Kotlin - Scope Function

There are several functions in the Kotlin standard library that help in the execution of a block of code within the context of an object. Calling these functions on an object with lambda expression creates a temporary scope. These functions are called Scope Functions. We can access the object of these functions without its name. Sounds confusing! Let's see an example,

Example: Without using scope function

Output:

GeeksforGeeks

Example: Using scope function

Output:

GeeksforGeeks

Explanation

You must have noticed that when we are not using the scope function, we need to write the object name every time to refer to members of the class. While using the scope function, we can directly refer to members without the object name. This is one of the ways of using the scope function. We will learn more about them in this article.

Scope Functions

Every scope function has well-defined use cases, although all of them have nearly the same outcome. Now let’s look at each scope functions and its use cases:

Application of using scope functions

Scope functions make code more clear, readable, and concise which are Kotlin language’s main features.

Types of scope functions

There are five types of scope functions:

1. let
2. run
3. with
4. apply
5. also

Each of these functions is quite similar in nature with minor differences. It's often confusing to decide which function to use and when. So, we need to know what are the differences between these functions and their use cases.

Differences in these functions:

There are mainly two differences among these functions:

1. Way of referring to a context object (i.e. using either ‘this’ or ‘it’ keyword)
2. return value (i.e. returns either ‘context object’ or ‘lambda result’)

Note: Context object refers to the object on which we are using the scope functions. As in our previous example - ‘gfg’ is our context object

Scope functions table:

Function	Object Reference	Return Value
let	it	Lambda result
run	this	Lambda result
with	this	Lambda result
apply	this	Context object
also	it	Context object

1. let function

Context object : it 
Return value : lambda result

Use Case:

let function is often used to provide null safety calls. Use safe call operator(?.) with ‘let’ for null safety. It executes the block only with the non-null value.

Example:

Output:

2

Explanation:

As you see when the value of ‘a’ is ‘null’ let function simply avoid the code block. Hence, solving the biggest nightmare of programmers - NullPointerException.

2. apply function

Context object : this
Return value : context object

Use Case:

As the name implies - “Apply these to the object”. It can be used to operate on members of the receiver object mostly to initialize members.

Example:

3. with function

Context object : this
Return value : lambda result

Use Case:

Recommended use of ‘with’ for calling functions on context objects without providing the lambda result.

Example:

Output:

GeeksforGeeks

4. run function

Context object : this
Return value : lambda result

‘run’ function can be said as the combination of ‘let’ and ‘with’ functions.

Use Case:

Used when the object lambda contains both initialization and the computation of the return value.  Using run we can perform null safety calls as well as other computations.

Example:

Output:

Company Name : 

Company Name : GeeksforGeeks

Explanation:

When the 'company' value is null, the body of the run is simply ignored. When it is non-null, the body executes.

5. also function

Context object : it
Return value : context object

Use Case:

It is used where we have to perform additional operations when we have initialized the object members.

Example:

Output:

[1, 3, 4]

Object References

There are two ways of object referencing in scope functions:

1. this

We can refer to the context object by a lambda receiver keyword - this. this keyword does object reference in ‘run’, ‘with’, and ‘apply’ functions.

Example:

Note: We can exclude this keyword to refer to members of the class.

2. it

‘let’ and ‘also’ functions refer to the object's context as a lambda argument.

Example:

Return values

There are two types of return values that a scope functions can return:

1. Lambda result

If we write any expression at the end of the code block, it becomes the return value for the scope function. Return value for ‘let’, ‘run’, and ‘with’ functions is the lambda result.

Example:

Output:

GfG's Founder : Sandeep Jain

2. Context object

‘apply’ and ‘also’ functions return the context object itself. In this case, we don’t need to specify the return value. The context object is automatically returned.

Example:

Output:

GfG's Founder : Sandeep Jain

Summary

• Scope functions make code more readable, clear and concise.
• Object reference - 'this' and 'it'.
• Return value - context object and lambda result.
• let : working with nullable objects to avoid NullPointerException.
• apply : changing object configuration.
• run: operate on nullable object, executing lambda expressions.
• also : adding additional operations.
• with : operating on non-null objects.