Moreover, because the entire language and development system are available, unrestricted, in the repl, you can define the missing function bar, resume foo, and get a sensible result.
This seems like one of the key points. The ability to edit computations while running. Type holes with resuming gets you most of the way there but there's probably also modifications. I wonder how you can keep it from getting confusing. Something similar to FRP?
suppose when you needed to make a permanent edit to the style sheet on your homepage, you opened up the CSS viewer, made the edit, and the result persists—not just in your browser, but by changing the very style sheet itself
it doesn't mean that we actually want all the instances in the heap to be reinitialized
It's conceivable that people would find it overly cumbersome having to deal with this when prototyping from a REPL—enough to consider it a bad experience to keep them from trying.
A live IDE could recognize that the user probably doesn't want that to happen for a given object, since it only just appeared a little while ago, so it should offer some fixup to save time for the user.