Kotlin Lambdas Expressions and Anonymous Functions

In this article, we are going to learn lambdas expression and anonymous function in Kotlin. While syntactically similar, Kotlin and Java lambdas have very different features. Lambdas expression and Anonymous function both are function literals means these functions are not declared but passed immediately as an expression. 

Lambda Expression

As we know, the syntax of Kotlin lambdas is similar to Java Lambdas. A function without a name is called an anonymous function. For a lambda expression, we can say that it is an anonymous function.

Syntax of Lambda expression

val lambda_name : Data_type = { argument_List -> code_body }

A lambda expression is always surrounded by curly braces, argument declarations go inside curly braces and have optional type annotations, the code_body goes after an arrow -> sign. If the inferred return type of the lambda is not Unit, then the last expression inside the lambda body is treated as return value.

Example: 

val sum = {a: Int , b: Int -> a + b}

In Kotlin, the lambda expression contains an optional part except for the code_body. Below is the lambda expression after eliminating the optional part. 

val sum:(Int,Int) -> Int = { a, b -> a + b}

Note: We don't always require a variable because it can be passed directly as an argument to a function.

Accessing the Lambda expression

fun main(args: Array<String>) {
    val company = { println("GeeksforGeeks")}

    // invoking functions:
    
    // Method1
    company()  

    // Method2
    company.invoke() 
}

Output: 

GeeksforGeeks
GeeksforGeeks

Kotlin program of using lambda expression

// with type annotation in lambda expression
val sum1 = { a: Int, b: Int -> a + b }

// without type annotation in lambda expression
val sum2:(Int,Int)-> Int  = { a , b -> a + b}

fun main(args: Array<String>) {
    val result1 = sum1(2,3)
    val result2 = sum2(3,4)
    println("The sum of two numbers is: $result1")
    println("The sum of two numbers is: $result2")

    // directly print the return value of lambda
    // without storing in a variable.
    println(sum1(5,7))
}

Output: 

The sum of two numbers is: 5
The sum of two numbers is: 7
12

Type inference in lambdas

Kotlin’s type inference helps the compiler evaluate the type of a lambda expression. Below is the lambda expression using which we can compute the sum of two integers.  

val sum = {a: Int , b: Int -> a + b}

Here, Kotlin compiler self evaluate it as a function which take two parameters of type Int and returns Int value. 

(Int,Int) -> Int

If we wanted to return String value than we can do it with help of toString() inbuilt function. 

val sum1 = { a: Int, b: Int ->
    val num = a + b
    num.toString()     //convert Integer to String
}
fun main(args: Array<String>) {
    val result1 = sum1(2,3)
    println("The sum of two numbers is: $result1")
}

Output: 

The sum of two numbers is: 5

In above program, Kotlin compiler self evaluate it as a function which takes two integer values and returns String. 

(Int,Int) -> String  

Type declaration in lambdas

We must explicitly declare the type of our lambda expression. If lambda returns no value, then we can use: Unit.

Pattern: (Input) -> Output

Lambda examples with return type:

val lambda1: (Int) -> Int = {a -> a * a}
val lambda2: (String,String) -> String = { a , b -> a + b }
val lambda3: (Int)-> Unit = {print(Int)}

Lambdas can be used as a class extension: 

val lambda4: String.(Int) -> String = {this + it} 

Here, it represents the implicit name of the single parameter, and we will discuss it later.

Kotlin program when lambdas are used as a class extension 

val lambda4 : String.(Int) -> String = { this + it }

fun main(args: Array<String>) {
    val result = "Geeks".lambda4(50)
    print(result)
}

Output: 

Geeks50

Explanation: 

In the above example, we are using the lambda expression as class extension. We have passed the parameters according to the format given above. this keyword is used for the string and it keyword is used for the Int parameter passed in the lambda. Then the code_body concatenates both the values and returns to variable result. 

it: implicit name of a single parameter

In most of cases lambdas contains the single parameter. Here, it is used to represent the single parameter we pass to lambda expression.

Kotlin program using the shorthand form of the lambda function

val numbers = arrayOf(1,-2,3,-4,5)

fun main(args: Array<String>) {
      println(numbers.filter {  it > 0 })
}

Output:

[1, 3, 5]

Kotlin program using the longhand form of the lambda function

val numbers = arrayOf(1,-2,3,-4,5)

fun main(args: Array<String>) {
     println(numbers.filter {item -> item > 0 })
}

Output: 

[1, 3, 5] 

Returning a value from a lambda expression

After executing a lambda function, the final value returned can be any of the following: Integer, String, or Boolean.

Kotlin program to return a String value by a lambda function 

val find =fun(num: Int): String{
if(num % 2==0 && num < 0) {
    return "Number is even and negative"
   }
    else if (num %2 ==0 && num >0){
    return "Number is even and positive"
    }
    else if(num %2 !=0 && num < 0){
    return "Number is odd and negative"
    }
    else {
    return "Number is odd and positive"
    }
}
fun main(args: Array<String>) {
    val result = find(112)
    println(result)
}

Output: 

Number is even and positive

Anonymous Function

An anonymous function is very similar to a regular function except for the name of the function, which is omitted from the declaration. The body of the anonymous function can be either an expression or a block.

Example 1: Function body as an expression  

fun(a: Int, b: Int) : Int = a * b

Example 2: Function body as a block 

fun(a: Int, b: Int): Int {
    val mul = a * b
    return mul
}

Return type and parameters: 

1. The return type and parameters are also specified in same way as for regular function but we can omit the parameters if they can be inferred from the context.
2. The return type of the function can be inferred automatically from the function if it is an expression and has to be specified explicitly for the anonymous function if it is a body block.

Kotlin program to call the anonymous function 

// anonymous function with body as an expression
val anonymous1 = fun(x: Int, y: Int): Int = x + y

// anonymous function with body as a block
val anonymous2 = fun(a: Int, b: Int): Int {
            		val mul = a * b
            		return mul
       			}

fun main(args: Array<String>) {
    //invoking functions
    val sum = anonymous1(3,5)
    val mul = anonymous2(3,5)
    println("The sum of two numbers is: $sum")
    println("The multiply of two numbers is: $mul")
}

Output: 

The sum of two numbers is: 8
The multiply of two numbers is: 15

Difference between lambda expressions and anonymous functions

The only difference is the behavior of non-local returns. A return statement without a label always returns from the function declared with the fun keyword. This means that a return inside a lambda expression will return from the enclosing function, whereas a return inside an anonymous function will return from the anonymous function itself.