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https://issues.apache.org/jira/browse/LUCY-5?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12682036#action_12682036
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Marvin Humphrey commented on LUCY-5:
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> How easy will it be to subclass in the host language? EG, for
> PyLucene I have to make a 'stub' class in Java first:
The stub class won't be necessary. You can write your subclass in pure Python.
Boilerplater auto-generates all the routines that "stub" class would have
provided at core build time. You bloat up the shared object that way
somewhat, but it's not the kind of bloat that would matter in the context of a
dynamic language.
Then at run-time, the VTable class's constructor installs these callback
pointers
whenever it finds that you've overridden a public method.
> I assume Lucy has a similar requirement, ie we must decide up front
> which methods are "dynamically dispatchable" and ensure Lucy always
> invokes those methods dynamically.
That seems to me like a weird way of putting it, so maybe I'm not grokking
you. I think the answer is yes -- but isn't that true for vtable-based
subclassing in general? Only invocations of "final" methods can be resolved
to a function address at compile-time. All other method invocations have to
go through the double-derefence to find the function address in the vtable.
However, it isn't necessary for the core to define methods that always call
back into the host. Abstract methods do that by default, but other methods
don't have to:
* The score() slot in the VTable for Scorer points at a function that
invokes lucy_Host_callback_i to call back to the host.
* The score() slot in the VTable for TermScorer points directly at the core
C function lucy_TermScorer_score.
* The score() slot in the dynamically generated VTable for the user-defined
pure-Python subclass "MyTermScorer" points at a function that invokes
lucy_Host_callback_i to call back to the host.
> Boilerplater compiler
> ---------------------
>
> Key: LUCY-5
> URL: https://issues.apache.org/jira/browse/LUCY-5
> Project: Lucy
> Issue Type: New Feature
> Components: Boilerplater
> Reporter: Marvin Humphrey
> Assignee: Marvin Humphrey
>
> Boilerplater is a small compiler which supports a vtable-based object model.
> The output is C code which adheres to the design that Dave Balmain and I
> hammered out a while back; the input is a collection of ".bp" header files.
> Our original intent was to pepper traditional C ".h" header files with no-op
> macros to define each class's interface; the code generator would understand
> these macros but the C compiler would ignore them. C source code files would
> then pound-include both the ".h" header and the auxiliary, generated ".bp"
> file.
> The problem with this approach is that C syntax is too constraining. Because
> C does not support namespacing, every symbol has to be prepended with a prefix
> to avoid conflicts. Futhermore, adding metadata to declarations (such as
> default values for arguments, or whether NULL is an acceptable value) is
> awkward. The result is ".h" header files that are excessively verbose,
> cumbersome to edit, and challenging to parse visually and to grok.
> The solution is to make the ".bp" file the master header file, and write it in
> a small, purpose-built, declaration-only language. The
> code-generator/compiler chews this ".bp" file and spits out a single ".h"
> header file for pound-inclusion in ".c" source code files.
> This isn't really that great a divergence from the original plan. There's no
> fixed point at which a "code generator" becomes a "compiler", and while the
> declaration-only header language has a few conventions that core developers
> will have to familiarize themselves with, the same was true for the no-op
> macro scheme. Furthermore, the Boilerplater compiler itself is merely an
> implementation detail; it is not publicly exposed and thus can be modified at
> will. Users who access Lucy via Perl, Ruby, Java, etc will never see it.
> Even Lucy's C users will never see it, because the public C API itself will be
> defined by a lightweight binding and generated documentation.
> The important thing for us to focus on is the *output* code generated by
> Boilerplater. We must nail the object model. It has to be fast. It has to
> live happily as a symbiote within each host. It has to support callbacks into
> the host language, so that users may define custom subclasses and override
> methods easily. It has to present a robust ABI that makes it possible to
> recompile an updated core without breaking compiled extensions (like Java,
> unlike C++).
> The present implementation of the Boilerplater compiler is a collection of
> Perl modules: Boilerplater::Type, Boilerplater::Variable,
> Boilerplater::Method, Boilerplater::Class, and so on. One CPAN module is
> required, Parse::RecDescent; however, only core developers will need either
> Perl or Parse::RecDescent, since public distributions of Lucy will
> contain pre-generated code. Some of Boilerplater's modules have kludgy
> internals, but on the whole they seem to do a good job of throwing errors
> rather
> than failing subtly.
> I expect to submit individual Boilerplater modules using JIRA sub-issues which
> reference this one, to allow room for adequate commentary.
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