TextMate's beautiful and flawed extension mechanism

This is about how TextMate’s bundle mechanism was brilliant, but subtly flawed. However, to make that point, I need to drag you through a dichotomy of developer tools.

Composition vs. context in developer tools

What's the difference between a tool that developers work with and a tool developers often end up working against? Is there a useful distinction between tools that seem great at first, but end up loathed as time goes on? Neal Ford has ideas. Why Everyone (Eventually) Hates (or Leaves) Maven:

I defined two types of extensibility/programability abstractions prevalent in the development world: composable and contextual. Plug-in based architectures are excellent examples of the contextual abstraction. The plug-in API provides a plethora of data structures and other useful context developers inherit from or summon via already existing methods. But to use the API, a developer must understand what that context provides, and that understanding is sometimes expensive.

Composable systems tend to consist of finer grained parts that are expected to be wired together in specific ways. Powerful exemplars of this abstraction show up in *-nix shells with the ability to chain disparate behaviors together to create new things.

Ford identifies Maven and IDEs like Eclipse as tools that rely on contextual extension to get developer started with specific tasks very quickly. On the other hand, a composable tool exchange task-oriented focus for greater adaptability.

Contextual systems provide more scaffolding, better “out of the box” behavior, and contextual intelligence via that scaffolding. Thus, contextual systems tend to ease the friction of initial use by doing more for you. Huge global data structures sometimes hide behind inheritance in these systems, creating a huge footprint that shows up in derived extensions via their parents. Composable systems have less implicit behavior and initial ease of use but tend to provide more granular building blocks that lead to more eventual power.

And thus, the crux of the biscuit:

Contextual tools like Ant and Maven allow extension via a plug-in API, making extensions the original authors envisioned easy. However, trying to extend it in ways not designed into the API range in difficultly from hard to impossible...

Contextual tools are great right up to the point you hit the wall of the original developer’s imagination. To proceed past that point requires a leap of one or two orders of magnitude in effort or complexity to achieve your goal which the original developer never intended.

Bundles are beautiful

Ford wrote this as a follow-on to a piece Martin Fowler wrote about how one extends their text edior. It turns out that the extension models of popular text editors, such as VIM and Emacs, are more like composable systems than extension-based systems.

All of this is a extremely elaborate setup for me to sing the praise of TextMate. Amongst the many things it got very right, TextMate brilliantly walked the line between a nerdy programmer’s editor and an opinionated everyday tool for a wide range of developers. It did this by exposing its extension mechanism through two tools that every developer knows: scripts and regular expressions.

To add functionality to TextMate, you make a bundle. A bundle is a convention for laying out a directory such that TextMate knows the difference between a template and a syntax definition through a convention. This works because developers know how to put things in the right folder. There were only ever five or so folders you needed to know about, so this was a simple mechanism that didn’t become a burden.

To tell TextMate how to parse a language and do nifty things like folding text, you wrote a bunch of regular expressions. If I recall, there were really only a few placeholders to wedge in these regular expressions. This worked great, as most languages, though the “serious” tools use lexers and parsers, are amenable to low-fidelity comprehension with a series of naive pattern matches. The downside was that languages that didn’t look like C were sometimes odd to work with.

In my opinion, the real beauty of TextMate’s bundles was that all of the behavioral enhancement was handled with shell scripts. Plain-old Unix. You could write them in Ruby, Python, bash, JavaScript, whatever fit your fancy. As long as you could read environment variables and output text (or even HTML), you could make TextMate do new things. This led to an absolute explosion of functionality provided by the community. It was a great thing.


Interestingly enough, TextMate is essentially a runtime for bundles. This is how VIM and Emacs are structured as well. TextMate just put a nicer interface around that bundle runtime. However, the way it did so was its downfall, at least for me.

Recall, a few hundred words ago, the difference between composable and contextual extensions. A contextual extension is easy to get going, but comes up short when you imagine something the creator of the extension point didn’t imagine. The phenomenal thing about TextMate was how well it chose the extension points and how much further those extension points took the program than what came before it. I’d estimate that about 90% of what TextMate ever needed to do, you could do with bundles. But the cost to find that last 10%, it was brutal.

Eventually, I bumped up against this limitation with TextMate. I wanted split windows, I wanted full-screen modes, I wanted better ctags integration. No one could add those (when TextMate wasn’t open source, circa 2010) because they required writing Objective-C rather than using TextMate’s extension mechanism. And so, I ended up on a different editor (after several months of wandering in a philosophical desert).

The moral of the story

If possible, you should choose a composable extension mechanism (a full-blown language, probably) and use that extension mechanism to implement your system, ala Vimscript/VIM and elisp/Emacs. If you can’t do that, you can get most of the benefit by doing a plugin API, but you have to choose the extension points really, really well.

Adam Keys @therealadam