Spec document for reified type parameters

2014-12-18 14:21:44 +03:00
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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`:
+
+``` kotlin
+
+inline fun foo<reified T>() {}
+```
+
+## 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.
+
+``` kotlin
+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<Array<String>>().type // returns '[Ljava.lang.String;'
+typeLiteral<List<*>>().type // returns 'java.util.List<? extends java.lang.Object>'
+```