Knowing and Doing

A couple of weeks ago, a former student emailed me after many years. Felix immigrated to the US from the Sudan back in the 1990s and wound up at my university, where he studied computer science. While in our program, he took a course or two with me, and I supervised his undergrad research project. He graduated and got busy with life, and we lost touch.

He emailed to let me know that he was about to defend his Ph.D. dissertation, titled "Efficient Reconstruction and Proofreading of Neural Circuits", at Harvard. After graduating from UNI, he programmed at DreamWorks Interactive and EA Sports, before going to grad school and working to "unpack neuroscience datasets that are almost too massive to wrap one's mind around". He defended his dissertation successfully this week.

Congratulations, Dr. Gonda!

Felix wrote initially to ask permission to acknowledge me in his dissertation and defense. As I told him, it is an honor to be remembered so fondly after so many years. People often talk about how teachers affect their students' futures in ways that are often hard to see. This is one of those moments for me. Arriving at the end of what has been a challenging semester in the classroom for me, Felix's note boosted my spirit and energizes me a bit going into the summer.

If you'd like to learn more about Felix and his research, here is his personal webpage The Harvard School of Engineering also has a neat profile of Felix that shows you what a neat person he is.

This is short story about a student finding something helpful in class and making my day, preceded by a long-ish back story.

In my programming languages course yesterday, I did a session on optimization. It's a topic of some importance, and students are usually interested in what it means for an interpreter or compiler to "optimize" code. I like to show students a concrete example that demonstrates the value of an optimization. Given where we are in the course and the curriculum, though, it would be difficult to do that with a full-featured language such as Python or Java, or even Racket. On the other end of the spectrum, the little languages they have been implementing and using all semester are too simple to benefit from meaningful optimization.

I found a sweet spot in between these extremes with BF. (Language alert!) I suppose it is more accurate to say that Eli Bendersky found the sweet spot, and I found Bendersky's work. Back in 2017, he wrote a series of blog posts on how to write just-in-time compilers, using BF as his playground. The first article in that series inspired me to implement something similar in Python and to adapt it for use with my students.

BF is well-suited for my purposes. It is very simple language, consisting of only eight low-level operators. It is possible to write a small interpreter for BF that students with only a background in data structures can understand. Even so, the language is Turing complete, which means that we can write interesting and arbitrarily complex programs.

The low-level simplicity of BF combines with its Turing completeness to create programs that are horribly inefficient if they are interpreted in a naive manner. There are many simple ways to optimize BF programs, including creating a jump table to speed up loops and parsing runs of identical opcodes (moves, increments, and decrements) as more efficient higher-level operators. Even better, the code to implement these optimizations is also understandable to a student with only data structures and a little background in programming languages.

My session is built around a pair of interpreters, one written in a naive fashion and the other implementing an optimization. This semester, we preprocessed BF programs to compute a table that makes jumping to the beginning or end of a loop an O(1) operation just like BF's other six primitives. The speed-up on big BF programs, such as factoring large numbers or computing a Mandelbrot set, is impressive.

Now to the story.

At the end of class, I talk a bit about esoteric languages more broadly as a way for programmers to test the boundaries of programming language design, or simply to have fun. I get to tell students a story about a four-hour flight back from OOPSLA one year during which I decided to roll a quick interpreter for Ook in Scheme. (What can I say; programming is fun.)

To illustrate some of the fun and show that programmers can be artists, too, I demo programs in the language Piet, which is named for the Dutch abstract painter Piet Mondrian. He created paintings that look like this:

That is not a Mondrian, but it is a legal program in the Piet language. It prints 'Piet'. Here is another legal Piet program:

It prints "Hello, World". Here's another:

That program reads an integer from standard input, determines whether it is prime or not, and prints 'Y' or 'N'. Finally, how about this:

If you are a certain age, you may notice something special about this image: It is made up exclusively of Tetris pieces. The program prints... "Tetris". Programming truly is an art!

One of my students was inspired. While reviewing the session notes, he searched for more information about Piet online and found this interactive editor. He then used it to create a Piet program in honor of a friend of his who passed away earlier this semester. It prints the Xbox gamertag of his late friend. In his email to me, he said that writing this program was therapeutic.

I'm not sure one of my class sessions has ever had a more important outcome. I'm also not sure that I have ever been happier to receive email from a student.

This has been a tough year for most everyone, and especially for students who are struggling with isolation and countermeasures against a nasty virus. I'm so glad that programming gave one student a little solace, at least for an evening. I'm also glad he shared his story with me.