It's actually really weird, if you think about it, that point 1 should involve the network. We should be able to just call a function in one language from a function in another language.
Actually this happened to me once. We had two components that needed to talk to each other - one with an Erlang server and C client library that communicated over a socket with a proprietary protocol - and the other in node.js. The first attempt was to write a C translator that took requests over another socket with a different proprietary protocol, but this one was proprietary to us so we could use it directly from node.js. The second, much better attempt was to learn node's C++ module interface and write a C++ node module wrapper around the C client library.
This third-party Erlang component benefited from being an independently restartable process and therefore needing some RPC, but we also had a mess of C/C++ components inter-connecting over RPC that in reality probably didn't need to be separate processes, but for some reason we'd already decided that architecture before we started writing them.
> It's actually really weird, if you think about it, that point 1 should involve the network. We should be able to just call a function in one language from a function in another language.
If you have two languages that both are not C or C++ , and have more involved runtimes, how well does this work? I know for some cases you have things like JRuby or IronPython, but say mixing a JVM language and a CLR language?
For those cases you have to bring the runtimes with you.
With JVM and CLR you can use JNI and COM to generate SOs/DLLs, and both of them can use any SOs/DLLs via FFI. There is also IKVM and Jni4Net that allowed Java code to run in .NET (or at least used to be, I last used it 15 years ago). Results may vary.
For other languages it can be a bit more involved: if there's no such thing as exposing as a library, you must embed the interpreter, which typically involves using C++.
It's not fun. This is why people end up using network requests.
If you can have a text-only interface, or even involve files, you can also just invoke the other app as a process.
Actually this happened to me once. We had two components that needed to talk to each other - one with an Erlang server and C client library that communicated over a socket with a proprietary protocol - and the other in node.js. The first attempt was to write a C translator that took requests over another socket with a different proprietary protocol, but this one was proprietary to us so we could use it directly from node.js. The second, much better attempt was to learn node's C++ module interface and write a C++ node module wrapper around the C client library.
This third-party Erlang component benefited from being an independently restartable process and therefore needing some RPC, but we also had a mess of C/C++ components inter-connecting over RPC that in reality probably didn't need to be separate processes, but for some reason we'd already decided that architecture before we started writing them.