I like that the comments mention another two implementations of a similar idea and all are a bit different.

Here's one I've written a few years ago: https://github.com/kiryk/mlisp/blob/master/vector.c

You may find it dirty, but it can be wrapped using macros, and used both on LHS and RHS like in "string(v, i) = ch"

(the macros BTW) https://github.com/kiryk/mlisp/blob/71d028738d2f9607fa7ce8c1...

(and a usecase) https://github.com/kiryk/mlisp/blob/71d028738d2f9607fa7ce8c1...