Early in my career I wrote Smalltalk in a GUI-based environment (VisualWorks/GemStone), with the browser, debugger, transcript, etc. It was an incredible experience.
Later, I used Common Lisp with SLIME and had a very similar experience, at least as productive, except that I was using my preferred IDE (Emacs) and saved the code to normal files so I could use more common tools like grep, SVN/Git, etc. I still built code into an image, which I could update live via the REPL.
I haven't used GNU Smalltalk personally, but I do like the idea of a Smalltalk that's less of an island, particularly if that makes it more palatable to a new generation of programmers.
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