Mutate string from an extension - kotlin

Question

I was trying to port a Swift script to Kotlin but it's not working as expected, what the code does is to consume a string while the condition is true (need it for a parser). In Swift it works as expected but in Kotlin it doesn't (I just started with Kotlin a month ago so maybe I'm missing something).
Swift
extension String {
#discardableResult public mutating func consumeWhile(test: (String) -> Bool) -> String {
var chars = [Character](self.characters)
var result = ""
while chars.count > 0 && test(String(chars[0])) {
result.append(chars.remove(at: 0))
}
self = String(chars)
return result
}
}
Kotlin
fun String.consumeWhile(test: (String) -> Boolean): String {
if (isEmpty()) return ""
val chars = toCharArray().toMutableList()
var result = ""
var i = -1
while (chars.isNotEmpty() && test(chars.first().toString())) {
result += chars.removeAt(0)
++i
}
removeRange(0..i)
return result
}
So the basic usage will look like
val myString = "--Test" // IntelliJ suggests change var to val
val consumedString = myString.consumeWhile{ it != "-" }
println("result: $myString consumedString: $consumedString")
// expected: "result: Test consumedString: --"
// but got: "result: --Test consumedString: --"
Edit: Thanks for all the answers, don't know if will be possible to do like I want because as mentioned string are immutable in Kotlin/Java (just using the same string).
I forgot to mention that I need the consumed string, basically b/c I'm doing a parser so I need to store the consumed chars and the mutated string. I will leave open this question but I ended up creating a class that implements only a few String class methods.
class Line(var string: String) {
val length: Int
get() = string.length
fun consumeWhile(test: (String) -> Boolean): String {
if (string.isEmpty()) return ""
val chars = string.toCharArray().toMutableList()
var result = ""
while (chars.isNotEmpty() && test(chars.first().toString())) {
result += chars.removeAt(0)
}
string = chars.joinToString("")
return result
}
fun isNullOrEmpty(): Boolean {
return string.isNullOrEmpty()
}
fun isNotEmpty(): Boolean {
return string.isNotEmpty()
}
private fun removeRange(range: IntRange) {
string = string.removeRange(range)
}
operator fun get(i: Int): Char {
return string[i]
}
}
Usage example
val line = Line(string)
if (line.isNotEmpty() && line[0].toString() == "(") {
line.consumeWhile { it == "(" }
while (line.isNotEmpty() && line[0].toString() != ")") {
line.consumeWhile { it == " " }
val key = line.consumeWhile { it != "=" }
line.consumeWhile { it == "\"" || it == "=" }
val value = line.consumeWhile { it != "\"" }
line.consumeWhile { it == "\"" }
attributes[key] = value
}
line.consumeWhile { it == ")" }
}

Answer 1

String is immutable in Kotlin & Java, so you can't modify its state anyway.
You should avoiding to makes the wheels repeatedly, there is an existing function String#dropWhile(Char) in Kotlin. one thing you need to do is invert the condition, for example:
val result = "--Test".dropWhile { it == '-' }
// ^--- "Test"

Answer 2

In both Java and Kotlin String is immutable and you cannot change it after it has been created.
In swift this presumably can be turned off through the mutating modifier. However in Kotlin removeRange(0..i) creates a new String object which you then discard.
To have it behave as you want you will need to either:
Create a wrapper object that contains a string that can be replaced.
Return both the split string and the rest as a Pair, you can then use the destructuring operators to assign it as [_, myString] = myString.consumeWhile {}

Answer 3

Kotlin Strings are immutable and cannot be modified in place. Instead you can create a new String and return it
fun String.consumeWhile(test: (String) -> Boolean): String {
if (isEmpty()) return ""
val chars = toCharArray().toMutableList()
while (chars.isNotEmpty() && test(chars.first().toString())) {
chars.removeAt(0)
// Do something with the char
}
return chars.joinToString(separator = "")
}
Additionally, unless I am misunderstanding, your test condition should be it == "-" to get the result you want:
val myString = "--Test"
val newString = myString.consumeWhile{ it == "-" }
println("result: $newString")

Answer 4

You use
myString.consumeWhile{ it != "-" }
which stops consuming as soon as it meets the first "-", and thus it's nothing more to do.
The code works as it should, if you use
myString.consumeWhile{ it == "-" }
you will get the expected output as is correct.

Answer 5

I ended up creating a class that implements only a few String class methods.
class Line(var string: String) {
val length: Int
get() = string.length
fun consumeWhile(test: (String) -> Boolean): String {
if (string.isEmpty()) return ""
val chars = string.toCharArray().toMutableList()
var result = ""
while (chars.isNotEmpty() && test(chars.first().toString())) {
result += chars.removeAt(0)
}
string = chars.joinToString("")
return result
}
fun isNullOrEmpty(): Boolean {
return string.isNullOrEmpty()
}
fun isNotEmpty(): Boolean {
return string.isNotEmpty()
}
private fun removeRange(range: IntRange) {
string = string.removeRange(range)
}
operator fun get(i: Int): Char {
return string[i]
}
}
Usage example
val line = Line(string)
if (line.isNotEmpty() && line[0].toString() == "(") {
line.consumeWhile { it == "(" }
while (line.isNotEmpty() && line[0].toString() != ")") {
line.consumeWhile { it == " " }
val key = line.consumeWhile { it != "=" }
line.consumeWhile { it == "\"" || it == "=" }
val value = line.consumeWhile { it != "\"" }
line.consumeWhile { it == "\"" }
attributes[key] = value
}
line.consumeWhile { it == ")" }
}
Obs: for now will mark as answered, till a better solution comes out