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Lisp for Web-Based Applications

One of the reasons to use Lisp in writing Web-based applications is that you *can* use Lisp. When you're writing software that is only going to run on your own servers, you can use whatever language you want.

For a long time programmers didn't have a lot of choice about what language to use for writing application programs. Until recently, writing application programs meant writing software to run on desktop computers. In desktop software there was a strong bias toward writing the application in the same language as the operating system. Ten years ago, for all practical purposes, applications were written in C.

With Web-based applications, that changes. You control the servers, and you can write your software in any language you want. You can take it for granted now that you have the source code of both your operating system and your compilers. If there does turn out to be any kind of problem between the language and the OS, you can fix it yourself.

This new freedom is a double-edged sword, however. Having more choices means that you now have to think about which choice to make. It was easier in the old days. If you were in charge of a software project, and some troublesome person suggested writing the software in a different language from whatever you usually used, you could just tell them that it would be impractical, and that would be the end of it.

Now, with server-based applications, everything is changed. You're now subject to market forces in what language you choose. If you try to pretend that nothing has changed, and just use C and C++, like most of our competitors did, you are setting yourself up for a fall. A little startup using a more powerful language will eat your lunch.

Incremental Development

There is a certain style of software development associated with Lisp. One of its traditions is incremental development: you start by writing, as quickly as possible, a program that does almost nothing. Then you gradually add features to it, but at every step you have working code.

I think this way you get better software, written faster. Everything about Lisp is tuned to this style of programming, because Lisp programmers have worked this way for at least thirty years. The Viaweb editor must be one of the most extreme cases of incremental development. It began with a 120-line program for generating Web sites that I had used in an example in a book that I finished just before we started Viaweb. The Viaweb editor, which eventually grew to be about 25,000 lines of code, grew incrementally from this program. I never once sat down and rewrote the whole thing. I don't think I was ever more than a day or two without running code.

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.

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.

Macros for Html

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.


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