How to use Kotlin when to find if a string is numeric?

Issue

I'd like to use a when() expression in Kotlin to return different values from a function. The input is a String, but it might be parsable to an Int, so I'd like to return the parsed Int if possible, or a String if it is not. Since the input is a String I can not use the is type check expression.

Is there any idiomatic way to achieve that?

Edit: My problem is how the when() expression should look like, not about the return type.

Solution

Version 1 (using toIntOrNull and when when as requested)

fun String.intOrString(): Any {
    val v = toIntOrNull()
    return when(v) {
        null -> this
        else -> v
    }
}

"4".intOrString() // 4
"x".intOrString() // x

Version 2 (using toIntOrNull and the elvis operator ?:)

when is actually not the optimal way to handle this, I only used when because you explicitely asked for it. This would be more appropriate:

fun String.intOrString() = toIntOrNull() ?: this

Version 3 (using exception handling):

fun String.intOrString() = try { // returns Any
   toInt()
} catch(e: NumberFormatException) {
   this
}

Answered By - Willi Mentzel