One Source to Rule Them All

Kotlin DSLs as a Single Source of Truth for Multiple Tasks

Ivan Ponomarev

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Ivan Ponomarev

  • Staff Engineer @ Synthesized.io

  • Teaching Java @ Northeastern University London

What do we need DSLs for?

  1. Code can be written by a subject-matter expert, not a programmer

What do we need DSLs for (in real life)?

  1. Sometimes code can be written by a subject-matter expert, not a programmer

  2. Often code can be read by a subject-matter expert

What do we need DSLs for (in real life)?

  1. Sometimes code can be written by a subject-matter expert, not a programmer

  2. Often code can be read by a subject-matter expert

  3. DSL code is the single source of truth for many tasks/aspects of the project

DSLs external and internal

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Martin Fowler, Rebecca Parsons. Domain-Specific Languages

fowler dsl

Types of external DSLs

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Designed from scratch

  • DOT (GraphViz), PlantUML, gnuplot…​

  • HCL (Hashicorp Configuration Language)

  • Gherkin (Cucumber framework’s language)

Types of external DSLs

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Based on general purpose markup language

  • YAML: OpenAPI, Ansible, GithubActions, k8s definitions…​

  • JSON: Vue i18n files

  • XML: XSLT, XSD

Internal DSLs

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Subset of a dynamically typed language

  • Lisp (historically first): Emacs Lisp, Symbolic Mathematics etc.

  • Ruby: Rails, RSpec, Chef…​

  • Groovy: Spock, Ratpack, Grails, Gradle, Jenkinsfile…​

Internal DSLs

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Subset of a statically typed language

  • Scala: Scalatest, Akka HTTP…​

  • Haskell: Parsec

  • Kotlin: Kotlinx.html, Ktor, Gradle,…​

Restrictions

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Restrictions

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Restrictions

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Restrictions

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IDE support

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IDE support

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IDE support

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IDE support

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Security Concerns

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* Does not mean that you are safe, e.g. google for "Billion_laughs_attack"

Kotlin language features for DSL building

Tool

DSL syntax

General syntax

Extension functions

mylist.first();
/* there isn’t first() method
in mylist collection*/
ListUtlis.first(mylist)

Infix functions

1 to "one"
1.to("one")

Operators overloading

collection += element
collection.add(element)

Type aliases

typealias Point = Pair

Creating empty inheritors classes and other duct tapes

Kotlin language features for DSL building (continued)

Tool

DSL syntax

General syntax

get/set methods convention

map["key"] = "value"
map.put("key", "value")

Destructuring declaration

val (x, y) = Point(0, 0)
val p = Point(0, 0)
val x = p.first
val y = p.second

Lambda out of parentheses

list.forEach { ... }
list.forEach({...})

Lambda with receiver

Person().apply { name = «John» }

N/A

Context control

@DslMarker

N/A

Demo time!

One source to be used in

  1. Execution of rules

  2. Documentation

  3. Visualization

  4. Validation

  5. Serialization ("free" JSON/YAML based DSL version for our Kotlin DSL)

Mixing DSL and code

We can leave extension points in our builder:

//In our example
customCondition { Random.nextDouble() < .88} invokes TransformationC


//Or in some other DSL
customBusinessRule { checkSmthProgramatically() }

Great for describing business rules (state-transition model, for example), especially if DSL defines only part of rules, which is usually the case.

Infix functions don’t work on this

    brightonKotlin {
        //Cannot get rid of paretheses (unlike Groovy)
        talk ("Talk 1") deliveredBy {
            speaker ("Speaker 1")
            speaker ("Speaker 2")
        }
        talk ("Talk 2") deliveredBy {
            speaker("ssd")
            speaker ("Speaker 3")
        }
    }

Infix functions don’t work on this: a workaround

    brightonKotlin {
        //Cannot get rid of paretheses
        + "Talk 1" deliveredBy {
            + "Speaker 1"
            + "Speaker 2"
        }
        + "Talk 2" deliveredBy {
            + "ssd"
            + "Speaker 3"
        }
    }

@DslMarker

    talk ("Talk 1") deliveredBy {
        talk (...) // ???
    }

@DslMarker

@DslMarker
annotation class MeetupDsl

@MeetupDsl
class MeetupBuilder { ... }

@MeetupDsl
class SpeakersBuilder { ... }

Reading/watching suggestions

jemerov kotlin

Examples for your inspiration

Conclusions

  • DSL combined with designed patterns is a powerful tool for solving multiple tasks

  • Creating DSLs in Kotlin is not scary. You can improve parts of your existing internal APIs today making them "DSL-like"

  • Internal Kotlin DSLs are not the only way to implement DSLs, but definitely not the worst one in many scenarious.

Thanks for listening!

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Code and slides are available on GitHub https://github.com/inponomarev/dsl-talk

ivan@synthesized.io

@inponomarev