Regex or Wildcard in Kotlin's when statement? - regex

Question

I'm working on a RESTful app in Kotlin and for the router, I'm using a when statement, as it's the most readable and good looking conditional.
Is there a way to use Regex or a wildcard in the when statement for a string?
(So that URIs like "/article/get/" would all be passed to the same controller)
The structure of my router is as follows:
when(uri) {
"some/url" -> return SomeController(config).someAction(session)
}

Answer 1

Yes.
import kotlin.text.regex
val regex1 = Regex( /* pattern */ )
val regex2 = Regex( /* pattern */ )
/* etc */
when {
regex1.matches(uri) -> /* do stuff */
regex2.matches(uri) -> /* do stuff */
/* etc */
}
You could also use containsMatchIn if that suits your needs better than matches.
Explanation:
The test expression of a when statement is optional. If no test expression is included, then the when statement functions like an if-else if chain, where the whenCondition of each whenEntry shall independently evaluate to a boolean.
EDIT:
So I thought about it for awhile, and I came up with a different approach that might be closer to what you want.
import kotlin.text.regex
when (RegexWhenArgument(uri)) {
Regex(/* pattern */) -> /* do stuff */
Regex(/* pattern */) -> /* do stuff */
/* etc */
}
Where RegexWhenArgument is minimally defined as:
class RegexWhenArgument (val whenArgument: CharSequence) {
operator fun equals(whenEntry: Regex) = whenEntry.matches(whenArgument)
override operator fun equals(whenEntry: Any?) = (whenArgument == whenEntry)
}
This approach lets you get as close as possible to the "argument-ful" when expression syntax. I think it's about as streamlined and readable as it can be (assuming that you define the RegexWhenArgument class elsewhere).
This approach uses something similar to the visitor design pattern in combination with Kotlin's operator overloading to redefine what constitutes a "match" between a when expression argument and a whenEntry. If you really wanted to, I suppose you could take this approach a step further and generify RegexWhenArgument into a general-purpose WhenArgument and WhenArgumentDecorator that allows you to specify custom "match" criteria in a when expression for any sort of type, not just Regex.

Answer 2

The typing of the when statement enforces to have compatible types between the whenSubject and the whenEntries. So we cannot compare a String whenSubject with a Regex directly.
We can use when with no subject, then branch conditions may be simply boolean expressions.
fun main() {
val uri: String? = "http://my.site.com/a/b/c"
val res = when {
uri == null -> "NULL"
uri == "http://my.site.com/" -> "ROOT"
uri.startsWith("http://my.site.com/a/") -> "A STUFF"
uri.matches(Regex("http://my.site.com/b/.*")) -> "B STUFF"
else -> "DEFAULT"
}
/* do stuff */
}
Alternatively, we can emulate a kind of when+regex with a dedicated class and few helper functions.
fun main() {
val uri: String? = "http://my.site.com/a/b/c"
val res2 = when(matching(uri)) {
null -> "NULL"
matchesLiteral("http://my.site.com/") -> "ROOT"
matchesRegex("http://my.site.com/a/.*") -> "A STUFF"
else -> "DEFAULT"
}
/* do stuff */
}
class MatchLiteralOrPattern(val value: String, val isPattern: Boolean) {
override fun equals(other: Any?): Boolean {
if (other !is MatchLiteralOrPattern) return false
if (isPattern && !other.isPattern) return Regex(this.value).matches(other.value)
if (!isPattern && other.isPattern) return Regex(other.value).matches(this.value)
return value == other.value
}
}
fun matching(whenSubject: String?) = whenSubject?.let { MatchLiteralOrPattern(it, false) }
fun matchesLiteral(value: String) = MatchLiteralOrPattern(value, false)
fun matchesRegex(value: String) = MatchLiteralOrPattern(value, true)

Answer 3

I tried the following on the kotlin playground and it seems to work as expected.
class WhenArgument (val whenArg: CharSequence) {
override operator fun equals(other: Any?): Boolean {
return when (other) {
is Regex -> other.matches(whenArg)
else -> whenArg.equals(other)
}
}
}
fun what(target: String): String {
return when (WhenArgument(target) as Any) {
Regex("source-.*") -> "${target} is-a-source"
Regex(".*-foo") -> "${target} is-a-foo"
"target-fool" -> "${target} is-the-target-fool"
else -> "nothing"
}
}
fun main() {
println(what("target-foo"))
println(what("source-foo"))
println(what("target-bar"))
println(what("target-fool"))
}
It works around the type compatibility problem by making the 'when' argument of type Any.