Common Lisp is the only language where I've seen other languages implemented on top. Today I saw an awk implementation! https://lemonodor.com/archives/000040.html
Racket comes close (and has great tooling) but I've not seen anything like the Python implementation on CL.
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I've heard of 'blub languages', where you don't realise that other languages have better abstractions until you've experienced them.
I think the same thing happens with individual features. I've seen several C++ folks miss variadic generics in Rust, but I've not written enough C++ to feel it.
I've seen "cons cells" and "cons pairs", but today I saw "conses", which is new to me.
(Clearly Lisp has no downsides, only pros and cons! 🙃)
TIL Tcl has a notion of 'safe interpreters', a mode where you can run untrusted code in a sandbox: https://www.tcl.tk/man/tcl8.4/TclCmd/safe.htm
Not many programming languages have this, but it's way safer to include in the implementation than try to build as a userland library.