# Understanding Higher-Order Functions and Lambdas in Kotlin

**This article is for anyone who is curious about the higher-order functions and lambdas in Kotlin but has no idea what they are exactly.**

My (**Amit Shekhar**) goal is to make you understand what are higher-order functions and lambdas which means that there are few simplifications done while writing this. If you understand what higher-order functions and lambdas in Kotlin are, then my mission will be accomplished.

Let's start with the Lambda Expressions and understand what they are.

### Lambdas Expressions

Lambdas Expressions are essentially anonymous functions that we can treat as values – we can, for example, pass them as arguments to functions, return them, or do any other thing we could do with a normal object.

Lambda Expressions look like below:

`val square : (Int) -> Int = { value -> value * value }`

`val nine = square(3)`

Now, let's learn it by a simple example in Kotlin:

`val doNothing : (Int) -> Int = { value -> value }`

This is a lambda expression that does nothing. Here the

** { value -> value }** is a complete function in itself. It takes an

**int**as a parameter and returns a value as an

**.**

`int`

In** (Int) -> Int **

** (Int)** represents input

**as a parameter.**

`int`

** Int** represents return type as an

**.**

`int`

So, the ** doNothing** is a function in itself that takes a value as an

**and returns the same value as an**

`int`

**.**

`int`

Now another example:

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

This is also a lambda expression that takes two ** int** as the parameters, adds them, and returns as an

**.**

`int`

** { a, b -> a + b }** is a function in itself that takes two

**as the parameters, adds them, and returns as an**

`int`

**int**.

In** (Int, Int) -> Int**

** (Int, Int)** represents two

**as the input parameters.**

`int`

** Int** represents return type as an

**.**

`int`

So, the ** add** is a function in itself that takes two

**as the parameters, adds them, and returns as an**

`int`

**.**

`int`

We can call it very similar as we do with the function like below:

`val result = add(2,3)`

Now, that we have understood the lambda expressions. Let's move to the Higher-order functions.

### Higher-Order Functions

A higher-order function is a function that takes functions as parameters or returns a function.

It's a function which can take do two things:

- Can take functions as parameters
- Can return a function

We will see both the thing one by one.

Let's start with the first one - A function can take functions as parameters.

```
fun passMeFunction(abc: () -> Unit) {
// I can take function
// do something here
// execute the function
abc()
}
```

This takes a function `abc: () -> Unit`

** abc** is just the name for the parameter. It can be anything. We just need to use this when we execute the function.

** () -> Unit**, this is important.

** ()** represents that the function takes no parameters.

** Unit** represents that the function does not return anything.

So, the ** passMeFunction** can take a function that takes zero parameters and does not return anything.

Let try passing that type of function to the ** passMeFunction**.

```
passMeFunction(
{
val user = User()
user.name = "ABC"
println("Lambda is awesome")
}
)
```

Here `{ }`

is a function in itself. See below highlighted one.

```
passMeFunction(
{
val user = User()
user.name = "ABC"
println("Lambda is awesome")
}
)
```

Which takes zero parameters and does not return anything. It just creates a user, set the name, and print something. This means it does not take any parameters and does not return anything.

Let's add ** fun abc()** just for the sake of understanding it. Actually, we do not need to write

**in our code. It is just for understanding purpose now.**

`fun abc()`

```
passMeFunction(
fun abc(){
val user = User()
user.name = "ABC"
println("Lambda is awesome")
}
)
```

Now, we can clearly see that we are passing a function to the ** passMeFunction**.

As Kotlin provides us a concise way for writing code, it can be changed to the following by removing the

.**()**

```
passMeFunction {
val user = User()
user.name = "ABC"
println("Lambda is awesome")
}
```

Now, that we have understood how to pass a function as a parameter to a function.

Let's move to the second one - A function can return a function.

### A function can return a function

Suppose we have a function ** add** which takes two parameters and returns a value as in int.

```
fun add(a: Int, b: Int): Int {
return a + b
}
```

And, we have a function ** returnMeAddFunction** which takes zero parameters and returns a function of the type

**.**

`((Int, Int) -> Int)`

```
fun returnMeAddFunction(): ((Int, Int) -> Int) {
// can do something and return function as well
// returning function
return ::add
}
```

`((Int, Int) -> Int)`

** (Int, Int) **means that the function should take two parameters both as the

**.**

`int`

** Int** means that the function should return value as an

**.**

`int`

Now, we can call the ** returnMeAddFunction,** get the add function, and call it like below:

```
val add = returnMeAddFunction()
val result = add(2, 2)
```

This is what higher-order functions are.

I hope you understood "What are Higher-Order Functions and Lambdas in Kotlin?"

That's it for now.

Happy Learning :)

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