Type aliases
Type aliases provide alternative names for existing types. If the type name is too long you can introduce a different shorter name and use the new one instead.
It's useful to shorten long generic types. For instance, it's often tempting to shrink collection types:
typealias NodeSet = Set<Network.Node>
typealias FileTable<K> = MutableMap<K, MutableList<File>>
You can provide different aliases for function types:
typealias MyHandler = (Int, String, Any) -> Unit
typealias Predicate<T> = (T) -> Boolean
You can have new names for inner and nested classes:
class A {
inner class Inner
}
class B {
inner class Inner
}
typealias AInner = A.Inner
typealias BInner = B.Inner
Type aliases do not introduce new types. They are equivalent to the corresponding underlying types. When you add typealias Predicate<T>
and use Predicate<Int>
in your code, the Kotlin compiler always expands it to (Int) -> Boolean
. Thus you can pass a variable of your type whenever a general function type is required and vice versa:
typealias Predicate<T> = (T) -> Boolean
fun foo(p: Predicate<Int>) = p(42)
fun main() {
val f: (Int) -> Boolean = { it > 0 }
println(foo(f)) // prints "true"
val p: Predicate<Int> = { it > 0 }
println(listOf(1, -2).filter(p)) // prints "[1]"
}
Last modified: 11 February 2021