Language design

Upcoming Change: Function Types Reform

Kotlin M12 will likely bring another change that is crucial for implementing a useful reflection library for Kotlin. In short, we are going to unify FunctionX and ExtensionFunctionX to be represented in the same way at runtime, but it will not affect our ability to create type-safe builders and other DSL-like constructs.


Currently, there are 23*2 = 46 class files related to FunctionX in the kotlin package kotlin-runtime.jar and 46 more class files related to ExtensionFunctionX (X being a number between 0 and 22). This is a lot of class files already, but there are 46*3 = 138 more class files in the kotlin.reflect package (KFunctionX, KMemberFunctionX, KExtensionFunctionX), which is way over the top :)

So, for one thing, we really need to reduce the number of class files in kotlin-runtime.jar.

Then, at the moment ExtensionFunctionX types are not related to FunctionX types, and we can not say, because the argument has type String.() -> Int, but map() expects (String) -> Int, which is sad, annoying and inconvenient.

So, we want to make extension functions coercible to normal functions (with an extra parameter).

While we are at it, we would also like to allow functions with more than 22 parameters, theoretically any number of parameters (in practice 255 on JVM).

An important constraint here is that implementing Kotlin functions in Java must remain easy: Java 8 lambdas should work and in earlier versions of Java new Function2() { ... } with only invoke() method in the body should be enough.


All 230 function class files will be replaced by a single interface Function which will represent all functions at runtime (+ we will keep 23 interfaces kotlin.jvm.FunctionX to facilitate easy creation of Kotlin functions in Java). A total win of over 200 class files.

Now, function values (“(A) -> B“) and extension function values (“A.() -> B“) will be represented by the same type at runtime, which will allow using them almost interchangeably. The static type system will only distinguish arities (e.g. the compiler will create fictitious “classes” FunctionX for any X, which will exist at compile time only and will never be emitted as class files).

Since the compile-time types will not be exactly the same as runtime types, several tweaks in operators like is and as are required, but they will not affect the cases that are not statically known to involve function types.

The syntactic distinction between function- and extension function types will be preserved (through desugaring A.() -> B to an annotated function type): when a lambda is type-checked against an expected type which is an extension function, type inference adds a this-receiver to its signature, and for a normal function it adds a parameter instead:

// Normal function:
fun call(callback: (Foo) -> Unit) { // callback has type `Function1<Foo, Unit>

// argument is a lambda with a normal parameter
call { foo -> println(foo) }

// Extension function
fun builder(body: Foo.() -> Unit) { // body has type `@extension Function1<Foo, Unit>`

// argument is an extension lambda: no parameters, but `this` is available
builder { // `this` has type Foo


These changes will make reflection on functions possible (i.e. KClass will finally have getFunctions()), and using function objects will be more intuitive.

You won’t need to change anything in your code unless you referenced ExtensionFunctionX types directly (by saying, e.g. ExtensionFucntion0<Foo, Unit>).

For more details, see this spec document (you can comment on the source, or press “View” for rendered markdown).