I think I had not described it fully (since had read it long ago). Searched for the article just now. The Beating the Averages article [1] does not have the point I mentioned. But a more detailed article [2] linked from it, does:
Lisp's interactive toplevel is a great help in developing software
rapidly. But the biggest advantage for us was probably in finding
bugs. As I mentioned before, with Web-based applications you have
the users' data on your servers and can usually reproduce bugs.
When one of the customer support people came to me with a report
of a bug in the editor, I would load the code into the Lisp
interpreter and log into the user's account. If I was able to
reproduce the bug I'd get an actual break loop, telling me exactly
what was going wrong. Often I could fix the code and release a
fix right away. And when I say right away, I mean while the user
was still on the phone.
Such fast turnaround on bug fixes put us into an impossibly tempting
position. If we could catch and fix a bug while the user was still
on the phone, it was very tempting for us to give the user the
impression that they were imagining it. And so we sometimes (to
their delight) had the customer support people tell the user to
just try logging in again and see if they still had the problem.
And of course when the user logged back in they'd get the newly
released version of the software with the bug fixed, and everything
would work fine. I realize this was a bit sneaky of us, but it
was also a lot of fun. ]
Incidentally, I seem to remember the following: some years ago, after playing around some with Lisp (Allegro CL from Franz, and the Lisp IDE from Lispworks, both of which supported this feature or something like it), I was trying out a then-current version of Visual Studio, and saw that it had a new feature called Edit-and-Continue, which I thought was similar to the Lisp feature.
[1] http://paulgraham.com/avg.html
[2] More Technical Details: http://paulgraham.com/lwba.html (note: redirects twice)
Here's the relevant section:
[ Interactive Toplevel
Lisp's interactive toplevel is a great help in developing software rapidly. But the biggest advantage for us was probably in finding bugs. As I mentioned before, with Web-based applications you have the users' data on your servers and can usually reproduce bugs.
When one of the customer support people came to me with a report of a bug in the editor, I would load the code into the Lisp interpreter and log into the user's account. If I was able to reproduce the bug I'd get an actual break loop, telling me exactly what was going wrong. Often I could fix the code and release a fix right away. And when I say right away, I mean while the user was still on the phone.
Such fast turnaround on bug fixes put us into an impossibly tempting position. If we could catch and fix a bug while the user was still on the phone, it was very tempting for us to give the user the impression that they were imagining it. And so we sometimes (to their delight) had the customer support people tell the user to just try logging in again and see if they still had the problem. And of course when the user logged back in they'd get the newly released version of the software with the bug fixed, and everything would work fine. I realize this was a bit sneaky of us, but it was also a lot of fun. ]