Enhancing the REPL to use it like a worksheet

[SuperFola]

Currently, the REPL is very crude. It counts lines, save code (but only when it compiles and runs successfully), adds coloration (but it's incomplete, due to the fact the standard library/builtins/keywords are changing) and provides some kind of completion (same thing as coloration).

It would be great to be able to write code, have a result, and then modify anything in your code and have a result (may it be an error or success), in something like a 2 way view ; eg IntelliJ with Scala and Ammonite does this:

It would be great to be able to quickly save the produced code to a file (or even have it saved to a file by default?), so that we can take the code outside the REPL to continue working on it (copying code from the REPL and pasting it outside is tedious as you have to remove the prompts on each line).

I'm open to any other suggestions related to the REPL and how we could improve it.