This might seem like nitpicking but it's an important distinction because you can't assume that C data structures can be passed between different platforms (e.g. over a network or even stored in a filesystem).
Symbian did not had a C ABI for example, nor do mainframes, in ChromeOS or Android you also don't have a C ABI exposed to userspace, in Windows a C ABI also won't help you much if you need to speak COM or now UWP.
I consider it significant code smell when a project has multiple languages, particularly when each language seems tied to a particular developer.
But if you want to be able to optimize both globally and locally, well that's just an engineering problem!
Two examples spring to mind:
1. Lisp. Lisp's metaprogramming facilities allow you to build the language into the specialized tools that you need, while still having the common runtime and base language.
2. Unix. There a hundreds (possibly even thousands) of little programs running on my Linux machine written in probably dozens of languages. For the most part, I have no idea (nor do I care) what language they are written in. That's because the Unix model puts a strong emphasis on the protocols for communicating between programs.
Unix doesn't put a strong emphasis on protocols. It just says "everything is text, except when it's not". It's not very helpful.
Those mechanisms aren't textual, they are byte-sequence based.
It turns out that many text-based protocols are more popular than binary/object alternatives. The penalty for textual redundancy is negligible/acceptable for many of them. Where not, Unix allows people to use unix/create new mechanisms to implement specialized protocols (rpc, protobuffers,...).
OK, I probably misspoke by claiming it put an emphasis in protocols, but I think it's fair to say that Unix does emphasize component integration by sharing data instead of trying to integrate through a common runtime.
I do think that having everything be (mostly) text is helpful though. It's true that the data formats are often poorly specified, but that is compensated for by the fact text is a format that has a very rich set of tooling. text editors, regular expressions, parser-generators, etc. all make it possible to capture, analyze, and manipulate the text data exchanged.
Perhaps a better example of protocol-based integration would be the internet. It has it's flaws as well, but it's also enabled collective engineering projects on a vast scale.
The reason C is so successful is named ABI.
C (and C++ to some extend) can be used to produce library that can be used from any higher level language in existence. And this is strictly required in the heterogeneous world we live in.
If you have to write a system components, long lifetime, implementing a protocol / database / format / any-low-level-staff: You do not have choice, safety or not. It is going to be C or C++, because it is the only thing reusable in most other environment.
Python has pybind11, Java has JNI, Lua operate beautifully with C/C++, Node is itself C++, go has cgo, Ruby is in C, any proper programming language can interface with C or C++.
They are the only languages allowing that, and that is why they are still alive, very well alive, even if they are unsafe.
As long as new language authors will be more obsessed about supporting new fancy feature instead of providing a well defined ABI (C compatible). The situation will not change.
Rust might be the new comer in this area that has its chance. But for the time being, it is still too young: you have to map every Rust API to a C one manually to make it exportable.
Unix is a reasonable example of a polyglot environment, but at a high cost. Lots of serialization/deserialization through text. That has a high cost in terms of bugs, complexity, lines of code, and inefficiency.
Yup, that's more or less the argument, though the argument need not only apply to lisp.
> Unix is a reasonable example of a polyglot environment, but at a high cost.
I'd rather say that Unix is an example of how to effectively drive down the cost of a polyglot environment to a point where it is outweighed by the benefits.
Now, of course the cost is still there, but I would argue that beyond a certain scale you cannot optimize globally on a single runtime (perhaps not even with something like lisp) and so the cost of global consistency is outweighed by cost of being unable to optimize locally.
To put it more succintly, you could not build a system as complex as Unix in a single language[1]
[1] Although Ala Kay's work at VPRI suggests you can, if you choose a sufficiently expressive base language. But even that may have it's limits.
This isn't really true.
Trait objects are a double pointer: one to the vtable, and one to the data. C++'s dynamic dispatch uses a single pointer to a structure that has the vtable and the data. The memory layouts are very different.
Clarification: the memory layouts between C++ dynamic dispatch and rust dynamic dispatch are very different.
Rust imposes no requirements on the structure layout, whereas C++ adds some magic data. The magic data means it's awkward to make C++ do dynamic dispatch on a C struct pointer; but rust can do dynamic dispatch on a C struct pointer with no problem.
Have a gnarly problem in a Python program that Haskell would be perfect for? Too bad. You'll spend more time figuring out how to transform the data and get the Python code to call into Haskell than you'll save by using the right tool. And in the process, the overhead, complexity, and bugs introduced in this process turn the right tool into the wrong tool.
And that's the reason why C is so important. It actually does work with pretty much any language, because it has a defined ABI, simple data types, minimal runtime, can work on any data formats unmodified, no GC. Very few languages have this superpower.
C++ mostly has that superpower, but some features don't entirely work with unmodified structures (RTTI and virtual functions both require adding magical fields to the class structure). Rust seems to have this superpower (does dynamic dispatch a different way than C++, so can always work on unmodified structures).
The JVM is an interesting case and the ecosystem of languages built on it do work better together because they have the exact same runtime (GC, etc.).
To summarize: we'd all like to use the right tool for the job. But we can't, except for the languages that go out of their way to make this effective, and that list of languages is very short.