Lambda with receiver concept in Kotlin - Step-by-Step Execution

Concept Flow - Lambda with receiver concept

Define lambda with receiver

↓

Call lambda on receiver object

↓

Inside lambda: 'this' refers to receiver

↓

Execute lambda body using receiver's members

↓

Return result or Unit

↓

End

The lambda is defined to work with a receiver object, so inside the lambda, 'this' means the receiver. When called, the lambda runs using the receiver's context.

Execution Sample

Kotlin

val result = "Hello".run {
    this.length + 5
}
println(result)

This code runs a lambda on the string "Hello" where 'this' is the string, and returns its length plus 5.

Execution Table

Step	Action	Receiver ('this')	Expression Evaluated	Result
1	Call run on "Hello"	"Hello"	Enter lambda	Lambda starts with 'this' = "Hello"
2	Evaluate this.length	"Hello"	5	Length is 5
3	Add 5 to length	"Hello"	5 + 5	10
4	Return result	"Hello"	10	Lambda returns 10
5	Assign to result	N/A	result = 10	result = 10
6	Print result	N/A	println(10)	Output: 10

💡 Lambda finishes after returning 10, which is assigned to result and printed.

Variable Tracker

Variable	Start	After Step 4	Final
this	N/A	"Hello"	"Hello"
result	N/A	N/A	10

Key Moments - 2 Insights

Why does 'this' inside the lambda refer to the string "Hello"?

Because the lambda is called with 'run' on the string, making the string the receiver. See execution_table step 1 where 'this' is set to "Hello".

What happens if we use 'it' instead of 'this' inside the lambda?

'it' is not defined in a lambda with receiver unless explicitly passed. Here, 'this' refers to the receiver object. See execution_table steps 2-3 where 'this.length' is used.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'this.length' at step 2?

A 5

B 10

C Hello

D Undefined

💡 Hint

Check the 'Expression Evaluated' column at step 2 in execution_table.

At which step does the lambda return its final value?

A Step 3

B Step 4

C Step 5

D Step 6

💡 Hint

Look for the step where 'Lambda returns' is mentioned in the Result column.

If we changed the lambda to add 10 instead of 5, what would be the final result?

A 10

B 5

C 15

D 20

💡 Hint

Add 10 to the length 5 of "Hello" as shown in variable_tracker.

Concept Snapshot

Lambda with receiver syntax:
val result = receiver.run {
  // 'this' refers to receiver
  expression
}

Inside the lambda, 'this' is the receiver object.
Useful to call multiple receiver methods cleanly.
Returns the lambda's last expression value.

Full Transcript

This example shows how a lambda with receiver works in Kotlin. We call the 'run' function on the string "Hello". Inside the lambda, 'this' refers to the string. We get the length of the string using 'this.length', which is 5. Then we add 5 to it, resulting in 10. The lambda returns 10, which is assigned to the variable 'result'. Finally, we print 'result', which outputs 10. The key point is that inside the lambda, 'this' means the receiver object, allowing us to access its members directly.

Practice

1. What is the main benefit of using a lambda with receiver in Kotlin?

easy

A. It disables access to the outer scope variables.

B. It automatically creates a new object for each lambda execution.

C. It forces all lambdas to return Unit type.

D. It allows you to call methods on an object without repeating the object name inside the lambda.

Solution

1. Step 1: Understand lambda with receiver purpose

   A lambda with receiver lets you write code blocks that behave as if they belong to an object, so you can call its methods directly.

2. Step 2: Identify the benefit

   This means you don't have to repeat the object name inside the lambda, making code cleaner and easier to read.

3. Final Answer:

   It allows you to call methods on an object without repeating the object name inside the lambda. -> Option D

4. Quick Check:

   Lambda with receiver = cleaner calls [OK]

Hint: Remember: receiver means 'this' inside lambda [OK]

Common Mistakes:

• Thinking it creates new objects automatically
• Assuming it restricts return types
• Believing it blocks outer variables

2. Which of the following is the correct syntax to declare a lambda with receiver of type StringBuilder in Kotlin?

easy

A. val lambda: () -> StringBuilder = { append("Hello") }

B. val lambda: (StringBuilder) -> Unit = { append("Hello") }

C. val lambda: StringBuilder.() -> Unit = { append("Hello") }

D. val lambda: StringBuilder -> Unit = { append("Hello") }

Solution

1. Step 1: Recall lambda with receiver syntax

   A lambda with receiver is declared by specifying the receiver type followed by a dot and then the function type, like StringBuilder.() -> Unit.

2. Step 2: Match syntax with options

   val lambda: StringBuilder.() -> Unit = { append("Hello") } matches this syntax and uses append("Hello") inside the lambda, which is valid for StringBuilder.

3. Final Answer:

   val lambda: StringBuilder.() -> Unit = { append("Hello") } -> Option C

4. Quick Check:

   Receiver syntax = Type.() -> ReturnType [OK]

Hint: Receiver lambdas use Type.() -> ReturnType syntax [OK]

Common Mistakes:

• Confusing parameter lambdas with receiver lambdas
• Missing the dot after the receiver type
• Using arrow without parentheses

3. What is the output of the following Kotlin code?

val buildString: StringBuilder.() -> Unit = {
    append("Hi")
    append(", ")
    append("there!")
}

val result = StringBuilder().apply(buildString).toString()
println(result)

medium

A. Compilation error

B. Hi, there!

C. Hi there!

D. StringBuilder@<hashcode>

Solution

1. Step 1: Understand the lambda with receiver usage

   The lambda buildString is a StringBuilder receiver lambda that appends "Hi", ", ", and "there!" to the StringBuilder.

2. Step 2: Apply lambda and convert to string

   The apply function runs the lambda on a new StringBuilder instance, then toString() converts it to the string "Hi, there!".

3. Final Answer:

   Hi, there! -> Option B

4. Quick Check:

   apply + receiver lambda = built string [OK]

Hint: apply runs lambda on object, returns object [OK]

Common Mistakes:

• Expecting default toString output of StringBuilder
• Missing commas in output
• Thinking lambda returns string directly

4. Identify the error in this Kotlin code using a lambda with receiver:

val printLength: String.() -> Unit = {
    println(length())
}

"Hello".printLength()

medium

A. Cannot call length() as a function; length is a property.

B. Lambda with receiver must return a value.

C. String cannot be a receiver type.

D. Missing parentheses when calling printLength.

Solution

1. Step 1: Check usage of length inside lambda

   In Kotlin, length is a property of String, not a function, so calling length() causes an error.

2. Step 2: Verify other parts

   The lambda correctly uses String as receiver and returns Unit, and calling printLength without parentheses is valid syntax.

3. Final Answer:

   Cannot call length() as a function; length is a property. -> Option A

4. Quick Check:

   Properties accessed without () [OK]

Hint: Remember: properties have no parentheses [OK]

Common Mistakes:

• Using parentheses with properties
• Thinking receiver type can't be String
• Forgetting lambda return type can be Unit

5. You want to create a function buildHtml that uses a lambda with receiver to build an HTML string with nested tags. Which approach correctly uses lambda with receiver to achieve this?

fun buildHtml(block: StringBuilder.() -> Unit): String {
    val sb = StringBuilder()
    sb.block()
    return sb.toString()
}

val html = buildHtml {
    append("")
    append("")
    append("Hello")
    append("")
    append("")
}
println(html)

hard

A. This code correctly uses a lambda with receiver to build the HTML string.

B. The lambda should have a receiver of type HtmlBuilder, not StringBuilder.

C. You must return the lambda result explicitly inside buildHtml.

D. The lambda cannot be called with sb.block(); use block(sb) instead.

Solution

1. Step 1: Analyze the function signature and call

   The function buildHtml takes a lambda with receiver StringBuilder.() -> Unit and calls it on sb using sb.block(), which is correct syntax.

2. Step 2: Check the lambda body and return

   The lambda appends HTML tags to StringBuilder, and the function returns the built string with sb.toString(), which is the expected behavior.

3. Final Answer:

   This code correctly uses a lambda with receiver to build the HTML string. -> Option A

4. Quick Check:

   Lambda with receiver called on object = correct [OK]

Hint: Call receiver lambda with object.lambda() syntax [OK]

Common Mistakes:

• Confusing lambda with receiver and lambda with parameter
• Forgetting to call lambda on the object
• Expecting lambda to return string directly