Dynamic IDEs for Dynamic Languages!

Dynamic languages are all the rage these days, and rightly so. Historically, most of the currently popular dynamic languages have had negligible IDE support (since they are scripting languages). More recently, we see a flurry of activity around IDE support for these languages. However, the mainstream IDEs are designed for, and implemented in, static languages. Consequently, even when dealing with a dynamic language, the dynamism stops when it comes to the IDE itself.

What do I mean by this? In an IDE written in a dynamic language (such as Smalltalk or Self or Lisp), the IDE code itself can be modified on the fly. This is a double edged sword, as I’ll explain shortly. I’ll start with the good edge.

Suppose you want to modify the IDE for some reason. It lacks a feature you need, or something is buggy. If it’s a proprietary system, you can file a bug with the vendor, and hope that they pay attention, so that in the next release, in a year or two, you’ll get the fix.

If the system is open source, you can just go ahead and implement the fix/feature. Now if that system is written in a mainstream, conventional language, you just need to recompile the compilation unit with the fix, and rebuild your system. Of course, in order to develop the fix, you had to load a project with the source for your IDE and probably go through several edit-compile-debug cycles. When it all worked, you replace your existing installation with your new build. I have the strange feeling that all this setting up the IDE as a project within itself, rebuilding and reinstalling could take quite a lot of time and effort.

In an IDE like Smalltalk (or Newspeak, or Self), the IDE source is always available from within itself. In such a system, if you change its source code, the change takes effect immediately. For example, the Newspeak class browser supports a pop up menu for methods, that lets you find references to the method and to the messages sent within it. In the screen shot below, the speech bubble on the right denotes this menu





However, the initial version did not support the same functionality for slots.
How do we fix this? Well, for starters, we need to find where the relevant source code is. How do you do this? The old fashioned way is to go on a trek through the source code trying to figure it out. What subdirectory should we start with? Which file should we open first? What should we grep for?

Fortunately, Newspeak makes it easy, as most IDE components have an Inspect Presenter menu item, that will open an object inspector on the actual presenter in question; a presenter on a presenter.

For example, if we want to find out how the references menu for methods works, we can look at a method like the one above. On the far right is a drop down menu:



This opens an object presenter, like this:




Toward the top, is a line marked class; it contains a hyperlink to the class of the object being inspected - the class ExpandableMethodPresenter. Now we know where the code that presents methods resides! If we click on the link, the Hopscotch browser will show us the class.

Having found the code that manages the presentation of methods, and the implementation of the references menu, we next want to find the code that presents slots, so we can modify it. We do the same thing again - invoke the Inspect Presenter menu item, but this time on a slot.

Once we’ve found the code and made the change, we can test it right away. Just hit refresh in the browser, and you’ll see the new reference bubble next to slots.



All of this is part of what Dan Ingalls, and Smalltalkers in general, mean when they say the system is alive. It’s the key property you seek in every system - its ability to respond to changes and mutate while running, which gives you the kind of interactivity you get in the real world with real objects. Live code in the IDE is the opposite of dead code - stuff written in files, which needs to be explicitly loaded into a runtime to do anything.

This all is very cool, but it also has a downside. Suppose you have a bug in your code, causing it to crash. You may be able to fix it in the debugger - but maybe the root of the problem is in another class. You need to browse it - but now, every time you try and browse a class, it crashes. You’ve shot yourself in the foot.

This is where you almost wish you were looking at dead code, by browsing files in an editor (or a conventional IDE). Fortunately, we know how to deal with this problem as well.

In this case, we want to manipulate the modified IDE from another, unmodified one, as a dynamic program. That way we still enjoy immediate feedback for our changes, while jkeeping our feet safe. Once it works, we want to be able to easily suck in the changes into the unmodifed version - without restarting, rebuilding, or going back to the big bang.

We haven’t implemented this yet - but much of it’s been done before, in the mid-90s, by Allen Wirfs-Brock in the FireWall project at ParcPlace. You do it using mirror based reflection.

One of the many things mirrors facilitate, is managing reflection in a distributed setting. Java’s JDI is an example of a mirror API was designed to do just that - though it has serious limitations because it has to work on a variety of JVMs. If you design the mirror API correctly, and build your IDE on it, the IDE can work almost identically on programs within the same process, in another process, or across the internet.

This ability to manage reflective changes to the IDE via a separate process is a luxury, which probably explains why it generally isn’t implemented, The reality is that shooting yourself in the foot is infrequent, and you can always recover (if only by saving your changes to a file before testing them). The benefits of liveness far outweigh the risks.

Nevertheless, in time, I expect to address this in the Newspeak IDE. In the meantime, it’s alive, even if support for death is lacking. Of course, the broader lesson is that IDEs especially, should be implemented using dynamic languages.

The above echoes my previous post from June, but I hope that the concrete examples are helpful. The liveness makes it easy to implement features like Inspect Presenter, which let you identify where to change the IDE, but even more critically, liveness allows you actually make the change and get instant feedback.

In closing, I want to emphasize that this is not an unprincipled approach. All of the above is possible because the IDE can reflect on itself. Self reference is at the heart of computing. It’s all about recursion. I find it strange that many theoretically oriented computer scientists, who can wax poetic about the Y combinator, eschew languages and systems that apply the same principles of self-application to themselves. I’m sure this is will change though; the good ideas win in the end.