Deep Dive into Kotlin DSL

by Matsuda Jumpei

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Deep Dive into Kotlin DSL Kotlin Fest 2019 @red_fat_daruma

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Self Introduction Jumpei Matsuda / Daruma / @red_fat_daruma ▸ Software Engineer @DeployGate ▸ GitHub: jmatsu / Twitter: red_fat_daruma ▸ JVM projects : Android apps/SDK, Scala server- side and Gradle plugins ▸ One of Kotlin users since M10 2

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Today's Goal Hopefully, Every Audience Will.... ▸ Know what is DSL and/or Kotlin DSL ▸ Understand what speciﬁcations/features of Kotlin support Kotlin DSL ▸ Have a motivation to develop the original Kotlin DSL and/or extend existing libraries through Kotlin DSL. 3

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Let me ask questions! 4

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Who thinks Kotlin DSL is for Gradle? ✋ 5

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I do not use a word *Kotlin DSL* to represent Kotlin DSL for Gradle Kotlin DSL Is Not Only Kotlin DSL for Gradle ▸ *Kotlin DSL* in this presentation does not mean Gradle's one ▸ Kotlin DSL for Gradle is just a DSL in Kotlin for Gradle ▸ Contracts, coroutines, etc. are also Kotlin DSL 6

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Who have used and/or are using Kotlin DSL? ✋ 8

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Who know what techniques will help Kotlin DSL? ✋ 9

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Who can explain the definition of Kotlin DSL? ✋ 10

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Who can explain the definition of DSL? ✋ 11

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If you said YES to all questions, you are already Kotlin DSL deep diver! 12

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Today's topics Agenda ▸ The base knowledge of DSL ▸ Speciﬁcations/techniques of Kotlin DSL based on levels ▸ Quick tips for DSL development 13

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The base knowledges of DSL Domain Specific Language ▸ A computer language specialized to a particular application domain ▸ In contrast: a general purpose language like Kotlin, Java 14 ref: https://en.wikipedia.org/wiki/Domain-speciﬁc_language

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Domain Specific Language ▸ A computer language specialized to a particular application domain ▸ In contrast: a general purpose language like Kotlin, Java ▸ What's a motivation to introduce/need DSL? 15 The base knowledges of DSL ref: https://en.wikipedia.org/wiki/Domain-speciﬁc_language

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Why People Want To Use DSL ▸ Simplify complex code ▸ More readable, understandable, ﬂuent ▸ Use smart and/or minimum APIs for the domain ▸ Improve productivity and reduce human errors ▸ Get a power of type system etc. 16 The base knowledges of DSL

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Why People Want To Use DSL 17 The base knowledges of DSL NOTE: pseudo code DSL

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Aspects of Domain Specific Language ▸ External DSL and Internal DSL ▸ External: A language that's parsed independently of the host general purpose language ▸ Internal: A particular form of API in the host general purpose language for the domain 18 The base knowledges of DSL ref: https://martinfowler.com/books/dsl.html

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Examples in the Real World ▸ External DSL ▸ SQL, CSS, Regular expressions ▸ Internal DSL ▸ HTML, Mock Libraries, DI container 19 The base knowledges of DSL

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Examples in the Real World ▸ External DSL ▸ SQL, CSS, Regular expressions ▸ Internal DSL ▸ HTML, Mocking Libraries, DI container ▸ Kotlin DSL 20 The base knowledges of DSL

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Examples in the Real World ▸ External DSL ▸ SQL, CSS, Regular expressions ▸ Internal DSL ▸ HTML, Mocking Libraries, DI container ▸ Kotlin DSL ▸ NOT a special edition of Kotlin 21 The base knowledges of DSL

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Techniques for Internal DSL Depends on... ▸ The host general purpose language ▸ Type checking ▸ Syntax ▸ Syntactic extensions ▸ Compiler extensibility and interpolation 22 The base knowledges of DSL

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Techniques for Internal Kotlin DSL Depends on... ▸ A general purpose language Kotlin ▸ Type checking Static type checking ▸ Syntax Lambda expressions, etc. ▸ Syntactic extensions Method extension, operator overloading, etc. ▸ Compiler extensibility and/or interpolation Compiler plugins etc. 23 The base knowledges of DSL

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e.g. kotlinx.html is one of Kotlin DSL ▸ DSL for HTML domain ▸ Generate DOM tree from a Kotlin code 24 The base knowledges of DSL https://github.com/Kotlin/kotlinx.html

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e.g. kotlinx.html is one of Kotlin DSL ▸ DSL for HTML domain ▸ Generate DOM tree from a Kotlin code 25 Type safe builder pattern, lambda expressions, functions with receiver, higher-order functions, operator overloading, DSLMarker, etc. The base knowledges of DSL

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Today's topics Agenda ▸ The base knowledge of DSL ▸ Speciﬁcations/techniques of Kotlin DSL based on levels ▸ Quick tips for DSL development 26 ▸ NOTE: DSL in following slides means Internal DSL unless speciﬁed

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Techniques of Kotlin DSL Techniques of Kotlin DSL ▸ Getting started ▸ Language speciﬁcations ▸ Intermediate ▸ Features for syntactic extensions ▸ Expertise ▸ Build environment stuff of Kotlin 27

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Techniques of Kotlin DSL ▸ Getting started ▸ Language speciﬁcations ▸ Intermediate ▸ Features for syntactic extensions ▸ Expertise ▸ Build environment stuff of Kotlin 28 Techniques of Kotlin DSL - Getting Started

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Language Specifications ▸ Type system ▸ Static type checking ▸ Syntax related speciﬁcations ▸ Lambdas, Functions literals with receiver 29 Techniques of Kotlin DSL - Getting Started

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Static Type Checking ▸ Code analysis based type safety veriﬁcation ▸ In contrast: Dynamic type checking ▸ Allow know type violations before executing 30 Techniques of Kotlin DSL - Getting Started - 1/3

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Static Type Checking ▸ Code analysis based type safety veriﬁcation ▸ In contrast: Dynamic type checking ▸ Allow know type violations before executing 31 Groovy : no error but fail at runtime Techniques of Kotlin DSL - Getting Started - 1/3

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Static Type Checking ▸ Code analysis based type safety veriﬁcation ▸ In contrast: Dynamic type checking ▸ Allow know type violations before executing 32 Groovy : no error but fail at runtime Kotlin : a compilation error Techniques of Kotlin DSL - Getting Started - 1/3

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Syntax-Related Specifications ▸ Lambda expressions ▸ Function literals with receiver 33 Techniques of Kotlin DSL - Getting Started - 1.5/3

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Lambda Expressions ▸ Syntax sugar for function type variables and SAM ▸ Represent a function by braces 34 Techniques of Kotlin DSL - Getting Started - 2/3

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Lambda Expressions - Example 35 Techniques of Kotlin DSL - Getting Started - 2/3

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Lambda Expressions - Example 36 Techniques of Kotlin DSL - Getting Started - 2/3

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Lambda Expressions - Example 37 Techniques of Kotlin DSL - Getting Started - 2/3

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Lambda Expressions - Example 38 Techniques of Kotlin DSL - Getting Started - 2/3

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Lambda Expressions - Example 39 Techniques of Kotlin DSL - Getting Started - 2/3

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Function Literals With Receiver ▸ Kotlin allows specify what is this scope of a function ▸ Groovy also can do this but Java cannot. ▸ Function Type : Receiver.(Argument) -> Return ▸ Great help for block-code and ﬂuent style coding 40 Techniques of Kotlin DSL - Getting Started - 3/3

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Function Literals With Receiver 41 Techniques of Kotlin DSL - Getting Started - 3/3

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A Primitive Kotlin DSL ▸ Techniques described before allow create a simple DSL 42 Techniques of Kotlin DSL - Getting Started - Closing

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A Primitive Kotlin DSL - Is It Enough for Your Better Development? ▸ Only techniques described before allow create a simple DSL 43 Techniques of Kotlin DSL - Getting Started - Closing

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Break ▸ We can build DSLs easily from scratch with knowledges I have explained ▸ It's enough if you want type-safe and/or lambda's elegant style. ▸ Let's check what techniques can improve usability of Kotlin DSL and how we can create Kotlin DSL for existing libraries/APIs. 44 Techniques of Kotlin DSL - Getting Started - Closing

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Break ▸ We can build DSLs easily from scratch with knowledges I have explained ▸ It's enough if you want type-safe and/or lambda's elegant style. ▸ Let's check what techniques can improve usability of Kotlin DSL and how we can create Kotlin DSL for existing libraries/APIs. 45 Techniques of Kotlin DSL - Getting Started - Closing

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Techniques of Kotlin DSL ▸ Getting started ▸ Language speciﬁcations ▸ Intermediate ▸ Features for syntactic extensions ▸ Expertise ▸ Build environment stuff of Kotlin 46 Techniques of Kotlin DSL - Intermediate

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Syntactic Extension ▸ Extension method ▸ Operator overloading ▸ Inﬁx notation ▸ Method convention ▸ Delegation 47 Techniques of Kotlin DSL - Intermediate - 0/5

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Syntactic Extension ▸ Extension method ▸ Operator overloading ▸ Inﬁx notation ▸ Method convention ▸ Delegation 48 Techniques of Kotlin DSL - Intermediate - 1/5

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Extension Method ▸ Deﬁne new member methods w/o inheritances ▸ However, we cannot avoid member visibility restrictions ▸ The beneﬁts of Extension Method for DSL ▸ Apply the techniques which I will explain later ▸ Can create DSLs without modifying 3rd party libraries 49 Techniques of Kotlin DSL - Intermediate - 1/5

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Syntactic Extension ▸ Extension method ▸ Operator overloading ▸ Inﬁx notation ▸ Method convention ▸ Delegation 50 Techniques of Kotlin DSL - Intermediate - 2/5

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Example - kotlinx.html - DOM tree generator in Kotlin ▸ + would become a text node 51 Techniques of Kotlin DSL - Intermediate - 2/5 https://github.com/Kotlin/kotlinx.html

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Example - kotlinx.html - DOM tree generator in Kotlin ▸ +"any text" means appending text to the node ▸ + is shorter than *text* 52 Techniques of Kotlin DSL - Intermediate - 2/5 https://github.com/Kotlin/kotlinx.html

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Operator Overloading ▸ Kotlin allow overload operators for developers ▸ We can assign a logic to several operators like +, -... ▸ The beneﬁts of Operator Overloading for DSL ▸ Basically, operators are shorter than method names ▸ A meaning of an operation is clear 53 Techniques of Kotlin DSL - Intermediate - 2/5

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Syntactic Extension ▸ Extension method ▸ Operator overloading ▸ Inﬁx notation ▸ Method convention ▸ Delegation 54 Techniques of Kotlin DSL - Intermediate - 3/5

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Example - DSL for Hamcrest API ▸ Not ﬂuent ▸ It might be unclear for non-hamcrest users 55 Techniques of Kotlin DSL - Intermediate - 3/5

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Example - DSL for Hamcrest API ▸ Looks ﬂuent ▸ Hamcrest's knowledge is not required 56 Techniques of Kotlin DSL - Intermediate - 3/5

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Infix Notation ▸ Allow call a function like *Receiver infix-function argument* ▸ Can omit a dot and parentheses from a method call ▸ The benefits of Infix Notation for DSL ▸ Would be more fluent style ▸ Can reduce domain knowledge 57 Techniques of Kotlin DSL - Intermediate - 3/5

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Syntactic Extension ▸ Extension method ▸ Operator overloading ▸ Inﬁx notation ▸ Method convention ▸ Delegation 58 Techniques of Kotlin DSL - Intermediate - 4/5

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Example - Kotlin DSL for Gradle ▸ debug is not an existing function which accepts closure ▸ It's dynamically generated and be conﬁgured by a passed closure 59 Techniques of Kotlin DSL - Intermediate - 4/5 Groovy code https://github.com/gradle/gradle

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Example - Kotlin DSL for Gradle ▸ NOTE: create(String) creates a configurable object for SigningConfig ▸ Prob.: configure is domain knowledge but hidden in Groovy DSL 60 Techniques of Kotlin DSL - Intermediate - 4/5 Groovy code Raw Kotlin code https://github.com/gradle/gradle

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Example - Kotlin DSL for Gradle ▸ Implement an invoke function which accepts a lambda ▸ Could hide conﬁgure from Kotlin DSL users 61 Techniques of Kotlin DSL - Intermediate - 4/5 https://github.com/gradle/gradle/blob/90974be832395cd5eabﬀ97c18842791d7bee3a8/subprojects/kotlin-dsl/src/main/kotlin/org/gradle/kotlin/dsl/ NamedDomainObjectContainerExtensions.kt#L36 https://github.com/gradle/gradle

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Method Convention ▸ This is a kind of operator overloading ▸ Operators/syntax which Java does not implement manually will be available if a method satisﬁes a convention ▸ Index-access, invoke, range operator... ▸ The beneﬁts of Method Convention for DSL ▸ Easy to create lambda expressions-based builder by overloading invoke ▸ Can hide domain knowledge bypass a common operator/syntax in Kotlin 62 Techniques of Kotlin DSL - Intermediate - 4/5

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Syntactic Extension ▸ Extension method ▸ Operator overloading ▸ Inﬁx notation ▸ Method convention ▸ Delegation 63 Techniques of Kotlin DSL - Intermediate - 5/5

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Example - Kotpref - Android SharedPreferences DSL ▸ SharedPreferences API is low-level ▸ A simple command-query API sequence is required 64 Techniques of Kotlin DSL - Intermediate - 5/5 https://github.com/chibatching/Kotpref

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Example - Kotpref - Android SharedPreferences DSL ▸ Hide low-level API and domain knowledge ▸ Writing a value to a property will save it to the preferences but it's hidden 65 Techniques of Kotlin DSL - Intermediate - 5/5 https://github.com/chibatching/Kotpref https://github.com/chibatching/Kotpref

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Delegation ▸ A property delegates to another object when accessed ▸ The beneﬁts of Delegations for DSL ▸ Can hide domain knowledge ▸ Effective for DRY when using read and write operations to properties 66 Techniques of Kotlin DSL - Intermediate - 5/5

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Techniques of Kotlin DSL ▸ Getting started ▸ Language speciﬁcations ▸ Intermediate ▸ Features for syntactic extensions ▸ Expertise ▸ Build environment stuff of Kotlin 67 Techniques of Kotlin DSL - Expertise

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Build Environment Stuff of Kotlin ▸ The plugin system for the compiler ▸ SAM with receiver compiler plugin ▸ Interpolators ▸ DslMarker 68 Techniques of Kotlin DSL - Expertise - 0/2

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Build Environment Stuff of Kotlin ▸ The plugin system for the compiler ▸ SAM with receiver compiler plugin ▸ Interpolators ▸ DslMarker 69 Techniques of Kotlin DSL - Expertise - 1/2

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Example - Kotlin DSL for Java Code ▸ Java cannot specify a receiver of SAM so Kotlin cannot use functions with receiver without extension methods 70 Java code Call Container#execute from Kotlin Techniques of Kotlin DSL - Expertise - 1/2

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▸ Just annotate SAM interfaces and register the annotation to the plugin ▸ An argument of SAM will be a receiver in Kotlin! 71 Techniques of Kotlin DSL - Expertise - 1/2 Example - Kotlin DSL for Java Code

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SAM With Receiver ▸ Control a receiver in SAM conversions ▸ For Kotlin code, but need to modify Java code ▸ The beneﬁts of SAM-with-receiver ▸ Useful when non-Kotlin libraries assume usability of Kotlin DSL ▸ Avoid a bit awkward API in Kotlin 72 Techniques of Kotlin DSL - Expertise - 1/2 https://github.com/JetBrains/kotlin/tree/master/plugins/sam-with-receiver

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SAM With Receiver - Example - Kotlin DSL for Gradle ▸ Action API is widely used so SAM-with-receiver is better than extensions 73 https://github.com/gradle/gradle/commit/18aebdba33cd9e1be966cc286a11d3ce86a28e4f Techniques of Kotlin DSL - Expertise - 1/2 https://github.com/gradle/gradle

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Build Environment Stuff of Kotlin ▸ The plugin system for the compiler ▸ SAM with receiver compiler plugin ▸ Interpolators ▸ DslMarker 74 Techniques of Kotlin DSL - Expertise - 1/2

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Example - Pseudo Dom Tree Builder ▸ head tag in head tag should be forbidden! 75 Techniques of Kotlin DSL - Expertise - 2/2

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▸ Add an annotation of @DslMarker to receiver classes or superclasses (and functions if necessary) 76 Techniques of Kotlin DSL - Expertise - 2/2 Example - Pseudo Dom Tree Builder

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▸ Show an error because of implicit receiver violation! 77 Techniques of Kotlin DSL - Expertise - 2/2 Example - Pseudo Dom Tree Builder

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DslMarker ▸ Kotlin's only DSL speciﬁc feature as far as I know ▸ Add a constraint of a caller scope and can raise errors if invalid ▸ The beneﬁts of DslMarker for DSL ▸ This is required to make your DSL safe ▸ Be developers' help because IntelliJ highlight marked functions 78 Techniques of Kotlin DSL - Expertise - 2/2

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DslMarker - Example - kotlinx.html ▸ Of course, kotlinx.html uses DslMarker to build DOM tree correctly ▸ html, head, body are marked functions so highlighted 79 Techniques of Kotlin DSL - Expertise - 2/2

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Build Environment Stuff of Kotlin - Limitations ▸ Compiler plugin stuff is undocumented ▸ Complex logic is hard to be implemented ▸ Maintainability... ▸ DslMarker needs be annotated to classes, so it's unavailable for existing libraries 80 Techniques of Kotlin DSL - Expertise - Closing

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Quick tips for DSL development 81

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Differences Between API and DSL ▸ Technically, nothing special but... ▸ The concepts of the interface design are different ▸ DSL is focusing on the domain ▸ DSL is well-designed API set for the domain but also limited ▸ We should not assume DSL can work as general APIs 82 Quick Tips 1/4

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Quick Tips 2/4 Adapt Kotlin DSL to Existing Libraries ▸ Aggregate existing APIs by extension method ▸ Use Operator overloading, Inﬁx operators, Method conventions ▸ Create DTOs if you would like to use DslMarker 83

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Quick Tips 3/4 How To Keep Both Usabilities of Kotlin DSL and Other JVM Languages ▸ Java -> Kotlin DSL ▸ Use SAM with receiver ▸ Kotlin DSL -> Other JVM languages ▸ Provide method chaining interface because of functions with receiver issue ▸ Provide alternative non-inline APIs because of inline features' availability ▸ Split API for other JVM languages and Kotlin DSL 84

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Quick Tips 4/4 For Advanced Steps ▸ Read Martin Fowler's DSL Patterns if you want to know DSL interfaces ▸ Good examples of Kotlin DSL ▸ See appendix! 86

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Closing 87

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Closing Summary 1/3 ▸ Kotlin DSL is ▸ A pure Kotlin code ▸ Not only for Kotlin libraries ▸ Built on several techniques 88

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Summary 2/3 ▸ Language specs ▸ Static type ▸ Lambda expressions w/ receiver control ▸ Language features ▸ Extension method, operator overloading, inﬁx notation, method convention, delegation ▸ Build environment features ▸ SAM-with-receiver, DslMarker 89 Closing

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Summary 3/3 ▸ DslMarker is the only DSL speciﬁc feature but improves safety! ▸ Kotlin DSL can extend existing libraries with only a few constraints ▸ We can co-exist usability for the both of Kotlin and other JVM languages ▸ If you work hard 90 Closing

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Appendix 1/2 ▸ Books ▸ Domain Speciﬁc Languages by M. Fowler with R. Parsons ▸ DSLs in Action by D. Ghosh ▸ Dsl Engineering: Designing, Implementing and Using Domain-speciﬁc Languages by M. Voelter ▸ Kotlin in Action by D. Jemerrov and S. Isakova ▸ Web/Blog ▸ Catalog of DSL Patterns https://martinfowler.com/dslCatalog/ ▸ Kotlin DSL: From Theory to Practice https://dzone.com/articles/kotlin-dsl-from-theory-to-practice ▸ Writing a Kotlin DSL (series) https://proandroiddev.com/writing-dsls-in-kotlin-part-1-7f5d2193f277 ▸ Idiomatic Kotlin. Best Practices. https://phauer.com/2017/idiomatic-kotlin-best-practices/ ▸ and more... 91 Closing

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Appendix 2/2 ▸ Libraries ▸ Build system : gradle/gradle/tree/master/subprojects/kotlin-dsl ▸ Android : android-ktx, Jetpack Compose, Kotlin/anko, JakeWharton/kotterknife, chibatching/Kotpref, infotech-group/ android-drawable-dsl, agoda-com/Kakao ▸ Web : Kotlin/kotlinx.html, perwendel/spark-kotlin, spring-projects/spring-fu, ktorio/ktor ▸ Testing libraries : MarkusAmshove/Kluent, winterbe/expekt, npryce/hamkrest, nhaarman/mockito-kotlin, kotlintest/ kotlintest, spekframework/spek ▸ DI containers : Kodein-Framework/Kodein-DI, Ekito/koin, authzee/kotlin-guice ▸ Otherwise: skrapeit/skrape.it, JetBrains/Exposed, x2bool/kuery, ruslanys/telegraff, ▸ Words ▸ Kotlin DSL, DSL, kotlish, type builder, kotlinize, idiomatic Kotlin 92 Closing

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Fin. Thank You for Listening! ▸ Jumpei Matsuda / daruma ▸ Software Engineer @DeployGate ▸ GitHub: jmatsu ▸ Twitter: red_fat_daruma ▸ Please feel free to ask questions ▸ ೔ຊޠŧŔŕŪũƄŝſ (Ӊ౎ٶҭͪͷ७೔ຊਓͰ͢) 93