Kotlin lambda expressions_20210608

1. Lambda Expressions Haim Michael June 8th , 2021 All logos, trade marks and brand names used in this presentation belong to the respective owners. life michae l www.lifemichael.com in Kotlin

11. © 2008 Haim Michael 20150805 Table of Contents  We are going to cover the following topics: Introduction to Lambda Higher Order Functions Function Types in Kotlin The Parameters' Names Combining Function Types Instantiating Function Types Compiler Inference Invoking Function Type Objects Function Literals Lambda Expressions Syntax The Function Last Parameter The it Implicit Parameter The :: Operator The Underscore Style

12. © 2009 Haim Michael All Rights Reserved 3.1.2021 12 Introduction  The functions in Kotlin can be first class, which means that we can handle objects that represent functionality. We can store the references for those object into variables, pass the references as arguments to other functions or get them as the returned value when calling a function.

14. © 2009 Haim Michael All Rights Reserved 3.1.2021 14 Function Types  Kotlin uses a family of function types for declarations that deal with functions. val onLoad: () → Unit = …  The function type has a parenthesized parameter types list and a return type: (A,B) -> C

15. © 2009 Haim Michael All Rights Reserved 3.1.2021 15 Function Types  The function type can optionally have an additional receiver type, specified before a dot. A.(B) -> C This type represents a function we can invoke on object of type A, with parameter of type B, and a returned value of type C.

16. © 2009 Haim Michael All Rights Reserved 3.1.2021 16 Function Types  Using the suspend modifier we can indicate the function is a suspending one. suspend () -> Unit suspend A.(B) -> C  Suspending functions can be called from within coroutines only.

17. © 2009 Haim Michael All Rights Reserved 3.1.2021 17 Function Types  In order to specify a function type is nullable we should place it all in parentheses and add the question mark. ((Int,Int)->Int)?  We can give a function type an alternative alias by using the typealias keyword. typealias Calculate = (Int,Int) ->Int

18. © 2009 Haim Michael All Rights Reserved 3.1.2021 18 Parameters Names  The function type notation can include names for the parameters. Using such names assists with the creation of accurate documentation. (height:Int, width:Int) -> Rectangle

19. © 2009 Haim Michael All Rights Reserved 3.1.2021 19 Combining Function Types  It is possible to combine function types and use parentheses in order to get code that is easier to understand. (Int) -> ((int)->Unit)  The arrow notation is right associative, which enables us to write alternatively the following: (Int) -> (Int) -> Unit `

20. © 2009 Haim Michael All Rights Reserved 3.1.2021 20 Instantiate Function Type  There are several ways for instantiating a function type. We can do it using a lambda expression. {a,b -> a+b}  We can do it using an anonymous function. fun(s:String):Int {return s.length}

21. © 2009 Haim Michael All Rights Reserved 3.1.2021 21 Instantiate Function Type  We can declare a class that implement a specific function type as an interface.  We can assign a function type variable with a reference for an object we instantiate from that class.

22. © 2009 Haim Michael All Rights Reserved 3.1.2021 22 Instantiate Function Type class X : (Int) -> Int { override fun invoke(x:Int):Int = 10*x } var f = X() fun main(args: Array<String>) { print(f(4)) }

24. © 2009 Haim Michael All Rights Reserved 3.1.2021 24 Function Type with Receivers  The values of function types with and without receiver are interchangeable. The receiver can stand for the first parameter. var f1:String.(Int)->String = {num -> this.repeat(num)} var f2:(String,Int)->String = f1

25. © 2009 Haim Michael All Rights Reserved 3.1.2021 25 Invoking Function Type Instances  We can invoke it either using the invoke(...) method or just by using simple parentheses. class X : (Int) -> Int { override fun invoke(x:Int):Int = 10*x } var f = X() fun main(args: Array<String>) { print(f(4)) print(f.invoke(4)) }

26. © 2009 Haim Michael All Rights Reserved 3.1.2021 26 Function Literals  Lambda expressions and anonymous functions are considered as function literals.  Functions literal are functions that are not declared and can be passed over. max (numbers, {a,b->a<b})

27. © 2009 Haim Michael All Rights Reserved 3.1.2021 27 Lambda Expressions Syntax  The full syntactic form of a lambda expression includes the use of the curly braces. The lambda expression is always surrounded by curly braces. val f = {x:Int,y:Int->x+y}

28. © 2009 Haim Michael All Rights Reserved 3.1.2021 28 Lambda Expressions Syntax  If the return type of the lambda expression is not Unit, then the last (or possibly single) expression inside the lambda body is treated as the returned value. val sum : (Int,Int)->Int = {x,y->x+y}; val sum : (Int,Int)->Int = {x,y-> var result = x + y result };

29. © 2009 Haim Michael All Rights Reserved 3.1.2021 29 The Function Last Parameter  If the last parameter of a function accepts a function, a lambda expression that is passed over as the corresponding argument to that parameter, can be placed outside of the parentheses: val ob = accounts.doSomething(0) {total,e → total *e}

30. © 2009 Haim Michael All Rights Reserved 3.1.2021 30 The it Implicit Parameter  When the lambda expression has one parameter only, it is allows to omit the arrow and implicitly name the parameter with it. numbers.filter(it%2==0)

31. © 2009 Haim Michael All Rights Reserved 3.1.2021 31 The :: Operator  When prepending a function name with the '::' operator we will get a reference for the object that represents the function.  We can then assign that referent to a variable that its type is a function type. fun add(a: Int, b: Int) = a + b var f: (n1:Int,n2:Int)->Int = ::add println(f(3,4)) //prints 7

32. © 2009 Haim Michael All Rights Reserved 3.1.2021 32 The Underscore Style  When writing a lambda expression and having a parameter we don't use we can place an underscore instead. map.forEach {_,v -> println(“$v is awesome!“}

Kotlin lambda expressions_20210608

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