(Programming) Language Interoperability

My advisor recently pointed me to an article in the ACM Queue about a familiar topic: interoperability between programming languages. My research for my thesis was very much in this area, and I am happy to see others thinking about the topic. I have not been able to find a whole lot of related work. Broadly speaking, I agree with the sentiment of the article and differ on a few minor points. For example, I don’t think that mutability is a significant barrier to language interoperability. Certainly, I would like to see immutable data become the norm in every language, but that is almost an orthogonal concern. Even in Haskell, mutable data is easily handled within the IO monad. Existing Haskell language interop tools can deal with this sensibly. Expecting every library in every language to be designed with purely functional languages in mind is unrealistic and not productive. In fact, sometimes mutation is the right tool for the job, and in some of those cases, a Haskell programmer might want to drop down into a more low-level language (e.g., rust) to more easily handle their mutable data. Of course, having mutability highlighted in the type system of a language interoperability solution would be highly desirable.

Instead of focusing on the article itself, I want to spend some time musing about a point raised in some of the comments: using C as a least-common denominator for gluing languages together. At the moment, this is certainly the best option for language interoperability. However, I think a distinction should be made in this context between using C the language and C the calling convention. The value of C in this domain is not related to the language itself. In fact, as the article and some of the comments point out, it is in some ways a difficult language to interoperate with due to its memory management, simplistic data model, and the ad-hoc nature of its error handling. That said, the underlying C calling convention is, if not simple, consistent for each platform. Nearly every language knows how to issue calls with the C calling convention. Libraries like libffi make this easy to provide for any language implementation.

Granted, the C calling convention is not very expressive. It is sufficient if you only need to pass pointers and machine-sized data types across language boundaries. If you require deeper integration, and in particular data interoperability, the C calling convention falls short for most languages. This is not insurmountable. Only agreement stands in the way of adopting more expressive calling convention. Even better if this common calling convention is a proper superset of the C calling convention. If the details of the calling convention were abstracted behind a library interface that various language implementations could use, it could even be evolved in a backwards compatible way to improve performance over time as more work is done in the area. Perhaps, as we have the “C” calling convention, this could be the “X” calling convention.

Ahead-of-time compiled languages may need to explicitly export certain functions under the “X” calling convention, much like a C++ function can be declared extern "C". If called from within their native language, the compiler could either use the extern "X" version of the function directly or possibly generate both the externalized version of the function and a “native” version of the function. Just-in-time compiled languages with a compiler available at run-time, like Java, could simply generate extern "X" variants of functions on-demand.

A useful cross-language calling convention would also need to expose some amount of introspection data so that callers can determine what functions the library provides. This is one area where Microsoft’s COM works well from the caller’s perspective. A language implementation aware of the “X” calling convention could automatically generate this information. A dynamic language run-time supporting the calling convention could allow the introspection data to be queried at run-time. Static languages would require a separate tool to load the (versioned) introspection data so that static type-safe call stubs could be created. This is roughly the client experience provided by gobject-introspection. In that case, however, the library interface descriptions are largely hand-generated. Keeping the compiler in the loop would simplify the process and reduce the number of errors.

I should note that it is unlikely that all features of every language could be exposed through a common interface. Languages have many differences, and some differences between languages may be incompatible. That is fine. Languages have differences, and we attempt to use the language with the most helpful quirks for any particular problem. A language interoperability layer that simplifies data transfer and exposes most functionality would still be a vast improvement over the status quo. We have this problem already with C and C++. A C++ function being made callable from C must obey a number of restrictions. This is still useful. It would be an interesting research question to figure out what subset of features is reasonable, and how language-specific extension could be handled.

There are many practical considerations for an “X” calling convention. Error handling is important for any library. While many languages use exceptions to report errors, others do not. Worse, nearly no implementations of exceptions are compatible. Any “X” calling convention would need to standardize error handling. One simple way would be to require an implementation generating an “X”-callable function to wrap it in a catch-all handler and return information about any exceptional conditions via a standard mechanism (e.g., an extra return value). Deeper integration is, of course, possible, but a simple solution may be best. It requires little of language implementors and does not require significant performance sacrifices for the common case of intra-language calls.

Another significant problem in language interoperability is resource management. Objects allocated by the run-time system of one language (malloc in C, the JVM in Java) must be managed by that run-time system. The “X” calling convention could expose resource management hooks. For example, a Python program calling a Java library would receive objects allocated by the JVM; the calling convention could expose pointers to the appropriate functions to call when the JVM object is no longer reachable from Python. The object would be pinned in the JVM until Python un-pinned it. Then it could be garbage collected as it would normally in a Java-only environment.

These are just a few assorted thoughts. Clearly, there would be many details to work out. These ideas are complementary to my earlier work. Instead of performing a deep analysis to make idiomatic bindings, the additional features of this hypothetical calling convention allow enough information to be exposed to require a bit less analysis. Furthermore, the extra information in this hypothetical calling convention would make it easier to issue cross-language calls into languages besides C. The C calling convention is our current lingua franca: we can do better, and doing better would make true polyglot programming simpler.