"Model 2" prototype status

John Altidor jgaltidor at gmail.com
Tue Sep 1 23:29:37 UTC 2015

Thanks for letting me know about the prototype and explaining many cases.
It will be interesting to see how interface Box$$any will be used with
wildcard capture or when an instance of Box<any> is passed to a generic
method that captures the actual type argument of Box.

- John

On Tue, Sep 1, 2015 at 12:17 PM, Brian Goetz <brian.goetz at oracle.com> wrote:

> Hi John;
> The prototype in the valhalla repo is fairly functional (though by no
> means complete).  So I'll answer these with some code.  Of course, this is
> still a prototype, so everything will probably change.
> If I have:
> class Box<any T> {
>     public T t;
>     public T get() { return t; }
> }
> Compilation (currently) produces a combination erased class / template
> file (Box.java), and a wildcard interface (Box$$any):
> brian:tmp$ javap -c Box\$\$any.class
> Compiled from "Box.java"
> interface Box$$any {
>   public abstract java.lang.Object get();
>   public abstract java.lang.Object t$get();
>   public abstract java.lang.Object t$set(java.lang.Object);
> }
> We see that methods, and accessors for the getter and setter for the field
> 't', are lifted to the interface.  Box<any> (represented at runtime by
> Box$$any) becomes the common supertype between all instantiations of Box<T>.
> Note that the signatures have some mangled types, which looks like erasure
> here (but is not exactly the same as erasure, and hence needs a new name --
> call it "anyrasure".)  Ideally, the anyrasure of a type would be a common
> supertype of all possible instantiations of that type, but there's no
> common supertype between "Object" and "int", so we'll settle for a common
> conversion target.
> The anyrasure of a bare type variable is its erasure; a reference
> instantiation need make no conversions, and a value instantiation needs to
> apply a boxing conversion to implement the methods of Box$$any. Clients
> accessing members through a concretely typed receiver bypass the boxing
> path since both client and implementation will already agree on member
> types.
> What about when Foo<T> shows up in a signature?  Compiling this:
> class Box<any T> {
>     Box<T> me() { return this; }
> }
> yields
> interface Box$$any {
>   public abstract Box$$any me();
> }
> showing that the anyrasure of Box<any T> is the wildcard type Box<any>
> (which is a common supertype).  As I mentioned in my talk, we are working
> on a story for the anyrasure of T[] as well (but is not yet implemented.)
> Mapping wildcards to interfaces is imperfect.  It works great for
> capturing subtyping using tools the VM already understands (which means
> lots of things Just Work).  It also leverages invokeinterface, which is
> good.  However, there are some obvious imperfections in the story, which
> we're currently working on:
>  - What about protected and package members?  (In theory we could support
> these accessibilities on interfaces -- the "consistency brigade" would
> certainly applaud this -- but these have issues too (i.e., what does it
> mean for a public interface to have a 'package' member?  Only classes in
> that package can implement the interface?  That's kind of weird.))
>  - Even if we did the above, what about private members?  Private members
> are not inherited, so dispatching through invokeinterface won't work, so
> that needs another story regardless.
>  - What about reflection?  Currently, the way to reflect over "all
> instantiations of Foo" is to reflect over Foo.class.  But Foo.class is only
> the erased instantiation, so people will want the equivalent of reflecting
> over Foo<any>.  (This is one place where lifting the members to the
> interface really shines; an indy-centric approach would require getting
> there another way.)
>  - How do we enforce finality?  There *are* multiple legitimate classes
> that implement Foo<any T> -- Foo<int> is a different class than
> Foo<erased>.  How do we enforce that only "legitimate" instantiations are
> allowed to implement Foo$$any?
>  - Even ignoring finality, how do we prevent impersonation?  If we
> represent Foo<any> by Foo$$any at runtime, how do we keep someone from
> simply implementing Foo$$any but not respecting the invariants of Foo, and
> passing the fake to a method expecting a Foo<any>?
>  - What about wildcard capture?  We need a story for this; we're not
> working on this quite yet.
>  - Non-anyfied superclasses.  The case "class Foo<any T> extends Moo" is
> an annoying corner case, which messes up our story for subtyping checks
> (member access is easily dealt with in one of several ways.)
> In the happy cases, many operations on anyfied types proceed quite nicely,
> leaning on existing VM mechanisms (subtyping, invokeinterface, reflection),
> so they "just work".  All the work is dealing with the unhappy cases.
> With a big enough crowbar(s), each of these issues can be dealt with, but
> we're mindful of the risk of ending up with a pile of unrelated hacks.
> OK, back to work!
> On 8/31/2015 10:04 PM, John Altidor wrote:
>> Hi Brian,
>> I am new to this mailing list.  My PhD dissertation covered subtyping with
>> variance and Java wildcards extensively, so the questions you raised in
>> this thread are very interesting to me.  I was wondering how you are
>> handling the translational aspects of wildcards and specialized generic
>> methods.
>> Your earlier post asked how to represent List<any> in bytecode.  Since
>> List<any> is a supertype of both List<int> and List<double>, for example,
>> type List<any> should only support operations that can be applied to both
>> List<int> and List<double>.  One such operation is counting the number of
>> elements using method List.size().  Which byte representation would
>> support
>> being able to dispatch List.size() on both an instance of List<int> and an
>> instance of List<double>?
>> It seems such a byte representation would need to be independent of
>> fields.
>>    The number of bytes needed to represent an instance differs among
>> primitive types (e.g. int and double).  As a result, it seems List<int>
>> and
>> List< double> may differ in the number of bytes needed for their fields.
>> In that case, one could not know the number of bytes in the instance of
>> List<any> returned from the following method:
>> List<any> func(int input_num) {
>>    if(input_num is odd)
>>      return new List<int>();
>>    else
>>      return new List<double>();
>> }
>> In addition to type-independent methods such as List.size(), another
>> operation that is type safe to allow on an instance of List<any> is
>> wildcard capture.  Consider the generic method, swapFirstTwo, below that
>> just swaps the order of the first two elements in the input list.  It is
>> type safe to pass an instance of List<any> to this method (because no
>> runtime type error would occur).
>> <any T> void swapFirstTwo(List<T> list) {
>>    T first = list.getAndRemoveFirst();
>>    T second = list.getAndRemoveFirst();
>>    list.addToBeginning(first);
>>    list.addToBeginning(second);
>> }
>> Would two calls to method swapFirstTwo, one with a List<int> as input and
>> the other method call with a List<double> as input, result in two
>> specialized copies of method swapFirstTwo in byte code?  If that is the
>> case, what is the byte representation of method swapFirstTwo when the
>> input
>> is an instance of List<any>?
>> Thank you,
>> John Altidor
>> http://jgaltidor.github.io/

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