Does Kotlin have an identity function?

问题:

Scala has a generic identity function in the Predef:

def identity[A](x: A): A

Does Kotlin have a similar facility in the standard library? Of course I could simply use { it } instead, but I find identity easier to read, and instantiating all those lambdas is a little wasteful.

I must be able to use this identity function in places where a function (Foo) -> Foo is expected, for any type Foo. Is such a function even possible in Kotlin's type system? (In the case of Scala, there is an implicit conversion that wraps the method inside a function object or something.)

回答1:

There's no such function at the moment, but you can easily define it yourself:

fun <T> identity(x: T): T = x

If you think there are enough use cases for this function to be declared in Kotlin standard library, please file an issue at youtrack.jetbrains.com. Thanks!

回答2:

If you need to pass the identity function as a parameter to another function, you can simply use { it }. For example, it you have a List<List<String>> and want to flatten it to a List<String>, you could use:

list.flatMap(identity)

where identity is the identity function. This can be written as:

list.flatMap { it }

This is equivalent to:

list.flatMap { x -> x }

The alternative would be to define the identity function somewhere, such as:

val identity: (List<String>) -> List<String> = { it }

But we can't create a generic val, so we would have to define an identity function for each type. The solution, (as it is done in Java Function interface) is to define it as a constant function:

fun <A> identity(): (A) -> A = { it }

and use it as:

list.flatMap(identity)

Of course, it is much easier to write:

list.flatMap { it }

Declaring an identity function once for all (that would work for all types) is not possible because it would have to be parameterized. What is possible is to use a function returning this identity function:

fun <T> identity(): (T) -> T  = { it }

Although it does the job, it is not very helpful since one has now to write:

list.flatMap(identity())