For programming languages to survive indefinitely, they have to either:
Be simple enough to be implemented with little cost.
Become irreplaceable critical infrastructure of many large organizations.
C++ will survive indefinitely. Even though it is so complex that no single person understands the language in its entirety, many Fortune 500 companies have multi-million SLOC C++ codebases as part of their critical infrastructure, and they have neither the desire nor the ability to rewrite them. The mystery box that nobody can characterize the behaviour of is irreplaceable until its business function is obsolete. C++ is, in a sense, a sunk cost. Large organizations can fund the maintanance of C++ compilers indefinitely into the future.
Forth, as a family of languages, will exist forever since you can write an interpreter in an evening. Scheme will exist indefinitely since there are many implementations, built largely by a single author, that conform to the latest standards. If all of them bitrot, a new implementation can be written for little cost. The only tricky part of Scheme is continuations.
Consider Ada. It’s a beautiful, very unfairly maligned language, but it is undeniably very big. If AdaCore folds, who’s going to maintain GCC Ada? The open source Ada community is very small and the commercial vendors have done little to promote it. Writing an open source Ada compiler from scratch is a tremendous undertaking. Ada can die.
Consider Common Lisp: it has many existing implementations, but the only one that matters is SBCL, because it’s the most widely-used open source one. SBCL is actively maintained by a small team. Google contributes a lot because Google Flights is internally an immense Common Lisp codebase they bought from ITA Software.
But when Google moves flight search to something else, and if the SBCL maintainers retire, who’s going to pick up the maintenace of a ~460,000 SLOC compiler written in a dynamically-typed language?
How about writing a new implementation? Common Lisp is a huge language: the spec has over 1000 pages. While much of that can be implemented in userspace, Common Lisp has significant implementation complexity, mostly in the object system and the condition system. Building a high-performance, new implementation is a Herculean task. Though, after a discussion with a fellow lisper, I think CLISP could be cleaned up and turned into a pedagogical/bootstrapping implementation. But, at present, Common Lisp can die if a small number of SBCL contributors retire and the compiler starts bit-rotting and nobody picks up the work.
A central constraint in the design of Austral is the language should be easy to implement. Not just because I was the only person writing the compiler, but because I want Austral to be a hundred year language, where you can reliably run code from decades ago (Common Lisp is like this: it is possible). If, for whatever reason, the source code of the bootstrapping compiler was lost, it would be trivial to rewrite it again from the ~100 pages of spec (roughly half of which describes the rationale for various design decisions).
Anyone who has built a compiler and reads the spec can build an Austral compiler, the language is designed such that it is obvious what any piece of code will compile down to. Because it’s statically typed, it’s more complex than building a minimal Scheme interpreter, but it’s infinitely less complex than a C++, Ada, Common Lisp, Scala, etc. compiler.
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