How to simplify when expression in kotlin - kotlin

Question

I'd like to simplify this expression, especially "isDigit" and "isLetter" case. How to do it?
smoothInput.forEach { char ->
when {
char.isValidOperator() -> {
output.push(char)
}
char.isDigit() -> {
if (output.isNotEmpty() && output.last()!!.isNumeric()) output.addToLast(char)
else output.push(char)
}
char.isLetter() -> {
if (output.isNotEmpty() && output.last()!!.isValidVariableName()) output.addToLast(char)
else output.push(char)
}
else -> {
throw InvalidIdentifierException()
}
}
}
I think, that it isn't important, but it's much better to add code here than in comment
output is InputStack Type:
class InputStack : Stack<String> {
override val storage = mutableListOf<String>()
fun push(e: Char) = push(e.toString())
fun push(e: Operator) = push(e.toString())
fun addToLast(e: Char) {
storage[storage.size - 1] += e.toString()
}
}
Stack Interface:
interface Stack<T> {
val storage: MutableList<T>
fun asString(): String = buildString {
appendLine("----top----")
storage.asReversed().forEach {
appendLine(it)
}
appendLine("-----------")
}
fun push(element: T) = storage.add(element)
fun pop(): T {
if (storage.size == 0) throw EmptyStackException()
return storage.removeAt(storage.size - 1)
}
fun isEmpty(): Boolean = storage.isEmpty()
fun isNotEmpty(): Boolean = !isEmpty()
fun last(): T? = storage.lastOrNull()
fun forEach(action: (T) -> Unit) {
for (element in storage) action(element)
}
}

Answer 1

You can extract some common parts in the following way:
fun addCharToOutputConditionally(char: Char, output: InputStack, conditionOnLast: (String) -> Boolean) {
if (output.isNotEmpty() && conditionOnLast(output.last()!!)) output.addToLast(char)
else output.push(char)
}
smoothInput.forEach { char ->
when {
char.isValidOperator() -> {
output.push(char)
}
char.isDigit() -> {
addCharToOutputConditionally(char, output) {
it.isNumeric()
}
}
char.isLetter() -> {
addCharToOutputConditionally(char, output) {
it.isValidVariableName()
}
}
else -> {
throw InvalidIdentifierException()
}
}
}
However, in cases like this, I don't think it's usually worth spending the time to refactor it this way, considering that there's little to gain by doing so: the resulting code is even longer and arguably harder to read than the original one.

Answer 2

The new when expression:
smoothInput.forEach { char ->
when {
char.isValidOperator() -> output.push(char)
char.isDigit() -> output.addToLastConditionally(char) { it.isNumeric() }
char.isLetter() -> output.addToLastConditionally(char) { it.isValidVariableName() }
else -> throw InvalidIdentifierException()
}
}
I've change the addToLast function in InputStack for addToLastConditionally
fun addToLastConditionally(char: Char, condition: (String) -> Boolean) {
if (isNotEmpty() && condition(last()!!)) storage[storage.size - 1] += char.toString()
else push(char)
}