The Power of Types, KotlinConf 2019

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Copenhagen Denmark THE POWER OF TYPES DANNY PREUSSLER @PreusslerBerlin

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Do you enjoy debugging?

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In an ideal world, in the right language, buggy code is impossible. The compiler simply refuses to build

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In an ideal world, in the right language, buggy code is impossible. The compiler simply refuses to build

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In an ideal world, in the right language, buggy code is impossible. The compiler simply refuses to build

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In an ideal world, in the right language, buggy code is impossible. The compiler simply refuses to build

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No language can prevent us from writing bugs!

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But we can avoid as many as possible, as early as possible

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TYPES

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Well-typed expressions do not go wrong A theory of type polymorphism in programming. Journal of Computer and System Sciences, Robert Milner, 1978

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WHAT IS A TYPE?

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abstraction mechanisms over data Several Types of Types in Programming Languages Simone Martini, Universit`a di Bologna

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A type is a concise, formal description of the behavior of a program fragment Type systems for programming languages, Didier Rémy, INRIA Paris

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THE HISTORY OF TYPES

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1950 1960 1970 1980 1990 2000 2010 2020

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Types are pretty much absent 1950 1960 1970 1980 1990 2000 2010 2020

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1950 1960 1970 1980 1990 2000 2010 2020 Fortran

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“Two types of constants are permissible: fixed points (restricted to integers) and floating points” FORTRAN manual, 1956 But at same time, “mode” is used instead 1950 1960 1970 1980 1990 2000 2010 2020 Fortran

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1950 1960 1970 1980 1990 2000 2010 2020 Fortran Algol 60 Types were born

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Integers are of type integer. All other numbers are of type real. Algol 60 1950 1960 1970 1980 1990 2000 2010 2020 Fortran Algol 60

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the results of any program [..] should be comprehensible without knowing anything about the machine or its storage layout. Tony Hoare 1960 1950 1960 1970 1980 1990 2000 2010 2020 Fortran Algol 60

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I realised that [types] were essential not only for determining memory requirements, but also for avoiding machine-dependent error Tony Hoare 1960 1950 1960 1970 1980 1990 2000 2010 2020 Fortran Algol 60

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1950 1960 1970 1980 1990 2000 2010 2020 Fortran Algol 60 Algol W 1966 Algol W

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One missing piece 1950 1960 1970 1980 1990 2000 2010 2020 Fortran Algol 60 Algol W

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the type system (including user-defined types) [should] guarantee that only the prescribed operations on a type could operate on its values James H. Morris. Types are not sets, New York, NY, USA, 1973. ACM 1950 1960 1970 1980 1990 2000 2010 2020 Fortran Algol 60 Algol W

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1950 1960 1970 1980 1990 2000 2010 2020 Fortran Algol 60 Algol W Pascal

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Types became a central feature of programming languages as we understand them today 1950 1960 1970 1980 1990 2000 2010 2020 Fortran Algol 60 Algol W Pascal

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The dawn of Object Orientation 1950 1960 1970 1980 1990 2000 2010 2020 Fortran Algol 60 Algol W Pascal C++

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Type structure is a syntactic discipline for enforcing levels of abstraction Types, abstraction and parametric polymorphism, John C. Reynolds, 1983 1950 1960 1970 1980 1990 2000 2010 2020 Fortran Algol 60 Algol W Pascal C++

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The age of the internet languages 1950 1960 1970 1980 1990 2000 2010 2020 Fortran Algol 60 Algol W Pascal C++ Java

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Object-oriented languages had made static typing so painful that many preferred to give up and just wing it. Ben Lynn, https://benlynn.blogspot.com/ 1950 1960 1970 1980 1990 2000 2010 2020 Fortran Algol 60 Algol W Pascal C++ Java

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Dynamic Typed languages 1950 1960 1970 1980 1990 2000 2010 2020 Fortran Algol 60 Algol W Pascal C++ Java JS Ruby

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It is important to note that this is diﬀerent than being typeless Dynamically Typed Languages, Laurence Tratt, Bournemouth University 1950 1960 1970 1980 1990 2000 2010 2020 Fortran Algol 60 Algol W Pascal C++ Java JS Ruby

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the chief technical diﬀerence between them being when types are enforced Dynamically Typed Languages, Laurence Tratt, Bournemouth University 1950 1960 1970 1980 1990 2000 2010 2020 Fortran Algol 60 Algol W Pascal C++ Java JS Ruby

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After you ship, it costs you about 10,000 times as much to ﬁx a software bug Bill Joy, Java-One, 1999 1950 1960 1970 1980 1990 2000 2010 2020 Fortran Algol 60 Algol W Pascal C++ Java JS Ruby Java Generics

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Functional Programming arises from the grave 1950 1960 1970 1980 1990 2000 2010 2020 Fortran Algol 60 Algol W Pascal C++ Java JS Ruby Scala Java Generics

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Functional programming languages are traditionally typed (Scheme and Erlang are exceptions) Type systems for programming languages, Didier Rémy, INRIA Paris 1950 1960 1970 1980 1990 2000 2010 2020 Fortran Algol 60 Algol W Pascal C++ Java JS Ruby Scala Java Generics

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Anders Hejlsberg, 2012 you can write large programs in JavaScript. You just can’t maintain them. 1950 1960 1970 1980 1990 2000 2010 2020 Fortran Algol 60 Algol W Pascal C++ Java JS Ruby Scala Typescript Java Generics

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The typed v.s. untyped ﬂavor [..] should not be confused with [..] whether types [..] are explicit or implicit. Type systems for programming languages, Didier Rémy, INRIA Paris 1950 1960 1970 1980 1990 2000 2010 2020 Fortran Algol 60 Algol W Pascal C++ Java JS Ruby Scala Typescript Java Generics

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type reconstruction starts spreading 1950 1960 1970 1980 1990 2000 2010 2020 Fortran Algol 60 Algol W Pascal C++ Java JS Ruby Scala Kotlin Typescript Java Generics

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KOTLIN AND TYPES

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The type system is Kotlin’s super power Nullability, 1st class functions, Nothing

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If Kotlin uses the power of Types So can you!

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PRIMITIVE OBSESSION

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Usage of primitives instead of small objects for simple tasks

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Usage of constants for coding information

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Don’t put in name what can be in type

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Encoding the type [..] into the name [..] is brain damaged the compiler knows the types Linus Torvalds about Hungarian notation

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actually...

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actually...

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Don’t put in name what could be in type!

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Express your Domain knowledge

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Sample: At Soundcloud a URN uniquely identiﬁes an entity: a user, track playlist… soundcloud:tracks:1234

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Anytime you ﬁnd yourself writing code of the form if the object is of type T1, then do something, but if it’s of type T2, then do something else, slap yourself. Scott Meyers, Eﬀective C++

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“type” properties/functions are a smell

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Depending properties might be hint for hidden type

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Factory methods are hint for diﬀerent types

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Booleans are a smell: 2 things in 1 place

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Don’t let the creep in

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COSTS

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ALLOCATION COSTS

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On JVM, objects allocated in Eden space

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To avoid thread synchronization Eden is split into Thread Local Allocation Buﬀer

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Allocating space in TLAB is often just a pointer bump

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The compiler might even inline the whole class: Allocation elimination with escape analysis Urban performance legends, revisited, Brian Goetz 2005

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Allocations on JVM are very cheap

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A word on Android ART is optimized for allocations

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Allocations are OK Types are OK Yes, even enums Modern Android development: Android Jetpack, Kotlin, and more (Google I/O 2018)

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MEMORY COSTS

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Memory A plain Object takes 8 bytes On Sun's JDK 1.3.1 for Windows

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Memory Any wrapper will be 8 bytes larger than wrapped primitive On Sun's JDK 1.3.1 for Windows

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But careful

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Memory An empty String takes 40 bytes On Sun's JDK 1.3.1 for Windows

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Memory Boolean: header has 12 bytes + 1 for the boolean + 3 for the granularity On HotSpot JVM 1.7.0_51

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Memory its "size" isn't something that's precisely deﬁned https://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html

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Kotlin compiler translates to int

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Kotlin compiler translates to java.lang.Integer

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Kotlin compiler can use primitive int inside Age

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Kotlin compiler translates to int

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Kotlin keeps Age with int

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GARBAGE COSTS

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the performance of the garbage collector is not determined by the number of dead objects but rather by the number of live ones Analyzing Java Memory, Java enterprise performance book

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Short lived objects are cheap

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SUM UP

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● Types improve readability (documentation) ● Types help compile time correctness (safety, machine checked documentation) ● Types improve abstraction (modularity) -> Types are enabler Sum up

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Watch out for “hidden type” smells: ● Types information in names ● Type attributes ● Booleans ● Primitives Sum up

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My goal was to ensure that all use of references should be absolutely safe, with checking performed automatically by the compiler Tony Hoare about Algol W

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But I couldn't resist the temptation to put in a null reference, simply because it was so easy to implement. Tony Hoare about Algol W

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call it my billion-dollar mistake Tony Hoare about Algol W

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Readability Let the compiler do the work!

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TYPE ALL THE THINGS!

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● Several Types of Types in Programming Languages Simone Martini, Universit`a di Bologna https://arxiv.org/pdf/1510.03726 ● History and Philosophy of Computing Third International Conference, HaPoC 2015, Pisa, Italy ● Type systems for programming languages Didier Rémy, INRIA Paris, http://gallium.inria.fr/~remy/mpri/cours1.pdf ● A theory of type polymorphism in programming. Journal of Computer and System Sciences, Robert Milner, 1978 https://homepages.inf.ed.ac.uk/wadler/papers/papers-we-love/milner-type-polymorphism.pdf ● Dynamically Typed Languages Laurence Tratt,Bournemouth University, 2009 https://tratt.net/laurie/research/pubs/html/tratt__dynamically_typed_languages/ Sources History of types

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● Type Wars, The Clean Code Blog, Uncle Bob http://blog.cleancoder.com/uncle-bob/2016/05/01/TypeWars.html ● From Primitive Obsession to Domain Modelling, Mark Seemann https://blog.ploeh.dk/2015/01/19/from-primitive-obsession-to-domain-modelling/ ● Phantom types in Kotlin, Danny Preussler https://proandroiddev.com/phantom-types-in-kotlin-afd3f59fde10? ● Primitive Data types, Oracle https://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html Sources Types

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● The cost of object creation in Java, including garbage collection https://www.bettercodebytes.com/the-cost-of-object-creation-in-java-including-garbage-collection/ ● Thread-Local Allocation Buﬀers in JVM https://alidg.me/blog/2019/6/21/tlab-jvm ● Urban performance legends, revisited https://www.ibm.com/developerworks/java/library/j-jtp09275/index.html#artrelatedtopics ● E-book: Java enterprise performance https://www.dynatrace.com/resources/ebooks/javabook/ ● Do you know your data size? https://www.javaworld.com/article/2077496/java-tip-130--do-you-know-your-data-size-.html ● The impact of Garbage Collection on Application Performance https://www.dynatrace.com/resources/ebooks/javabook/impact-of-garbage-collection-on-performance/ Sources Costs

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#KotlinConf THANK YOU AND REMEMBER TO VOTE Danny Preussler @ PreusslerBerlin