40. Type Aliases

A typealias gives an existing type a shorter or more meaningful name. It introduces no new type — just an alternative spelling — which is handy for function types and verbose generics.

typealias Handler = (String) -> Unit
typealias StringMap = Map<String, String>

fun onEvent(name: String, handler: Handler) = handler(name)

fun main() {
    val log: Handler = { event -> println("event: $event") }
    onEvent("click", log)

    val config: StringMap = mapOf("host" to "localhost", "port" to "8080")
    println(config["host"])
}

Handler reads better than (String) -> Unit in a signature, and StringMap keeps Map<String, String> from cluttering the code. At runtime they are exactly their underlying types.

Running it:

$ kotlin run
event: click
localhost

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