Instruction

Lambda Expressions

In Kotlin, a lambda expression is a function without a name. It’s used to define a code block that can be passed as an argument to a function or stored in a variable.

Defining a Lambda Expression

To define a lambda expression, you enclose a block of code in curly braces: {}. Then, you can assign the code block to a variable, such as the following:

val myFirstLambda = { println("This is my first lambda") }

The code above has two parts: a variable, myFirstLambda, on the left and a lambda expression on the right.

Now, you call the lambda from the main() function:

fun main() {
  myFirstLambda()
}

When you run this code, it prints the following on the screen:

This is my first lambda

You call a lambda just like you call a regular function. When you call the lambda it will ultimately execute the code in the curly braces. Neat!

Lambda Expressions With Arguments

A lambda expression can take arguments and return a value. The syntax of a lambda expression taking two arguments of type Int looks like this:

The last expression of the code enclosed in the curly braces is the value that will be returned when the lambda is executed. For example:

val myLambda = { num1: Int, num2: Int ->
  val sum = num1 + num2
  // The value stored in sum is the value that will be returned when the `myLambda` lambda expression is executed since it's the last statement in the lambda.
  sum
}

Type Inference

Kotlin’s type inference enables the compiler to evaluate the type of a lambda, such as in the myLambda expression:

val myLambda = { num1: Int, num2: Int ->
  val sum = num1 + num2
  sum
}

The type of the lambda is evaluated to (Int, Int) -> Int, meaning it takes two parameters of type Int and has a return type of Int.

Sometimes, the compiler can’t infer all of the types in a lambda. In such cases, you must declare the type for the lambda expression yourself. An example of this is a lambda expression that takes an argument of type String and returns an Int. You write it like this:

val stringLambda: (String) -> Int = { name: String ->
  name.length
}

If a lambda expression does not return a value, specify Unit as the return type.

You’ve learned about lambda functions and the syntax of defining a lambda. Now, it’s time to create and call lambda functions in a demo.