Functions With Expression Body

When a function returns a single expression, the curly braces can be omitted and the body is specified after a = symbol.

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fun double(x: Int) = x * 2

fun sum(a: Int, b: Int) = a + b

fun main(args: Array<String>) {
    println(double(21))
    // 21 * 2 = 42
    println(sum(36, 6))
    // 36 + 6 = 42
}

Explicitly declaring the return type is optional when this can be inferred by the compiler.