Neither.
* I meant s-types.
* There is no such thing. The problem, as Clinton: "It depends on what the meaning of the word 'is' is."
S-types WERE a thing, but never a major one.
I ran across them as a high school student on a random research project at a university as a summer internship of some kind. They were used by a handful of research groups for some data interchange format. Literally the only person who would find the interesting is myself, and not for what they were, but because they were my first exposure to Lisp / Scheme concepts, which I had not heard of before, and which triggered a series of (high school grade) epiphanies for me (which would probably be obvious to 50+% of the people here).
As far as I know, S-types no longer exist. A quick web search brought up zero references. There is absolutely no reason to know about them in 2023. However, if you want to know about them:
- They were quite literally S-expressions. All S-types were S-expressions
- However, they were well-specified, so useful as an import/export format
- In particular, they had research-grade libraries to do the above from at least two languages (I would guess Java and LISP, but I no longer recall; perhaps it was C++ and Scheme).
- There was some attempt to simplify, but I no longer recall what it was. I recall that I was told it had something to do with lists versus function calls, but looking back on it, the explanation I recall being told makes zero sense. That's more likely a function of my memory than the explanation.
I believe on that internship, my direct use of them was probably limited to looking at some data in that format. This was really at the outskirts of the project I was doing. They did inspire a lot of work during the school year, when I schemed for the first time, not really knowing what I was doing but figuring out a lot of stuff in the process.
Looking back at it, though, having a standardized Python / JavaScript / etc. import/export library for some standardized version of S-expressions might be helpful. Or it might not be. Who knows.