[wavemode]

Well, you can "source" a script, so its effects persist in your current shell session. That's a feature of the script and your shell sharing the same language.

[zbentley]

Sure, but “shell scripts” as an (admittedly imprecise) term usually imply separate executable files, not shell libraries/functions that are sourced.

[freehorse]

Then it seems that what you want is running a repl of your chosen language rather than shell/"executable" scripts.