This is nice and everything, but I don't see why you would write fish scripts in the first place. For anything like that I'd use python, and imo the only reason to do POSIX shell scripting is that a compliant shell is probably preinstalled on your target platform.
Some scripting example : https://nicolas-van.github.io/programming-with-fish-shell
(I use zsh as shell, as enough customization can do much of what fish can achieve with bit more stability but maybe they fixed with 3.0 which I haven't tried yet.)