Disappearing Code

:: dear diary, by Ben Greenman

Two experiences at SPLASH 2018 reminded me that software gets thrown away and replaced.

Story 1

The first reminder came near the end of a talk by Martin Rinard. Once upon a time, Martin was working as a consultant and a firm asked him to review a software package. (The firm wanted a second opinion about how the software computed its results.) The firm sent a zipfile; Martin found six versions of the code inside; the firm said “well, please check all six versions”; and it turned out:

  • Version 1 : the source code was written in a domain-specific language (DSL) that generated code for the application
  • Version 2 : the DSL source was the same as version 1, but the generated code was slightly modified
  • Version 6 : the generated code was the source code and the DSL was gone

The moral of Martin’s story was: (1) the creators of a software system are often different from the maintainers, and (2) researchers need to build tools to help these maintainers.

Story 2

The second reminder came from a teaching assistant who said the functional programming course at their institution was currently using a Python script to test students’ code. Once upon a time, I was a teaching assistant for the same course at the same institution. We had trouble testing students’ code via the Python script left by the pre–2013 course staff, so I wrote a command-line tool to handle the tests and other compile/run/grade tasks. To keep history from repeating itself, I used the same language the course teaches (OCaml) and wrote some documentation — but it seems like that was not enough. At any rate, writing the tool was a good exercise.

In the end, everybody must understand for himself.Per Martin-Löf


In each story, the maintainers of a software system threw away some old code to make their job easier in the short term. How can we stop this “re-inventing the wheel” from happening?

Martin Rinard’s solution is to let maintenance programmers keep their current habits, but provide tools to make the short-term, pragmatic solutions into a more robust systems. Search for "failure-oblivious computing" to learn more (this was the topic of his talk).

In Story 1, the maintainers were able to avoid the DSL by modifying an inherited blob of DSL-generated code. If the DSL did not generate code, history might have taken a different course; it might be best to start with a language that offers tools for linguistic re-use, and to build a DSL from these tools — so there is no generated code. The Racket programming language is exploring this path. For a recent example, see the video-lang paper.

The Story 2 test harness, however, was not generating code. Its maintainers discarded a “big” program written in a typed functional language in favor of a script. Perhaps we need a language that allows mixing statically-typed and dynamically-typed code (shouts out to my own research).

The best solution is probably to start with a team and keep the culture alive. Always pair program!

Addendum: comment from Mitch Wand

The best solution is probably to start with a team and keep the culture alive. Always pair program!

Ermm, this works better for sourdough bread than for people.

Even in the not-so-real world of checking student solutions, there’s often no way of guaranteeing that one half of a pair will be around for the second round. They may be on co-op. Or the course will not be offered the next semster/year/etc. Or the course will change at the next offering (from OCaml to Python or from Racket to Java) so that large chunks of the infrastructure will have to be discarded or rewritten.

The “real” solution is to write literate code (as we preached incessantly in PDP), so that the next reader will have at least some clue as about what you wrote. This just may be sufficient incentive to modify rather than rebuild from scratch.

Ever the optimist, —Mitch