it’s nearest implicit receiver.
    @DslMarker
    @Target(AnnotationTarget.TYPE)
    annotation class ConfDSL
    

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75. fun RoomModel.speaker(block: SpeakerModel.() -> Unit) {
    // ...
    }
    fun ConferenceModel.room(block: RoomModel.() -> Unit) {
    // ...
    }
    fun conference(block: ConferenceModel.() -> Unit): ConferenceModel {
    // ...
    }
    

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76. fun RoomModel.speaker(block: (@ConfDSL SpeakerModel).() -> Unit) {
    // ...
    }
    fun ConferenceModel.room(block: (@ConfDSL RoomModel).() -> Unit) {
    // ...
    }
    fun conference(block: (@ConfDSL ConferenceModel).() -> Unit):
    ConferenceModel {
    // ...
    }
    

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77. To conclude, DSLs are
    awesome
    
    

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78. Questions?
    http://bit.ly/DevConfDSL
    

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79. Thank you
    SubhrajyotiSen
    iamsubhrajyoti
    

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