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.

1. Kotlin DSLs and How To Build Them Subhrajyoti Sen smallcase
2. DSL??
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
4. Examples - HTML <html> <body> <h1>My First Heading</h1> <p>My first paragraph.</p> </body> </html> <html> <body> <h2>An Unordered List</h2> <ul> <li>Coffee</li> <li>Tea</li> </ul> </body> </html>
5. Examples - CSS div { background-color: lightgrey; width: 300px; padding: 50px; } h1 { font-size: 40px; } h2 { font-size: 30px; }
6. Examples - SQL SELECT * FROM Customers WHERE Last_Name=”Sen” SELECT First_Name, Nickname FROM Friends WHERE Nickname like ‘%brain%’
7. Where’s my Kotlin?
8. Examples - HTML val html = document.create.html { head { title("Hello world") } body { h1("h1Class"){ +"My header1" } } } https://github.com/Kotlin/kotlinx.html
9. Examples - CSS val css = Stylesheet { a { width = 10.px color = 0xffffff hover { color = 0xf2cacf } } } https://github.com/olegcherr/Aza-Kotlin-CSS
10. Examples - SQL val row = from(Friends) .where { Nickname like "brian" } .select(Friends.First_name, Friends.Nickname) https://github.com/orangy/squash
11. Examples - SQL https://github.com/arunkumar9t2/transition-x
12. Advantages There must be a reason DSL exists and developers write customs DSLs
13. Advantages There must be a reason DSL exists and developers write customs DSLs • Expressive
14. Advantages There must be a reason DSL exists and developers write customs DSLs • Expressive • Readable
15. Advantages There must be a reason DSL exists and developers write customs DSLs • Expressive • Readable • Concise - Less Boilerplate
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" } } }
17. Why Kotlin? Kotlin provides certain features that let us develop expressive and structured DSL • Lambda with Receiver • Lambda as method parameter • Extension function
18. val speaker = speaker { name = "Subhrajyoti Sen" talkTitle = "Intro to Kotlin DSL" }
19. val speaker = speaker { name = "Subhrajyoti Sen" talkTitle = "Intro to Kotlin DSL" } Function
20. data class SpeakerModel( var name: String = “”, var talkTitle: String = “” )
21. fun speaker() { }
22. fun speaker(): SpeakerModel { }
23. fun speaker(): SpeakerModel { val speaker = SpeakerModel() }
24. fun speaker(): SpeakerModel { val speaker = SpeakerModel() return speaker }
25. fun speaker(block: (SpeakerModel) -> Unit): SpeakerModel { val speaker = SpeakerModel() return speaker }
26. Lambda
27. Lambda Lambdas are code blocks enclosed in curly braces.
28. Lambda Lambdas are code blocks enclosed in curly braces. val multiply: (Int, Int) -> Int
29. Lambda Lambdas are code blocks enclosed in curly braces. val multiply: (Int, Int) -> Int val multiply = { a: Int, b: Int -> a*b }
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))
31. fun speaker(block: (SpeakerModel) -> Unit): SpeakerModel { val speaker = SpeakerModel() return speaker }
32. fun speaker(block: (SpeakerModel) -> Unit): SpeakerModel { val speaker = SpeakerModel() block(speaker) return speaker }
33. val speaker = speaker({ it.name = "Subhrajyoti Sen" it.talkTitle = "Intro to Kotlin DSL" })
34. fun speaker(block: (SpeakerModel) -> Unit): SpeakerModel { val speaker = SpeakerModel() block(speaker) return speaker }
35. fun speaker(block: SpeakerModel.() -> Unit): SpeakerModel { val speaker = SpeakerModel() block(speaker) return speaker }
36. fun speaker(block: SpeakerModel.() -> Unit): SpeakerModel { val speaker = SpeakerModel() block(speaker) return speaker } Receiver Lambda
37. fun speaker(block: SpeakerModel.() -> Unit): SpeakerModel { val speaker = SpeakerModel() speaker.block() return speaker }
38. val speaker = speaker({ this.name = "Subhrajyoti Sen" this.talkTitle = "Intro to Kotlin DSL" })
39. val speaker = speaker({ name = "Subhrajyoti Sen" talkTitle = "Intro to Kotlin DSL" })
40. val speaker = speaker({ name = "Subhrajyoti Sen" talkTitle = "Intro to Kotlin DSL" }) Extra bracket?
41. Lambda as a Parameter fun modify(a: Int, lambda: (Int) -> Int)
42. Lambda as a Parameter fun modify(a: Int, lambda: (Int) -> Int) modify(3, someLambda)
43. Lambda as a Parameter fun modify(a: Int, lambda: (Int) -> Int) modify(3, someLambda) modify(3) { }
44. val speaker = speaker({ name = "Subhrajyoti Sen" talkTitle = "Intro to Kotlin DSL" })
45. val speaker = speaker { name = "Subhrajyoti Sen" talkTitle = "Intro to Kotlin DSL" }
46. room { roomNumber = 1 floor = 2 speaker { name = "Subhrajyoti Sen" talkTitle = "Intro to Kotlin DSL" } }
47. data class RoomModel( var roomNumber: Int = 0, var floor: Int = 99, var speaker: SpeakerModel? = null )
48. fun room(block: RoomModel.() -> Unit): RoomModel { val room = RoomModel() room.block() return room }
49. room { roomNumber = 1 floor = 2 speaker { name = "Subhrajyoti Sen" talkTitle = "Intro to Kotlin DSL" } }
50. But there’s a problem
51. room { roomNumber = 1 floor = 2 speaker { name = "Subhrajyoti Sen" talkTitle = "Intro to Kotlin DSL" } }
52. Extension Function A function that extends the functionality without modifying it fun Banner.isVisible(): Boolean { return this.visibility == Banner.VISIBLE }
53. Extension Function A function that extends the functionality without modifying it fun Banner.isVisible(): Boolean { return this.visibility == Banner.VISIBLE } Base class
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
55. fun speaker(block: SpeakerModel.() -> Unit): SpeakerModel { val speaker = SpeakerModel() speaker.apply(block) return speaker }
56. fun speaker(block: SpeakerModel.() -> Unit) { val speaker = SpeakerModel() speaker.apply(block) return speaker }
57. fun speaker(block: SpeakerModel.() -> Unit) { val speaker = SpeakerModel() speaker.apply(block) }
58. fun RoomModel.speaker(block: SpeakerModel.() -> Unit) { val speaker = SpeakerModel() speaker.apply(block) }
59. fun RoomModel.speaker(block: SpeakerModel.() -> Unit) { val speaker = SpeakerModel() speaker.apply(block) this.speaker = speaker }
60. room { roomNumber = 1 floor = 2 speaker { name = "Subhrajyoti Sen" talkTitle = "Intro to Kotlin DSL" } }
61. val conference = conference { name = "DevConfIN 19" location = "Bengaluru" room { roomNumber = 1 floor = 2 speaker { name = "Subhrajyoti Sen" talkTitle = "Intro to Kotlin DSL" } } }
62. data class ConferenceModel( var name: String = "", var location: String = "", var rooms: MutableList<RoomModel> = mutableListOf() )
63. data class ConferenceModel( var name: String = "", var location: String = "", var rooms: MutableList<RoomModel> = mutableListOf() ) Create an empty mutable list
64. fun conference(block: ConferenceModel.() -> Unit): ConferenceModel { val conference = ConferenceModel() conference.apply(block) return conference }
65. fun ConferenceModel.room(block: RoomModel.() -> Unit) { val room = RoomModel() room.apply(block) this.rooms.add(room) }
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" } } }
67. Not yet done
68. conference { name = "DevConfIN 19" location = "Bengaluru" room { roomNumber = 1 floor = 2 speaker { name = "Subhrajyoti Sen" talkTitle = "Intro to Kotlin DSL" } } }
69. conference { name = "DevConfIN 19" location = "Bengaluru" room { roomNumber = 1 speaker { name = "Subhrajyoti Sen" talkTitle = "Intro to Kotlin DSL" floor = 2 } } }
70. This actually compiles and works
71. conference { name = "DevConfIN 19" location = "Bengaluru" room { roomNumber = 1 speaker { name = "Subhrajyoti Sen" talkTitle = "Intro to Kotlin DSL" floor = 2 } } }
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
73. Scope Control - DSL Marker The lambda should be able to access only it’s nearest implicit receiver.
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
75. fun RoomModel.speaker(block: SpeakerModel.() -> Unit) { // ... } fun ConferenceModel.room(block: RoomModel.() -> Unit) { // ... } fun conference(block: ConferenceModel.() -> Unit): ConferenceModel { // ... }
76. fun RoomModel.speaker(block: (@ConfDSL SpeakerModel).() -> Unit) { // ... } fun ConferenceModel.room(block: (@ConfDSL RoomModel).() -> Unit) { // ... } fun conference(block: (@ConfDSL ConferenceModel).() -> Unit): ConferenceModel { // ... }
77. To conclude, DSLs are awesome
78. Questions? http://bit.ly/DevConfDSL
79. Thank you SubhrajyotiSen iamsubhrajyoti