Exploring Kotlin’s Local Declarations: Variables, Functions, and Classes

A valid use case for local declarations in Kotlin is when you need to calculate intermediate values in a for loop. While Kotlin includes features from other programming languages, it is important to note that Java 8 lambdas allow access to outer variables but cannot modify them. This limitation restricts the capabilities of functional programming in Java 8.


Solution:

Declarations placed within a function’s body (including constructors,

init

blocks, and property accessors) are referred to as local declarations.

The references to these declarations are limited to the lexical scope where they are initially declared.

fun foo() { 
    if (Random().nextInt() % 2 == 0) {
        fun bar() { 
            println("bar") 
        }
        bar() // OK
    } else {
        bar() // Error: unresolved reference
    }
}

As a result, it is impossible to utilize these declarations beyond the scope of the body. Consequently, visibility modifiers, which typically determine the accessibility of a declaration outside of the type or file, hold no significance for local declarations.

Local declarations are applicable to entities that possess significance solely within the function’s body and should not be utilized elsewhere.

A situation where local declarations can be applied effectively is when there is a need to define

data class

for interim results in a calculation.

fun getAndSaveEmails(people: List) { 
    data class PersonWithEmail(
        val person: Person, 
        val email: String
    )
    val peopleWithEmails = people.map { PersonWithEmail(it, requestEmail(it)) }
    peopleWithEmails.forEach { save(it.person, it.email) }
}

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