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reified-type-parameters.md

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Reified Type Parameters

Goal: support run-time access to types passed to functions, as if they were reified (currently limited to inline functions only).

Syntax

A type parameter of a function can be marked as reified:

inline fun <reified T> foo() {}

Semantics, Checks and Restrictions

Definition A well-formed type is called runtime-available if

  • it has the form C, where C is a classifier (object, class or trait) that has either no type parameters, or all its type parameters are reified, with the exception for class Nothing,
  • it has the form G<A1, ..., An>, where G is a classifier with n type parameters, and for every type parameter Ti at least one of the following conditions hold:
    • Ti is a reified type parameter and the corresponding type argument Ai is a runtime-available type,
    • Ai is a star-projection (e.g. for List<*>, A1 is a star-projection);
  • it has the form T, and T is a reified type parameter.

Examples:

  • Runtime-available types: String, Array<String>, List<*>;
  • Non-runtime-available types: Nothing, List<String>, List<T> (for any T)
  • Conditional: T is runtime-available iff the type parameter T is reified, same for Array<T>

Only runtime-available types are allowed as

  • right-hand arguments for is, !is, as, as?
  • arguments for reified type parameters of calls (for types any arguments are allowed, i.e. Array<List<String>> is still a valid type).

As a consequence, if T is a reified type parameter, the following constructs are allowed:

  • x is T, x !is T
  • x as T, x as? T
  • reflection access on T: javaClass<T>(), T::class (when supported)

Restrictions regarding reified type parameters:

  • Only a type parameter of an inline function can be marked reified
  • The built-in class Array is the only class whose type parameter is marked reified. Other classes are not allowed to declare reified type parameters.
  • Only a runtime-available type can be passed as an argument to a reified type parameter

Notes:

  • No warning is issued on inline functions declaring no inlinable parameters of function types, but having a reified type parameter declared.

Implementation notes for the JVM

In inline functions, occurrences of a reified type parameter T are replaced with the actual type argument. If actual type argument is a primitive type, it's wrapper will be used within reified bytecode.

open class TypeLiteral<T> {
    val type: Type
        get() = (javaClass.getGenericSuperclass() as ParameterizedType).getActualTypeArguments()[0]
}

inline fun <reified T> typeLiteral(): TypeLiteral<T> = object : TypeLiteral<T>() {} // here T is replaced with the actual type

typeLiteral<String>().type // returns 'class java.lang.String'
typeLiteral<Int>().type // returns 'class java.lang.Integer'
typeLiteral<Array<String>>().type // returns '[Ljava.lang.String;'
typeLiteral<List<*>>().type // returns 'java.util.List<?>'