Kotlin to Swift mapping

This document gives a quick superficial overview on how Swift export translates Kotlin declarations to Swift. Currently, Swift export supports only a subset of the Kotlin language; thus this list is intentionally incomplete.

❗️ Swift export is in very early stages of development. Things change very quickly, and documentation might not catch up at times.

Please note that this part of Swift export will change very often. Some features are currently implemented the way they are not because this is what we consider the best, but because it’s easier to start with some (even ugly) quick restricted solutions first, and then gradually improve it towards something beautiful.

Declarations

Classifiers

Classes

Swift export supports only final classes that directly inherit from Any (like class Foo()). They are translated to Swift classes that inherit from a special KotlinBase class.

class MyClass {
    val property: Int = 0
    
    fun method() {}
}

public class MyClass : KotlinRuntime.KotlinBase {
    public var property: Swift.Int32 {
        get {
            ...
        }
    }
    public override init() {
        ...
    }
    public func method() -> Swift.Void {
        ...
    }
}

Objects

Objects are translated to Swift classes with private init and static shared accessor.

object O

public class O : KotlinRuntime.KotlinBase {
    public static var shared: O {
        get {
            ...
        }
    }
    private override init() {
        ...
    }
}

Typealiases

Typealiases are exported "as is":

typealias MyInt = Int

public typealias MyInt = Swift.Int32

Callables

Functions

Swift export supports only simple top-level functions and methods: no suspend, inline, operator or extensions.

fun foo(a: Short, b: Bar) {}

fun baz(): Long = 0

public func foo(a: Swift.Int16, b: Bar) -> Swift.Void {
    ...
}

public func baz() -> Swift.Int64 {
    ...
}

Properties

Kotlin properties are translated to Swift properties.

val a: Int = 0
            
var b: Short = 15

const val c: Int = 0

public var a: Swift.Int32 {
    get {
        ...
    }
}
public var b: Swift.Int16 {
    get {
        ...
    }
    set {
        ...
    }
}
public var c: Swift.Int32 {
    get {
        ...
    }
}

Constructors

Constructors are translated to Swift initializers.

class Foo(val prop: Int)

public class Foo : KotlinRuntime.KotlinBase {
    public init(
        prop: Swift.Int32
    ) {
        ...
    }
}

Types

Built-in types

Primitive types

kotlin.Any

Any is translated to the special KotlinBase class.

kotlin.Unit

Unit is translated to the Void type.

kotlin.Nothing

Nothing is translated to the Never type.

fun foo(): Nothing = TODO()

fun baz(input: Nothing) {}

public func foo() -> Swift.Never {
    ...
}

public func baz(input: Swift.Never) -> Void {
    ...
}

Classifier types

Swift export supports only a limited number of reference types for now: final classes that directly inherit from Any.

Namespaces

Packages

Kotlin packages are translated to nested Swift enums to avoid name collisions.

// FILE: bar.kt
package foo.bar

fun callMeMaybe() {}

// FILE: baz.kt
package foo.baz

fun callMeMaybe() {}

public extension foo.bar {
    public func callMeMaybe() {}
}

public extension foo.baz {
    public func callMeMaybe() {}
}

public enum foo {
    public enum bar {}
    
    public enum baz {}
}