Trailing lambda convention in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the trailing lambda convention in Kotlin?

It is a Kotlin syntax feature that allows you to pass a lambda expression outside the parentheses of a function call if the lambda is the last argument.

beginner

How do you call a function with a trailing lambda in Kotlin?

You write the lambda expression after the function call parentheses instead of inside them, making the code cleaner and more readable.

beginner

Example: Convert this call to use trailing lambda:

list.filter({ it > 5 })

list.filter { it > 5 }

The lambda is moved outside the parentheses because it is the last argument.

intermediate

Can you use trailing lambda if the lambda is not the last argument?

No, trailing lambda syntax only works if the lambda is the last argument in the function call.

beginner

Why is trailing lambda convention useful?

It makes code easier to read and write, especially when lambdas are long or nested, by reducing parentheses clutter.

What does the trailing lambda convention allow in Kotlin?

A Calling functions without parentheses

B Using lambdas only inside parentheses

C Defining lambdas without curly braces

D Passing a lambda expression outside the parentheses if it is the last argument

Trailing lambda lets you put the lambda outside the parentheses when it is the last argument.

Which of these is a correct use of trailing lambda?

A list.map { it * 2 }

B list.map({ it * 2 })

C list.map it * 2

D list.map it => it * 2

Option A uses trailing lambda syntax correctly by placing the lambda outside parentheses.

Can you use trailing lambda if the function has multiple arguments and the lambda is not last?

A Yes, always

B No, only if lambda is last argument

C Only if lambda is first argument

D Only if function has one argument

Trailing lambda works only when the lambda is the last argument.

What is a benefit of using trailing lambda?

A Makes code more readable and less cluttered

B Makes code run faster

C Allows lambdas without curly braces

D Removes need for function names

Trailing lambda improves readability by reducing parentheses clutter.

Which Kotlin feature is demonstrated by this code?

button.setOnClickListener { println("Clicked") }

A Extension function

B Data class

C Trailing lambda convention

D Object declaration

The lambda passed outside parentheses shows trailing lambda convention.

Explain the trailing lambda convention in Kotlin and when you can use it.

Think about how lambdas are passed to functions.

Give an example of a function call using trailing lambda and explain why it is valid.

Try using list.filter or similar function.

Practice

1. What is the main purpose of the trailing lambda convention in Kotlin?

easy

A. To allow placing the lambda expression outside the parentheses when it is the last argument

B. To require all lambda expressions to be inside parentheses

C. To prevent using lambda expressions as function arguments

D. To force lambda expressions to be declared as variables before use

Solution

1. Step 1: Understand the trailing lambda convention and compare options

   The trailing lambda convention lets you write a lambda expression outside the parentheses if it is the last argument of a function. Only "To allow placing the lambda expression outside the parentheses when it is the last argument" correctly describes this behavior. Other options contradict the convention.

2. Final Answer:

   To allow placing the lambda expression outside the parentheses when it is the last argument -> Option A

3. Quick Check:

   Trailing lambda = lambda outside parentheses [OK]

Hint: Last lambda argument can go outside parentheses [OK]

Common Mistakes:

• Thinking lambdas must always be inside parentheses
• Confusing trailing lambda with lambda variable declaration
• Believing trailing lambda disallows lambdas as arguments

2. Which of the following Kotlin function calls correctly uses the trailing lambda convention?

easy

A. list.filter { it > 0 }

B. list.filter({ it > 0 })

C. list.filter(it > 0)

D. list.filter it > 0

Solution

1. Step 1: Identify correct trailing lambda syntax and check other options

   Trailing lambda means the lambda is placed outside the parentheses. list.filter { it > 0 } shows this correctly. list.filter({ it > 0 }) uses lambda inside parentheses (valid but not trailing). list.filter(it > 0) and D are syntax errors.

2. Final Answer:

   list.filter { it > 0 } -> Option A

3. Quick Check:

   Trailing lambda = lambda outside parentheses [OK]

Hint: Trailing lambda goes after parentheses, no extra parentheses needed [OK]

Common Mistakes:

• Putting lambda inside parentheses when trailing lambda is expected
• Omitting parentheses incorrectly
• Writing lambda without braces

3. What is the output of this Kotlin code?

fun greet(action: () -> String) = println(action())
greet { "Hello, Kotlin!" }

medium

A. No output

B. kotlin.Unit

C. Syntax error

D. Hello, Kotlin!

Solution

1. Step 1: Understand the function call and evaluate the output

   The function greet takes a lambda returning a String and prints its result. The call uses trailing lambda syntax. The lambda returns "Hello, Kotlin!" which is printed by println.

2. Final Answer:

   Hello, Kotlin! -> Option D

3. Quick Check:

   Trailing lambda call prints lambda result [OK]

Hint: Trailing lambda calls run the lambda and print result [OK]

Common Mistakes:

• Expecting syntax error due to trailing lambda
• Confusing return value with Unit
• Missing that println prints the lambda result

4. Identify the error in this Kotlin code using trailing lambda:

fun calculate(x: Int, operation: (Int) -> Int): Int {
    return operation(x)
}

val result = calculate(5) operation { it * 2 }

medium

A. Missing parentheses around lambda argument

B. Incorrect function declaration

C. Trailing lambda must be outside parentheses but parentheses are missing

D. No error, code is correct

Solution

1. Step 1: Analyze the function call syntax and correct trailing lambda usage

   The function call calculate(5) operation { it * 2 } is invalid because the trailing lambda must be placed immediately after the parentheses without extra tokens. The correct call is calculate(5) { it * 2 }. The word operation should not appear between parentheses and lambda.

2. Final Answer:

   Trailing lambda must be outside parentheses but parentheses are missing -> Option C

3. Quick Check:

   Trailing lambda goes right after parentheses [OK]

Hint: Trailing lambda must follow parentheses directly, no extra tokens [OK]

Common Mistakes:

• Inserting parameter name between parentheses and lambda
• Forgetting to remove parentheses when using trailing lambda
• Confusing function parameter names with call syntax

5. Given this Kotlin function:

fun processNumbers(numbers: List<Int>, filter: (Int) -> Boolean, action: (Int) -> Unit) {
    for (number in numbers) {
        if (filter(number)) action(number)
    }
}

Which is the correct way to call processNumbers using trailing lambda convention for both lambdas?

hard

A. processNumbers(listOf(1,2,3)) { it > 1 } { println(it) }

B. processNumbers(listOf(1,2,3), { it > 1 }) { println(it) }

C. processNumbers(listOf(1,2,3)) { println(it) } { it > 1 }

D. processNumbers(listOf(1,2,3), { println(it) }, { it > 1 })

Solution

1. Step 1: Understand trailing lambda rules for multiple lambdas and apply to function call

   Kotlin allows only the last lambda argument to be passed outside parentheses as a trailing lambda. Since action is the last lambda, it can be trailing. The first lambda filter must be inside parentheses. processNumbers(listOf(1,2,3), { it > 1 }) { println(it) } correctly places filter inside parentheses and action outside.

2. Final Answer:

   processNumbers(listOf(1,2,3), { it > 1 }) { println(it) } -> Option B

3. Quick Check:

   Only last lambda can be trailing [OK]

Hint: Only last lambda argument goes outside parentheses [OK]

Common Mistakes:

• Trying to put multiple lambdas outside parentheses
• Swapping lambda order incorrectly
• Putting non-lambda arguments outside parentheses