Software has gotten really big, with many systems — even, apparently, cars — running into the hundreds of millions of lines of code. The drawbacks of code bases this large are numerous: they are hard to understand, hard to modify, hard to test, and virtually guaranteed to contain huge numbers of bugs. My understanding is that up to this point, we have survived principally by burying mountains of code under abstraction layers, hiding much of the complexity from the next layer in the software stack. This is fine but it only gets us so far: large codes tend to export quirky, unclean abstractions and of course bugs cause even more abstraction leaks. We should be exploring alternative approaches that might be able to radically reduce the size of our code bases.
This annual report describes an ongoing effort to implement Frank, a full software stack for an interactive desktop machine, using less than 20,000 lines of code. This is a great project and I love the fact that they’ve made an annual report — typically an internal-only document required by a funding agency — publicly available. Another nice aspect of this project is that creating an innovative GUI is explicitly not a goal; they simply want to mimic existing systems using 1000 times fewer lines of code.
The technical approach, in a nutshell, is to design a number of domain-specific programming languages, each suited to concisely expressing some part of the system. For example, consider this text from the report which describes the core of Frank’s windowing system, which is written in the Nile language:
The several dozen standard compositing rules, shading, stroking, gradients, sampling, shadowing, etc.–457 lines in total–were written in Nile and debugged to make a working graphics system, strong enough to be used for Frank, and to do all the graphics required for personal computing (and hence this report).
The Nile code can be found here. What is really being talked about — thought the report doesn’t use the term — is executable specifications. Specifications describe how a system should operate; they are written in English, mathematics, pseudocode, etc. and in general they can’t be executed efficiently (or at all). On the other hand, if the domain and the specification language are constrained, it should be possible to create executable specifications. Even in cases where the simple version of the specification cannot be executed efficiently, this approach supports the separation of optimizations from the core specification, making it easier to reason about their correctness.
Although I love this research project, at some point we have to ask ourselves if the basic goal — a 1000x reduction in lines of code, or a full system in 20 KLOC — will survive contact with the enemy. To play devil’s advocate for a moment, I suspect that a minimal OS + GUI + applications could be written in 20,000 lines of code even in C++ if the design was careful, the feature set limited, and the implementers talented. To continue with that line of thought: How much of Frank’s elegance will survive performance tuning, porting to many platforms, feature creep, internationalization and related hacks, mediocre developers, security hardening, and all of the other things that happen to software as it is pushed into production and then used by real people to accomplish complex tasks?
My guess is that a significant fraction of Frank’s gains go away in the face of real-world engineering concerns. However, even if we are left with a 10x or 100x reduction in code size, instead of the original 1000x, the exercise is worthwhile. The thing that worries me most about pushing a system like Frank into the real world is that the hidden costs are larger than we might hope. Let’s make an analogy with microkernels. Speaking abstractly, there is much to love about, for example, a 10 KLOC kernel that has been proved correct. However, people whose judgement I trust have said that (1) debugging code running on a microkernel can be extremely hard and (2) creating high performance code requires very talented developers. The first problem stems mainly from concurrency, the second from the fact that in a microkernel-based system, an arbitrary piece of data is no longer a single pointer indirection away. It seems that a Frank-like system is likely to have similar problems. Creating a clean, coherent, and efficient system using a big pile of little languages, some of them highly mathematical, probably requires serious talent. In contrast, if I’m a medium-grade developer and you give me a big, crappy mountain of C++, I can just start hacking and probably I’ll eventually get the desired effect — even if it is fragile and impossible to maintain. The is a classic worse-is-better situation — which has nothing to do with “worse” or “better”, but rather is about the conflict between two value systems: one which prizes cleanliness and clarity, the other which values pragmatic code that works efficiently in practice and can be easily evolved to solve new problems.
In summary, I desperately hope that we won’t be seeing, in a few years, 50 billion lines of code on the desktop and in the automobile. This future seems all too plausible and we should be actively working to avoid it. Approaches such as Frank are promising. The Racket project has a significant amount of overlap with Frank (though its top-level goals are different, or at least differently stated); see for example the recent CACM article by my colleague Matthew Flatt.