The whole development process was one long series of gradual changes. This style of development fits well with the rolling releases that are possible with Web-based software. It's also a faster way to get software written generally.
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.
Lisp macros were another big win for us. We used them very extensively in the Viaweb editor. It could accurately be described as one big macro. And that gives you an idea of how much we depended on Lisp, because no other language has macros in the sense that Lisp does.
One way we used macros was to generate Html. There is a very natural fit between macros and Html, because Html is a prefix notation like Lisp, and Html is recursive like Lisp. So we had macro calls within macro calls, generating the most complicated Html, and it was all still very manageable.
Another big use for macros was the embedded language we had for describing pages, called Rtml. (We made up various explanations for what Rtml was supposed to stand for, but actually I named it after Robert Morris, the other founder of Viaweb, whose username is Rtm.)
Every page made by our software was generated by a program written in Rtml. We called these programs templates to make them less frightening, but they were real programs. In fact, they were Lisp programs. Rtml was a combination of macros and the built-in Lisp operators.
Users could write their own Rtml templates to describe what they wanted their pages to look like. We had a structure editor for manipulating these templates, a lot like the structure editor they had in Interlisp. Instead of typing free-form text, you cut and pasted bits of code together. This meant that it was impossible to get syntax errors. It also meant that we didn't have to display the parentheses in the underlying s-expressions: we could show structure by indentation. By this means we made the language look a lot less threatening.
We also designed Rtml so that there could be no errors at runtime: every Rtml program yielded some kind of Web page, and you could debug it by hacking it until it produced the page you meant it to. Initially we expected our users to be Web consultants, and we expected them to use Rtml a lot. We provided some default templates for section pages and item pages and so on, and the idea was that the users could take them and modify them to make whatever pages they wanted.
In fact it turned out that Web consultants didn't like Viaweb. Consultants, as a general rule, like to use products that are too hard for their clients to use, because it guarantees them ongoing employment. Consultants would come to our Web site, which said all over it that our software was so easy to use that it would let anyone make an online store in five minutes, and they'd say, there's no way we're using that. So we didn't get a lot of interest from Web consultants. Instead the users all tended to be end-users, the actual merchants themselves. They loved the idea of being in control of their own Web sites. And this kind of user did not want to do any kind of programming. They just used the default templates.
So Rtml didn't end up being the main interface to the program. It ended up playing two roles. First of all, it was an escape valve for the really sophisticated users, who wanted something our built-in templates couldn't provide. Somewhere in the course of doing Viaweb, someone gave me a very useful piece of advice: users always want an upgrade path, even though as a rule they'll never take it. Rtml was our upgrade path. If you wanted to, you could get absolute control over everything on your pages.
Only one out of every couple hundred users actually wrote their own templates. And this led to the second advantage of Rtml. By looking at the way these users modified our built-in templates, we knew what we needed to add to them. Eventually we made it our goal that no one should ever have to use Rtml. Our built-in templates should do everything people wanted. In this new approach, Rtml served us as a warning sign that something was missing in our software.
The third and biggest win from using Rtml was the advantage we ourselves got from it. Even if we had been the only people who used Rtml, it would have been very much worth while writing the software that way. Having that extra layer of abstraction in our software gave us a big advantage over competitors. It made the design of our software much cleaner, for one thing. Instead of just having bits of actual C or Perl code that generated our Web pages, like our competitors, we had a very high-level language for generating Web pages, and our page styles specified in that. It made the code much cleaner and easier to modify. I've already mentioned that Web-based applications get released as a series of many small modifications. When you do that you want to be able to know how serious any given modification is. By dividing your code into layers, you get a better handle on this. Modifying stuff in lower layers (Rtml itself) was a serious matter to be done rarely, and after much thought. Whereas modifying the top layers (template code) was something you could do quickly without worrying too much about the consequences.
Rtml was a very Lispy proposition. It was mostly Lisp macros, to start with. The online editor was, behind the scenes, manipulating s-expressions. And when people ran templates, they got compiled into Lisp functions by calling compile at runtime. Rtml even depended heavily on keyword parameters, which up to that time I had always considered one of the more dubious features of Common Lisp. Because of the way Web-based software gets released, you have to design the software so that it's easy to change. And Rtml itself had to be easy to change, just like any other part of the software. Most of the operators in Rtml were designed to take keyword parameters, and what a help that turned out to be. If I wanted to add another dimension to the behavior of one of the operators, I could just add a new keyword parameter, and everyone's existing templates would continue to work. A few of the Rtml operators didn't take keyword parameters, because I didn't think I'd ever need to change them, and almost every one I ended up kicking myself about later. If I could go back and start over from scratch, one of the things I'd change would be that I'd make every Rtml operator take keyword parameters.
We had a couple embedded languages within the editor, in fact. Another one, which we didn't expose directly to the users, was for describing images. Viaweb included an image generator, written in C, that could take a description of an image, create that image, and return its url. We used s-expressions to describe these images as well.