There is a significant, qualitative difference between using a language that designed for REPL use and one that isn’t. Those boundaries you talk of are the cru

There is a significant, qualitative difference between using a language that designed for REPL use and one that isn’t. Those boundaries you talk of are the crux of the issue. A highly dynamic, completely expression based language is going to enable a much different experience. Homoiconicity also plays an important role here, because you can ispect and parse code within the language, with the same functions and algorithms as everything else.