Kotlin: How to use if statements in a lambda expression

Error Type mismatch: inferred type is Unit but String was expected is because you defined the return datatype as String, but you didn't return anything. Two reasons for that:

• the -> in the condition are not necessary, they are too much
• the condition has to be in parentheses

Putting all together, this works:

fun main() {
    val addExclamationMark: (String) -> String = { if (it.contains("!")) it else it + "!"}
    println(addExclamationMark("Hallo Welt"))
}

Check here

You have to pass a string, didn't return anything,

fun main() {
    Val addExclamationMark: (String) -> String = {
                if (it.contains("!")) {
                    it
                } else {
                    "$it!"
                }
            }

        println(addExclamationMark("Hallo Welt"))
}

• You can use a lambda that take a String and returns a String
• In some situation, you can also use a method that implements a function by inference. The method have to respect the signature of the function.

import java.util.*

fun main() {
    // Lambda with String -> String
    val addExclamationMarkConst: (String) -> String = { if (it.contains("!")) it else "$it!" }
    println(addExclamationMarkConst("Hallo Welt"))
    
    // Method that implements the function with String -> Unit
    Optional.of("Hallo Welt").ifPresent(::printExclamationMark)
}

fun printExclamationMark(input: String): Unit {
    val elementToPrint: String = if (input.contains("!")) input else "$input!"
    println(elementToPrint)
}