leongeorgi / kotlin-when-matches

Fun project: an improvement for when statements in Kotlin

Better when statements

Regular when statements

When statements in Kotlin can be defined in two ways. Both have disadvantages.

With subject

val string = "Hello World!"
val i = when (string) {
    "Hello" -> 1
    "World" -> 2
    in listOf("foo") -> 3
    else -> 4
}

This syntax only allows equality, contains and instance checks. However it is not possible to check anything different like checking if the length of the string is even. For that we have to use when without argument.

Without subject

val string = "Hello World!"
val i = when {
    string == "Hello" -> 1
    string.length % 2 == 0 -> 2
    string in listOf("foo") -> 3
    else -> 4
}

This syntax gives you more freedom. You can check anything you want. But it's a bit verbose, since you have to use the variable string in every line.

when statements with the extension

The extension makes when statements work as follows:

val string = "Hello World!"
val i = when (string.matches) {
    "Hello" -> 1
    String::isNotEmpty -> 2
    { it: String -> it.length % 2 == 0 } -> 3
    else -> 4
}

When you call the getter of the property matches on an object, you can also use method references to check a predicate. As consequence you can also use lambdas to check if a predicate is fullfilled.

You may ask why this works. This feature is an abuse of the equals function. The function matches returns an object, which overrides the equals method such that it returns true, if this value equals the other value or if the other value is a predicate (a function taking the value's type as argument and returns a Boolean) which returns true for this value.

For the exact implementation have a look at the file Match.kt.