Kotlin Academy · Lesson

it: The Implicit Parameter

Use the implicit 'it' parameter in single-parameter lambdas.

Lesson 3 of 413 steps

it: The Implicit Parameter is a free Kotlin Academy lesson on CoddyKit. This is lesson 3 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Kotlin Academy learning path, and your progress syncs across the web and the CoddyKit app. The Kotlin Academy course includes 4 lessons in total.

The Implicit it

When a lambda has exactly one parameter, you can omit declaring it and reference it as it. Saves typing for short pipelines.

it in forEach

The classic example: forEach passes each element as it.

fun main() {
    listOf(1, 2, 3).forEach { println(it) }
}

it in map

Transform each element using it as the input.

fun main() {
    val doubled = listOf(1, 2, 3).map { it * 2 }
    println(doubled) // [2, 4, 6]
}

it in filter

filter keeps elements for which the lambda returns true. it stands for the current element.

fun main() {
    val evens = listOf(1, 2, 3, 4, 5).filter { it % 2 == 0 }
    println(evens) // [2, 4]
}

When NOT to Use it

If the lambda body is non-trivial, name the parameter explicitly. Naming improves clarity over abbreviated it.

fun main() {
    val users = listOf("alice", "bob")
    // OK
    users.forEach { println(it) }
    // Clearer with a name when body is bigger
    users.forEach { name ->
        println("User: $name length=${name.length}")
    }
}

it in Nested Lambdas

Be careful with nested lambdas: each it binds to the innermost lambda. Naming parameters avoids ambiguity.

fun main() {
    val nested = listOf(listOf(1, 2), listOf(3, 4))
    // Both lambdas use it — confusing!
    nested.forEach { outer ->
        outer.forEach { inner ->
            println("$outer -> $inner")
        }
    }
}

it with String Methods

Common pattern: filter/map strings using it.

fun main() {
    val names = listOf("Alice", "bob", "Charlie")
    val upper = names.map { it.uppercase() }
    println(upper)
}

it with Properties

it.property is a clean way to access fields when chaining operations.

data class Person(val name: String, val age: Int)
fun main() {
    val people = listOf(Person("Ana", 20), Person("Ben", 35))
    val names = people.map { it.name }
    println(names) // [Ana, Ben]
}

Predicates with it

Predicates (lambdas returning Boolean) read naturally with it.

fun main() {
    val nums = listOf(1, 4, 7, 9)
    println(nums.any { it > 5 }) // true
    println(nums.all { it > 0 }) // true
    println(nums.none { it < 0 }) // true
}

Custom Functions with it

Any higher-order function with a single-parameter lambda gives you it automatically.

fun <T> List<T>.firstWhere(predicate: (T) -> Boolean): T? {
    for (item in this) if (predicate(item)) return item
    return null
}
fun main() {
    val n = listOf(3, 8, 4, 5).firstWhere { it > 5 }
    println(n) // 8
}

it Inside Scope Functions

Some scope functions (let, also) use it as the receiver; others (apply, run) use this. Pick based on whether you want to reference the receiver explicitly.

fun main() {
    val name: String? = "Kotlin"
    name?.let { println("hello $it") } // it = name
}

Quick Check

When can you use it instead of naming a lambda parameter?

Answer it in the interactive lesson

Recap

it is the implicit name for the sole parameter of a one-parameter lambda. Use it to shorten short lambdas. Name parameters explicitly when bodies grow or when nesting could cause confusion.

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Is the “it: The Implicit Parameter” lesson free?

Yes — any 3 lessons of the Kotlin Academy learning path, including “it: The Implicit Parameter”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Kotlin Academy course includes 4 lessons in total.

What will I learn in “it: The Implicit Parameter”?

Use the implicit 'it' parameter in single-parameter lambdas. You practise Kotlin Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

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