All my opinion, of course: The time (cost) required to create such easily available introspection is too high for the comparatively small gains. REPLs in modern languages (when even implemented) are just so often a completely separate mode of operation.
But a person designing and implementing programming-language tools has to deliberately choose to provide the needed features because implementing them after the fact raises the costs and lowers the benefits. That means such a person must believe that the gains are worth the cost. The only people likely to believe that are people already familiar with those kinds of systems.
I actually like to use the 'repl' module in nodejs. With a little bit of effort, you can make the dev experience much easier.
That basic assumption is a giant knife-switch that pervasively affects everything in the language and environment. Consider, for example, the generic functions CHANGE-CLASS and UPDATE-INSTANCE-FOR-CHANGED-CLASS. Those functions are in the Common Lisp language standard. That makes no sense unless the standard assumes that a Common Lisp system can modify a program while it runs.
Only a minority of programmers want such systems, but to be fair, only a minority of programmers have worked with them enough to understand them.
Not everyone prefers that style of programming, but most programmers will probably never have the opportunity to find out whether they prefer it. I did have the opportunity, and I learned that I did prefer it. I still prefer it thirty years later.
Get back to me with a Clojure or JS environment that does what I miss from a great Lisp or Smalltalk environment. Let me trigger an error and walk the call stack to inspect parameters and intermediate results. Give me a keystroke to get a list of every other piece of code that calls the function with the error--don't make me wait more than about a second for the list. Let me interactively edit the values on the stack in the suspended computation or, better yet, change their types (and have them properly reinitialized), then resume the computation. Let me redefine a function that's on the call stack and then restart the call using the new definition. Let me serialize the entire dynamic state of the program to disk, copy it to a colleague's machine, and start it up to show my colleague the same error in the same dynamic environment. Let me do all of that interactively without needing to leave the original break loop triggered by the original error. Oh, and package the tools as a single modest-sized executable that I can run with a simple command line or a double-click, that launches in a couple seconds or less.
I don't know of such a Clojure or JS environment. If you do, I would be grateful to learn of it.
I have a Discord bot written in CL. If I want to add a new command or feature, I write it, hit a few buttons, and it seamlessly integrates with my bot without interrupting anything it's doing.
If I have a problem with a macro, I can hit a button to have it expand right there in my code, and when I see the problem I hit a button to unexpand it.
The editor will even catch errors and ask what you want to do, without interfering with other threads, which has been very useful while developing my bot.
It's strange from my perspective to see REPLs as a separate mode of operation. Yes, there's a little overhead, and it's something you might want to strip or disable from a production build, but the overhead is small in the context of a modern application language. Contrast with something like Electron.
This style of development makes the feedback loop between the programmer's mental model of how things work and a reality check extremely tight. It's one of those things like TDD that often changes what kind of programs you write.
