Revisiting types in Kotlin

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REVISITING TYPES IN KOTLIN “PACO” ESTEVEZ

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FROM CLASSES TO TYPES EAT YOUR VEGGIES

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CLASSES ENCAPSULATE DATA AND BEHAVIOUR. Classic OOP

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DATA

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DATA CHALLENGES ▸ Beans and not Beans ▸ Immutability ▸ Nullables and validation ▸ equals(), hashCode(), and toString()

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BEHAVIOUR

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HERE BE MONSTERS BEHAVIOUR IN JAVA

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BUT WHAT IS A TYPE ANYWAY? Someone at the back

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WHAT IS A TYPE ▸ A ﬁnite set of encapsulated operations ▸ A representation of either value or behaviour ▸ Some types can be represented with classes, some need language support ▸ They can be checked at compile time, so sometimes they’re called proofs ▸ Remember this

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PRIMITIVES No implicit type coercion. Boxing is hidden from you.

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FUNCTIONS

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FUNCTIONS IN KOTLIN ▸ Functions are runtime objects too! ▸ Create an instance using operator :: ▸ Deﬁne them using lambda nomenclature ▸ A function as last parameter to another function gets syntactic sugar ▸ Great for DSLs

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COLLECTIONS AND COMBINATORS

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COLLECTIONS IN KOTLIN ▸ Saner version of Java’s, and fully compatible ▸ Two ﬂavours: mutable and immutable ▸ Operations with function parameters: map, ﬁlter, reduce… https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/

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DATA CLASSES

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DATA CLASSES IN KOTLIN ▸ Synthetic accessors ▸ val for immutable, var for mutable ▸ Nullable types and Default values ▸ Named parameters ▸ data qualiﬁer: all of the above plus equals(), hashCode() and toString()

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OPEN CLASSES AND INTERFACES

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HERE BE JAVA

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ENUMS

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ENUM CLASSES IN KOTLIN ▸ Similar to Java ▸ values ▸ ordinal position ▸ Implement Enum interface if you’re a generic abstractions kind of person ▸ Look into Kotlin’s reiﬁed generics when you get home

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SEALED CLASSES

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DOMAIN DRIVEN DESIGN WITH TYPES ▸ Separate data from behaviour

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DOMAIN DRIVEN DESIGN WITH TYPES ▸ Separate data from behaviour ▸ Express execution ﬂows as data

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DOMAIN DRIVEN DESIGN WITH TYPES ▸ Separate data from behaviour ▸ Express execution ﬂows as data ▸ Encode behaviour in types

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DOMAIN DRIVEN DESIGN WITH TYPES ▸ Separate data from behaviour ▸ Express execution ﬂows as data ▸ Encode behaviour in types ▸ Make illegal states unrepresentable

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DOMAIN DRIVEN DESIGN WITH TYPES ▸ Separate data from behaviour ▸ Express execution ﬂows as data ▸ Encode behaviour in types ▸ Make illegal states unrepresentable ▸ Push validation to edges

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PART TWO

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SEALED CLASSES IN KOTLIN ▸ Closed inheritance ▸ Accessing ﬁelds and methods requires dereferencing using when() and is -> ▸ The compiler checks that all cases are handled! ▸ Data classes and implement interfaces (1.1) ▸ Type alias (1.1)

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MAKE THE COMPILER WORK FOR YOU! Me

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WHAT IF I CAN’T USE KOTLIN? JAVA 8 ▸ JavaSealedUnions JAVA 6 ▸ RxSealedUnions SCALA ▸ You should be on stage pacoworks.com/ @fe_hudl github.com/pakoito tinyurl.com/KMobi16