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) }