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

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Inside lambda: 'this' refers to receiver

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Execute lambda body using receiver's members

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Return result or Unit

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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.