[日本語注釈つき] Deep Dive into Kotlin DSL

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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 M10 Ҏདྷͷ Kotlin Ϣʔβʔ

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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 DSL ͱ͸Կ͔ɺKotlin DSL ͱ͸Կ͔Λ஌Δ ͲΜͳ࢓༷/ػೳ͕ Kotlin DSL Λࢧ͍͑ͯΔͷ͔ཧղ͢Δ ಠࣗͷ Kotlin DSL ͷ࡞੒΍طଘϥΠϒϥϦΛ Kotlin DSL Ͱ֦ு͢ΔϞνϕʔγϣϯΛಘΔ ͜ͷൃදΛฉ͍ͨਓ͕ͨͪɺ ͱ͍͍ͳ͊

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Let me ask questions! 4

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Who thinks Kotlin DSL is for Gradle? ✋ 5 Kotlin DSL == Gradle ͷ΋ͷͩͱࢥ͍ͬͯΔਓʁ

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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 Kotlin DSL ͸ Kotlin DSL for Gradle ͷ͜ͱ͚ͩ͡Όͳ͍ ͜ͷൃදͰͷ Kotlin DSL ͸ Gradle ͷ΋ͷ͚ͩΛࢦ͢Θ͚͡Όͳ͍ Kotlin DSL for Gradle ͸ Kotlin Ͱॻ͔Εͨ Gradle ༻ͷ DSL Ͱ͋Δͱ͍͏͚ͩ Contracts, coroutines ͱ͔΋ Kotlin DSL

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Who have used and/or are using Kotlin DSL? ✋ 8 ࠓ·Ͱʹ Kotlin DSL ࢖ͬͨΑɺࠓ΋࢖ͬͯΔΑɺͬͯਓʁ

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Who know what techniques will help Kotlin DSL? ✋ 9 ͲΜͳٕज़͕ Kotlin DSL Λࢧ͑ͯΔ͔஌ͬͯΔਓʁ

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Who can explain the definition of Kotlin DSL? ✋ 10 Kotlin DSL ͷఆٛΛઆ໌Ͱ͖Δͻͱ?

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Who can explain the definition of DSL? ✋ 11 DSL ͷఆٛΛઆ໌Ͱ͖Δͻͱʁ

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If you said YES to all questions, you are already Kotlin DSL deep diver! 12 શ෦ͷ࣭໰ʹʮ͸͍ʯͱճ౴Ͱ͖ͨํ͸΋͏͢Ͱʹ Kotlin DSL Deep diver Ͱ͢ɾɾɾ

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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 DSL ʹؔ͢Δجૅ஌ࣝ Kotlin DSL ͷ࢓༷΍ٕज़ΛϨϕϧผͰ DSL ։ൃͷͨΊͷͪΐͬͱͨ͠ tips

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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 υϝΠϯݻ༗ݴޠ ಛఆͷΞϓϦέʔγϣϯυϝΠϯʹಛԽͨ͠ίϯϐϡʔλݴޠ ରরతͳ΋ͷ͸ Kotlin, Java ͷΑ͏ͳ൚༻ݴޠ

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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 DSL Λಋೖ͢Δɺඞཁͱ͢ΔϞνϕʔγϣϯ͸ͳʹ͔

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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 ͳΜͰ DSL Λ࢖͏ͷ͔ ෳࡶͳίʔυΛγϯϓϧʹ͢ΔͨΊ ಡΈ΍͍͢ɺཧղ͠΍͍͢ɺྲྀெͳ interface ΁ ಛఆυϝΠϯʹର͢ΔεϚʔτͰɺඞཁ࠷௿ݶͳ API ͕ར༻Ͱ͖Δ ੜ࢈ੑͷ޲্΍ώϡʔϚϯΤϥʔͷݮগͷͨΊ ܕγεςϜͷྗΛಘͨΓͩͱ͔

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

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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 υϝΠϯݻ༗ݴޠಛੑ ֎෦ DSL ͱ಺෦DSL ϗετͱͳΔ൚༻ݴޠͱ͸ಠཱͯ͠ύʔε͞ΕΔ DSL ϗετͱͳΔ൚༻ݴޠͷதͰɺಛఆͷ API ͷܗࣜΛఏڙ͢Δ DSL

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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 ࣮ྫ SQL, CSS, ਖ਼نදݱ HTML, ϞοΫϥΠϒϥϦɺDI

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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 Kotlin ͷಛผͳ೿ੜͱ͔͍͏Θ͚Ͱ͸ͳ͍

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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 ಺෦ DSL Λࢧ͑Δٕज़͸ҎԼʹґଘ ϗετͱͳΔ൚༻ݴޠ ܕݕࠪ جૅߏจ ߏจ֦ு ίϯύΠϥ֦ுੑ΍ิؒࢠ

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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 Kotlin DSL ͷ࢓༷΍ٕज़ΛϨϕϧผͰ DSL ։ൃͷͨΊͷͪΐͬͱͨ͠ tips ͔ͬ͜Β໌هͳ͘ DSL ͱ͍ͬͨΒ಺෦ DSL Ͱ͢

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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 ॳڃ தڃ ্ڃ

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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 ݴޠ࢓༷

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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 ܕγεςϜ ੩తܕݕࠪ ߏจʹؔ͢Δ࢓༷ ϥϜμࣜɺϨγʔό͖ͭؔ਺Ϧςϥϧ

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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 ίʔυղੳϕʔεͷܕ҆શੑݕࠪ ରর : ಈతܕݕࠪ ࣮ߦલʹܕҧ൓Λ஌Δ͜ͱ͕Ͱ͖Δ

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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 Τϥʔ͸ͳ͍͕ɺ࣮ߦ࣌ʹམͪΔ ίʔυղੳϕʔεͷܕ҆શੑݕࠪ ରর : ಈతܕݕࠪ ࣮ߦલʹܕҧ൓Λ஌Δ͜ͱ͕Ͱ͖Δ

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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 ίϯύΠϧΤϥʔ Τϥʔ͸ͳ͍͕ɺ࣮ߦ࣌ʹམͪΔ ίʔυղੳϕʔεͷܕ҆શੑݕࠪ ରর : ಈతܕݕࠪ ࣮ߦલʹܕҧ൓Λ஌Δ͜ͱ͕Ͱ͖Δ

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

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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 ؔ਺ܕม਺΍ SAM (Single abstract method) ͷ౶ҥߏจ தׅހͰؔ਺ΛදݱͰ͖Δ

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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 ؔ਺ͷ this είʔϓ(receiver)Λࢦఆ͢Δ͜ͱ͕Ͱ͖Δ e.g. Groovy ͸Ͱ͖Δ͚ͲɺJava ͸Ͱ͖ͳ͍ ؔ਺ܕදه ϒϩοΫίʔυදݱ΍ྲྀெͳදݱʹศར

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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 ͜͜·ͰͷςΫχοΫͰγϯϓϧͳ DSL ͸࡞Δ͜ͱ͕Ͱ͖Δ ݪ࢝తͳ Kotlin DSL

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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 ྑ͍։ൃͷͨΊʹ͸͜ΕͰे෼ͩΖ͏͔

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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 ࠓ·Ͱͷ஌ࣝͰɺεΫϥον͔Β؆୯ʹ DSL ͸࡞ΕΔ ܕ҆શͰ͋ͬͨΓɺϥϜμࣜʹΑΔελΠϧͰॻ͖͍͚ͨͩͳΒे෼

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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 Ͱ͸ͲΜͳςΫχοΫ͕ Kotlin DSL ͷརศੑ΍දݱྗΛ͋͛Δ͔ɺͲ͏΍ͬͯ Kotlin DSL Λ࡞͍͚ͬͯΔͷ͔ΛݟͯΈ·͠ΐ͏

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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 தڃ ߏจ֦ுͷػೳ

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Syntactic Extension ▸ Extension method ▸ Operator overloading ▸ Inﬁx notation ▸ Method convention ▸ Delegation 47 Techniques of Kotlin DSL - Intermediate - 0/5 ֦ுؔ਺ ԋࢉࢠΦʔόʔϩʔυ தஔه๏ ϝιουن໿ ҕৡ

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Syntactic Extension ▸ Extension method ▸ Operator overloading ▸ Inﬁx notation ▸ Method convention ▸ Delegations 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 ▸ Can apply the techniques which I will explain later ▸ Can create DSLs without modifying 3rd party libraries 49 Techniques of Kotlin DSL - Intermediate - 1/5 ܧঝͳ͠ʹ৽͍͠ϝϯόʔϝιουΛఆٛͰ͖Δ ͨͩ͠ϝϯόʔՄࢹੑ͸ӽ͑ΒΕͳ͍ ར఺ for DSL ͜ͷ͋ͱઆ໌͢ΔςΫχοΫΛ͋ͱ͔Βద༻͢Δ͜ͱ͕Ͱ͖Δ 3rd party ϥΠϒϥϦΛमਖ਼ͤͣʹ DSL ΛߏஙͰ͖Δ

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

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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 + ͷه๏ͰςΩετϊʔυΛ࡞੒Ͱ͖Δ

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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 +<೚ҙͷςΩετ> ͱ͍͏ه๏ͦͷ΋ͷ͕ϊʔυ΁ͷςΩετ௥Ճͱ͍͏ૢ࡞Λද͍ͤͯΔ + ͸ text ΑΓ type ਺͕গͳ͍

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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 ։ൃऀ͸ԋࢉࢠΛΦʔόʔϩʔυ͢Δ͜ͱ͕Ͱ͖Δ ͭ·ΓɺಠࣗͷϩδοΫΛطଘͷԋࢉࢠʹ͕͋ͯ͑Δ རӹ for DSL ͍͍ͩͨͷέʔεͰԋࢉࢠͷํ͕ϝιου໊ΑΓ୹ͯ͘ࡁΉ ԋࢉࢠ͔Β͘Δૢ࡞ͷҙຯ͕໌ྎ

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

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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 ྲྀெͰͳ͍ɺ͗ͪ͜ͳ͍ Hamcrest ͷϢʔβʔҎ֎ʹ͸΍΍ෆ໌ྎ

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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 ࣗવݴޠʹ͍ۙɺྲྀெ Hamcrest ͷυϝΠϯ஌͕͍ࣝΒͳ͍

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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 Recever infix-function argument ͷܗͰݺͼग़͕͠Մೳ υοτ΍ׅހΛলུͯ͠ϝιουίʔϧͰ͖Δ རӹ for DSL ΋ͬͱྲྀெͳදݱΛ௥ٻͰ͖Δ υϝΠϯ஌ࣝΛinterface͔ΒݮΒ͢͜ͱ͕Ͱ͖Δ

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

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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 debug ͸ΫϩʔδϟΛड͚औΔΑ͏ͳطଘͷؔ਺Ͱ͸ͳ͍ ಈతʹੜ੒͞Εɺ౉͞ΕͨΫϩʔδϟΛݩʹઃఆ͞ΕΔ

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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 NOTE: create(String) ͸ ߏ੒ՄೳͳΦϒδΣΫτΛฦ͢ configure ؔ਺͸υϝΠϯ஌͕ࣝͩɺGroovy DSL Ͱ͸Ӆ͞Ε͍ͯΔͱ͍͏໰୊఺

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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 ϥϜμΛड͚औΕΔ invoke ؔ਺Λఆٛ͢Δ configure Λ Kotlin DSL users ͔ΒӅͤΔ

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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 ԋࢉࢠΦʔόʔϩʔυͷҰछ ϝιου͕ن໿Λຬͨ͢͜ͱͰɺJava ͕ಠࣗͰ͸ఆٛͰ͖ͳ͍Α͏ͳԋࢉࢠ΍ߏจ͕࢖͑ΔΑ͏ʹͳΔ IndexϕʔεͷΞΫηεɺinvokeɺൣғԋࢉࢠ རӹ for DSL invoke Λఆٛ͢Δ͜ͱͰɺϥϜμࣜϕʔε(ϒϩοΫίʔυ)ͷϏϧμʔͳͲ͕࡞Γ΍͍͢ Kotlin Ͱ͸Α͘࢖ΘΕΔԋࢉࢠ΍ߏจͷதʹɺυϝΠϯ஌ࣝΛӅṭ͢Δ͜ͱ͕Ͱ͖Δ

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

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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 SharedPreferences API ͸௿ڃ ୯७ͳίϚϯυΫΤϦAPI Λॱ൪ʹୟ͘ඞཁ͕͋Δ

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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 ௿ڃ API ΍υϝΠϯ஌ࣝΛӅͤΔ ྫ͑͹propertyʹ஋Λ୅ೖ͢Δͱ SharedPreferences ʹॻ͖ࠐΉ͕ɺͦͷϩδοΫ͸׬શʹӅṭ͞Ε͍ͯΔ

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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 ϓϩϖςΟ͸ΞΫηε͞ΕͨΒଞͷΦϒδΣΫτʹॲཧΛҕৡ͢Δ རӹ for DSL υϝΠϯ஌ࣝΛӅͤΔ ϓϩύςΟʹର͢ΔಡΈॻ͖ૢ࡞ʹؔ͢Δ DRY ʹศར

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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 ্ڃ Ϗϧυ؀ڥͱ͔

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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 ίϯύΠϥͷͨΊͷϓϥάΠϯγεςϜ SAM-with-receiver ίϯύΠϥϓϥάΠϯ ิؒࢠ

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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 Java ͸ SAM ͷϨγʔόΛࢦఆͰ͖ͳ͍ɻͦͷͨΊ Kotlin ͸Ϩγʔό͖ͭؔ਺Λͦͷ··(֦ுؔ਺ͳ͠)Ͱ͸ѻ͏͜ͱ͕Ͱ͖ͳ͍ɻ

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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 SAM interface ʹΞϊςʔγϣϯ͚ͭͯɺϓϥάΠϯʹ͓஌Βͤ͢Δ͚ͩ SAM ͷҾ਺͕ Kotlin Ͱ͸Ϩγʔόʔʹ

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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 SAM ม׵࣌ͷϨγʔόʔΛ͍͡ΕΔ Kotlin code ͷͨΊͷػೳ͚ͩͲɺJava ଆΛमਖ਼͢Δඞཁ͕͋Δ རӹ for DSL Kotlin ͡Όͳ͍ϥΠϒϥϦ͕ Kotlin DSL ͷͨΊͷརศੑΛߟ͑Δͱ͖ʹ༗༻ Kotlin Ͱͪΐͬͱ͗ͪ͜ͳ͍ײ͡ͷ API ʹͳΔͷΛճආͰ͖Δ

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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 Action API ͸ΊͬͪΌ࢖ΘΕͯΔͷͰɺSAM-with-receiver ͷํ͕֦ுؔ਺ΑΓྑ͍

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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 head tag ͷதʹ head tag ͸ஔ͍ͪΌ͍͚ͳ͍

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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 DslMarker ͷΞϊςʔγϣϯΛϨγʔόʔͷΫϥε΍εʔύʔΫϥεʹ͚ͭΔ

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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 ͢Δͱɺ҉໧తʹղܾ͞ΕͨϨγʔόʔ͕ҧ൓ѻ͍ͱͳͬͯΤϥʔ͕දࣔ͞ΕΔ

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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 ஌ΔݶΓɺKotlin ʹ͓͚Δ DSLಛԽͨ͠།Ұͷػೳ ݺͼग़͠ͷείʔϓʹ੍໿Λ͚ͭΔ͜ͱ͕Ͱ͖ͯɺҧ൓ͯͨ͠ΒΤϥʔͱͰ͖Δ རӹ for DSL DSL ͷ҆શੑΛߴΊΔͨΊʹ͸ඞਢ ։ൃऀͷॿ͚ʹͳΔɻIntelliJ ͸ϚʔΫ͞Εͨؔ਺ʹରͯ͠ಠࣗͷϋΠϥΠτΛ͔͚ͯ͘ΕΔ

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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 kotlinx.html ͸΋ͪΖΜར༻͍ͯͯ͠ɺ͜ΕͰਖ਼͘͠ DOM tree Λ࡞ΕΔΑ͏ʹ͍ͯ͠Δ html, head ͱ͔͕ϋΠϥΠτ͞ΕͯΔΑͶ

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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 ίϯύΠϥʔϓϥάΠϯ͸υΩϡϝϯτ͞Εͯͳ͍ ੍ݶ ෳࡶͳϩδοΫ͸ݫ͍͠ อकੑΣɾɾɾ DslMarker ͸ΫϥεʹΞϊςʔγϣϯΛ͚ͭΔඞཁ͕͋ͬͯɺͭ·ΓطଘϥΠϒϥϦͰ͸࢖͑ͳ͍

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Quick tips for DSL development 81 DSL ։ൃͷͨΊͷͪΐͬͱͨ͠ tips

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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 ٕज़తʹͳΜ͔ҧ͍ͱ͔͸ͳ͍ API ͱ DSL ͷҧ͍ interface ͷઃܭίϯηϓτ͕ҟͳΔ DSL ͸υϝΠϯʹॏ͖Λஔ͍ͯΔ DSL ͸υϝΠϯʹରͯ͠Α͘࿅ΒΕͨ API ηοτ͚ͩͲɺͦͷ෼੍ݶ΋͋Δ DSL ͕൚༻తͳ API ͱͯ͠ಈ͘ͱ૝ఆ͢Δ΂͖Ͱ͸ͳ͍

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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 ֦ுؔ਺Ͱطଘ API Λू໿͢Δͱ͜Ζ͔Βߟ͑Δ طଘϥΠϒϥϦ΁ͷ Kotlin DSL ͷద༻ ԋࢉࢠΦʔόʔϩʔυɺதஔه๏ɺϝιουن໿Λ࢖͍͖ͬͯ·͠ΐ͏ DslMarker Λ࢖͍͍ͨͳΒࣗલͰ Data transfer object Λ࡞ͬͯରԠ͠·͠ΐ͏

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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 Kotlin DSL ΛJava ϥΠϒϥϦʹద༻͢Δέʔε Ͳ͏΍ͬͯ Kotlin DSL ͱଞJVMݴޠͱͰརศੑΛҡ࣋͢Δ͔ ͪΌΜͱ sam-with-receiver Λ࢖͍·͠ΐ͏ Kotlin DSL ͷ interface ΛଞJVMݴޠ͔Β࢖͏έʔε Ϩγʔόʔ͖ͭؔ਺Ϧςϥϧ໰୊͕͋ΔͷͰɺϝιουνΣʔϯࣜͷ interface ͳͲΛผ్༻ҙ͢Δ inline ܥػೳͷՄࢹੑ໰୊͕͋ΔͷͰɺinline ͡Όͳ͍ API Λผ్ఏڙ͢Δ ଞJVMݴޠ༻్ͷ API ͱ Kotlin DSL ͸෼͚࣮ͯ૷͢Δ

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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 ͦͷ·ͨઌ΁ DSL ͷΠϯλʔϑΣʔεʹ͍ͭͯ஌Γ͍ͨ৔߹͸Martin Fowler ͷ DSL ύλʔϯ͸ಡΈ·͠ΐ͏ Kotlin DSL ͷྑ͍ྫ ༨ന͕ͳ͔ͬͨͷͰ Appendix Λݟ͍ͯͩ͘͞

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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 ७ਮͳ Kotlin ίʔυ Kotlin Ͱॻ͔ΕͨϥΠϒϥϦͷͨΊ͚ͩ͡Όͳ͍Α ͍͔ͭ͘ͷٕज़ͷ্ʹ੒ΓཱͬͯΔΑ

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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 ੩తܕ෇͚ ϥϜμࣜͱϨγʔόʔͷ؅ཧ ݴޠ࢓༷ ݴޠػೳ ֦ுؔ਺ɺԋࢉࢠΦʔόʔϩʔυɺதஔه๏ɺϝιουن໿ɺҕৡ Ϗϧυ؀ڥܥ

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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 DslMarker ͚͕ͩ Kotlin DSL ༻ͷಛԽػೳɻ҆શੑΛ޲্Ͱ͖Δ(࢖͍·͠ΐ͏) Kotlin DSL ͸طଘϥΠϒϥϦΛͪΐͬͱ੍ͨ͠໿͖͚ͭͩͲ֦ுͰ͖Δ Kotlin DSL ͱଞJVMݴޠͰͷརศੑ͸ڞଘՄೳ(ͨͩ͠ؤுΕ͹)

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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