=encoding utf8

=head1 Introduction

This infodump describes aspects of the most desirable interface for Java
interop functionality in Rakudo, based on experience with CLR interop
functionality in Niecza.  No aspects of implementation I<per se> are discussed. 

=head1 Unsolved questions

Mainly, "should wrapper types be invariant or covariant?".  A wrapper of type
C<Subclass> is not necessarily substitutable for a wrapper of type
C<Superclass>, because the subclass may have added methods which cause an
ambiguous overload (in the CLR we have the additional issue of shadowing, but
that doesn't apply here).  Invariant wrappers (all wrappers are assignable to
C<JavaObject> and C<Any>) are the safer option, but covariant wrappers
(reflecting the Java type DAG) would be more convenient in some cases.  I did
covariant wrappers for Niecza and was not quite happy with the associated
traps; I'd like to see if invariant wrappers can be made to work.

=head1 General object features

We'd probably like fields to be reflected as rw methods, returning C<Proxy>
objects that handle get and set.  (This works well in Niecza).

Methods, including C<new> aka C<< <init> >>, should be bundled by short name
and then turned into overload-resolving sets.  This can be done either by
creating a composite method that implements Java overload rules, or by building
a Perl 6 multi-dispatcher that wraps a set of methods.  Using a
multi-dispatcher would probably require covariant wrappers.

Java objects should be made to implement core Perl 6 roles whenever reasonable.
C<MethodHandle>s and single-method interfaces (including the new single-method
interfaces being added to Java 1.8 for use with lambdas) should function as
Callable with a C<< postcircumfix:<( )> >>.  Arrays and objects assignable to
C<java.util.List> should present a C<Positional> interface.  Objects assignable
to C<java.util.Map> should present an C<Associative> interface.
C<java.util.Iterable> things should probably be iterable, somehow.

It may additionally be useful to provide semantics for C<java.util.Collection>
and C<java.util.Set>, but that's I<probably> overkill.

It shall be possible to throw and catch Java exceptions as Perl 6 exceptions.
(The current implementation is limited such that you can throw OR catch Java
exceptions - a thrown Java exception is treated as a direct control operator to
the innermost callback, and cannot be caught by Perl 6 exception handlers until
it propagates to a callout point.)

Since a C<java.lang.ClassLoader> is effectively the root of a class namespace,
it shall be possible to obtain a PsuedoStash-like object for a class loader
which can be used to fetch type objects representing the classes defined in it.
Each such PsuedoStash holds a class loader and a package prefix; fetches
attempt a lookup and return a concatenated stash, along with either a type or
some kind of C<Uninstantiatable> thing.

=head1 Marshal-in (boxing) rules

Most objects should box as C<JavaObject>s.  In order for the interface to be
most generally usable, it must be possible to get the exact same Java object
out of the box - boxings that lose object identity are probably unacceptable.
Primitive types have no such restriction and should be translated into Perl 6
types freely.  C<BootJavaObject> already translates all primitives that
correspond to 6model types in the obvious way, additionally C<char> is returned
as a one-character C<str> and C<boolean> is mapped to an C<int> of 0 or 1.  The
former is probably correct for Perl 6, but the latter should be overridden to
be C<True> or C<False>.

It is acceptable to map reference types in cases where the Perl 6 object wraps
the corresponding type, so it can be recovered identically from the box.  This
is the case for C<java.lang.String> / C<Str> and C<java.math.BigInteger> /
C<Int>, and of course C<SixModelObject>.

=head1 Marshal-out (unboxing / coercion) rules

In all cases it is acceptable to pass a JavaObject wrapper to code which
expects a reference type.  The type of the wrapper is ignored, except possibly
for overload resolution.  Primitive types and naturally-boxed types, including
C<SixModelObject>, are marshaled out in the obvious way.

It may be useful to provide special handling for functions which take an
argument of type C<ThreadContext> or C<GlobalContext>; note that
C<BootJavaInterop> specially allows passing C<nqp::null()> in such cases with
an interpretation of "current".  Alternatively mechanisms can be provided in
the library for retrieving the current context objects.

Wherever an Iterator or Iterable is needed, it shall be acceptable to pass an
iterable Perl 6 object; a wrapper Iterable shall be constructed which iterates
the underlying Perl 6 object.

Wherever a List is needed, it shall be acceptable to pass any Perl 6 object
that does Positional.  Note that implementing the List interface faithfully
requires C<splice> and C<elems> in addition to C<at_pos>, while C<exists> and
C<delete> are not used.

Wherever a Map is needed, it shall be acceptable to pass any Perl 6 object that
does Associative.

Wherever any non-collection interface type is needed, it shall be acceptable to
pass a hash mapping string method names to code references, which results in
the creation of a proxy class (as per C<BootJavaInterop.proxy>).  If the
interface has a single method, passing a single code reference is acceptable.
Collection interfaces must be proxied explicitly, to avoid ambiguity with the
(presumably more common) case where the programmer wishes to treat a hash as a
collection itself.

=head1 Access

It is inevitable that some general utilities not tied to a specific class will
be needed.  A setting module (C<JavaUtils::foo>) is probably the sanest way to
approach this.

Access to the class hierarchy is bootstrapped through a special symbol in the
setting which is bound to a psuedo stash representing the combination of the
NQP class loader (the class loader through which C<nqp-runtime.jar>,
C<rakudo-runtime.jar>, and (through delegation) C<rt.jar> are visible) and the
empty package prefix.  Since the NQP class loader itself cannot and should not
be serialized, it will probably be necessary to represent this in a special
way, for instance by allowing a null class loader to represent the NQP class
loader.  (Note that this is inconsistent with the usual behavior of Java core
methods to treat a null class loader as a reference to the bootstrap class
loader.  That might be a problem.)

C<use> can be wired up to bind class and package symbols into the current
scope.  Additionally we can support adverbs which trigger dynamic loading of
jars.
