In Kotlin, lambdas and anonymous functions are fundamental features used for functional programming. They allow you to write concise and flexible code. Here’s an overview on how to use them:

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1. Lambda Expressions

A lambda is an anonymous function defined using curly braces {}. It’s a short way to declare a function without explicitly naming it.

Syntax:

{ parameter(s) -> body }

Example:

// Lambda that takes two integers and returns their sum
val sum = { a: Int, b: Int -> a + b }

// Using the lambda
println(sum(3, 5)) // Output: 8

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2. Single-Parameter Lambdas

If a lambda has only one parameter, you can omit the parameter declaration and use it instead (an implicit name for the parameter).

Example:

val square: (Int) -> Int = { it * it } // 'it' is the implicit name for the parameter

println(square(4)) // Output: 16

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3. Passing Lambdas to Higher-Order Functions

You can pass lambdas as arguments to functions that take other functions as parameters, referred to as higher-order functions.

Example:

fun calculate(a: Int, b: Int, operation: (Int, Int) -> Int): Int {
    return operation(a, b)
}

// Passing a lambda
val result = calculate(4, 5) { x, y -> x + y }
println(result) // Output: 9

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4. Anonymous Functions

An anonymous function is similar to a lambda but explicitly uses the function keyword. It allows you to specify return types.

Example:

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

println(multiply(3, 4)) // Output: 12

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5. Differences Between Lambdas and Anonymous Functions

• Lambdas implicitly infer the return type (using the last expression), whereas anonymous functions can have explicitly declared return types.
• Lambdas cannot use a return keyword for the enclosing function, while anonymous functions can.

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6. Inline Lambda Usage

For functions like map, filter, or forEach, lambdas can be used to process collections concisely. These functions come from Kotlin’s standard library.

Example:

val numbers = listOf(1, 2, 3, 4, 5)

// Transform each element using a lambda
val doubled = numbers.map { it * 2 }
println(doubled) // Output: [2, 4, 6, 8, 10]

// Filter using a lambda
val evens = numbers.filter { it % 2 == 0 }
println(evens) // Output: [2, 4]

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7. Lambda as Return Type

You can assign functions returning lambdas to variables.

Example:

fun createMultiplier(factor: Int): (Int) -> Int {
    return { number -> number * factor }
}

val timesThree = createMultiplier(3)
println(timesThree(5)) // Output: 15

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8. Default Parameters in Lambda

While lambdas themselves don’t support default arguments, you can achieve a similar effect by wrapping them in a function that provides default behavior.

Example:

fun greet(name: String, message: (String) -> String = { "Hello, $it!" }) {
    println(message(name))
}

// Using the default lambda
greet("John") // Output: Hello, John!

// Customizing the lambda
greet("John") { "Hi, $it! Welcome back!" } // Output: Hi, John! Welcome back!

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9. Higher-Order Functions Inline and Crossinline

When using lambdas in performance-critical situations, consider using the inline or crossinline modifier, which instructs the compiler to inline the lambda directly into the calling function.

Example:

inline fun perform(action: () -> Unit) {
    action()
}

perform {
    println("This lambda was inlined!")
}

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Summary

• Lambdas: { parameter(s) -> body }
• Single-parameter lambdas can use it as the implicit name.
• Anonymous functions use the fun keyword and can declare explicit return types.
• You can pass lambdas to higher-order functions for concise and flexible processing.
• Use collections functions like map, filter, and forEach to apply lambdas efficiently.

The combination of lambdas and Kotlin’s higher-order functions lets you write clear and concise functional code!