Lambda syntax and declaration in Kotlin - Step-by-Step Execution

Concept Flow - Lambda syntax and declaration

Start

↓

Declare Lambda

↓

Assign Lambda to Variable

↓

Call Lambda

↓

Execute Lambda Body

↓

Return Result

↓

End

This flow shows how a lambda is declared, assigned to a variable, called, and executed to produce a result.

Execution Sample

Kotlin

val sum: (Int, Int) -> Int = { a, b -> a + b }
val result = sum(3, 4)
println(result)

This code declares a lambda that adds two integers, calls it with 3 and 4, and prints the result.

Execution Table

Step	Action	Evaluation	Result
1	Declare lambda 'sum' with parameters a, b	Lambda created	sum holds lambda (a, b) -> a + b
2	Call sum(3, 4)	Invoke lambda with a=3, b=4	Execute body: 3 + 4
3	Calculate 3 + 4	Addition	7
4	Assign result	result = 7	result holds 7
5	Print result	println(7)	Output: 7

💡 Lambda executed and result printed, program ends.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 4	Final
sum	undefined	lambda (a, b) -> a + b	lambda (a, b) -> a + b	lambda (a, b) -> a + b	lambda (a, b) -> a + b
result	undefined	undefined	undefined	7	7

Key Moments - 3 Insights

Why do we write the lambda parameters before the arrow '->' inside the braces?

The parameters before '->' define inputs to the lambda. This is shown in Step 1 where 'a, b' are declared before '->' to receive values when called.

How does the lambda get called and executed?

In Step 2, calling sum(3, 4) passes 3 and 4 to the lambda parameters, then Step 3 executes the body 'a + b' with these values.

What type does the variable 'sum' have?

In Step 1, 'sum' is declared with type '(Int, Int) -> Int', meaning it takes two Ints and returns an Int, matching the lambda signature.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'result' after Step 4?

A 7

B 3

C undefined

D sum lambda

💡 Hint

Check the 'result' variable in variable_tracker after Step 4.

At which step is the lambda body 'a + b' actually executed?

A Step 1

B Step 2

C Step 3

D Step 5

💡 Hint

Look at the 'Evaluation' column in execution_table where addition happens.

If we change the lambda to { x, y -> x * y }, what changes in the execution table?

A Step 5 output changes to 7

B Step 3 result changes to 12

C Step 2 changes to call sum(3, 4)

D No changes

💡 Hint

Multiplying 3 and 4 gives 12 instead of 7, check Step 3 result.

Concept Snapshot

Lambda syntax in Kotlin:
val name: (Type1, Type2) -> ReturnType = { param1, param2 -> expression }
- Parameters before '->' inside braces
- Expression after '->' is the body
- Call lambda like a function: name(arg1, arg2)
- Lambda returns the last expression value
- Useful for short, inline functions

Full Transcript

This visual trace shows how to declare and use a lambda in Kotlin. First, a lambda is declared with parameters and a body expression. It is assigned to a variable with a function type. Then, calling the lambda passes arguments to parameters, executes the body, and returns the result. The variable 'result' stores this output, which is printed. Key points include the syntax of parameters before the arrow, calling the lambda like a function, and the lambda returning the last expression. The execution table and variable tracker help visualize each step clearly.