>In CPython they are written in C (at the moment). Other Python implementations are written in other languages.

True. E.g.:

1. PyPy:

https://en.wikipedia.org/wiki/PyPy

>The PyPy interpreter itself is written in a restricted subset of Python called RPython (Restricted Python).

2. Jython:

https://en.wikipedia.org/wiki/Jython

>Implementation language: Python, Java

The article says the last release of Jython was 2 years ago. I wonder how much it is used these days. It did seem like a good idea when it was first released. I had only tried it out a little at that time.

>It depends on what you mean by 'anything doable'. Eg Haskell compilers can in principle do lots of crazy optimisations that a C compiler would not be able to safely do, just because they don't have enough information. Even more so for Lean compilers, which can _know_ which of your loops are terminating, instead of making crude assumptions like C compilers.

Interesting. What are some of those optimisations that Haskell and Lean can do, that a C compiler would not be able to safely do?

A very simple one: in Haskell the compiler has knowledge about which functions cause side-effects and which don't. The compiler can skip calling pure functions whose return value ain't used; similarly, the compiler can call a pure function multiple times, instead of saving the return value in memory.

The Haskell compiler can also fairly freely re-arrange in what order things are computed.

A fairly common idiom in Haskell is to just return a tuple of two values, if they are 'thematically' linked; even if computing one is much more expensive than the other. The compiler is smart enough, to only compute one of the values, if you only use one of them. (When I say 'tuple', it could be literally (a, b), or it could be in a struct or whatever.)

Lean knows even more about your code, so you could do more optimisations. But you need to ask Claude for details, I haven't used it as much nor thought as much about it.

Thanks.