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Isn't what you're describing basically a unix "shell" like bash, csh, zsh, etc?

I agree that something like bash leaves a lot to be desired in REPL functionality. Its ubiquity is convenient, however.

BusyBox has Ash sh and a number of other binaries all compiled into a multiply-symlinked executable.

BusyBox and Ash (and Bash) in Rust would be neat. IDK that docstring parity would be a good thing?

There's also RustPython.