Status: New
Owner: ----
New issue 657 by [email protected]: Type-safe means to build
parameterized TypeLiterals from existing TypeLiterals/class tokens
http://code.google.com/p/google-guice/issues/detail?id=657
Various framework code based on Guice needs to bind a parameterized type
where some of the type arguments contain in-scope type variables, which
obviously must be reified by existing type tokens of some kind. A
contrived example:
abstract class FancyModule extends AbstractModule {
<T> AnnotatedBindingBuilder<List<T>> bindListOf(Class<T> classT) {
TypeLiteral<List<T>> tl_LT = /* ??? */;
return bind(tl_LT);
}
}
One would like to just do:
TypeLiteral<List<T>> tl_LT = new TypeLiteral<List<T>>() {};
but there is no way to pass in classT, so Guice will inevitably raise the
error "List<T> cannot be used as a key; It is not fully specified" when it
discovers the information is missing. The following works, but is clunky
and incurs an unchecked warning:
TypeLiteral<List<T>> tl_LT = (TypeLiteral<List<T>>) \
TypeLiteral.get(Types.newParameterizedType(List.class, classT));
I propose to introduce new variants of the TypeLiteral class that take
additional type parameters and corresponding TypeLiteral tokens, like this:
abstract class TypeLiteralV1<T, A> extends TypeLiteral<T> {
protected TypeLiteralV1(TypeLiteral<A> tokA) { ... }
}
TypeLiteral<T> tl_T = TypeLiteral.get(classT);
TypeLiteral<List<T>> tl_LT = new TypeLiteralV1<List<T>, T>(tlT) {};
The implementation then reads the anonymous subclass's extends
declaration "TypeLiteralV1<List<T>, T>" to learn that T is reified by tl_T,
and thus uses tl_T to construct a reification of List of whatever T
actually is.
I have sample code for this construction from a Google internal project,
and the code has just been approved for open-source release. I will be
submitting a patch based on this code to Guice.
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