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My week at Recurse Center

In late April I served a residency at Recurse Center, formerly known as Hacker School. I want to write up what I did before I forget.

Recurse Center bills itself as being like a writer's retreat, but for programming. Recursers get better at programming four days a week for three months. There are some full-time instructors there to help, and periodically a resident, usually someone notable, shows up for a week. It's free to students: RC partners with companies that then pay it a fee if they hire a Recurser.

I got onto the RC chat system and BBS a few weeks ahead and immediately realized that it was going to be great. I am really wary about belonging to groups, but I felt like I fit right in at RC, in a way that I hadn't felt since I went off to math camp at age 14. Recurse Center isn't that different from math camp now that I think about it.

The only prescribed duty of a resident is to give a half-hour talk on Monday night, preferably on a technical topic. I gave mine on the history and internals of lightweight hash structures in programming languages like Python and Perl. (You can read all about that if you want to.)

Here's what else I did:

1. I gave a bunch of other talks: two on Git, one on calculating with continued fractions, one on how the Haskell type inferencer works, one on the topology of data types, one on the Unix process model, one on Alien Horrors from the Dawn of Unix. This was too many talks. I didn't have enough energy and time to prepare all of them properly. On the other hand, a lot of people were very complimentary about the talks and said they were very glad that I gave so many. Also, giving talks is a great way to get people familiar with you so that they won't be shy about talking to you or asking you to work with them. But I think I'll cut it down to one per day next time.

2. Alex Taipale was inspired by my hash talk to implement hashes synthetically in Python, and I paired with her on that for the first part and reviewed her code a couple of times after. It was really fun to see how she went about it.

3. Libby Horacek showed me around the text adventure game she wrote in Haskell. I had the first of several strokes of luck here. Libby had defined an input format to specify the room layout and the objects, and I observed that it was very similar to Asherah, a project that another Recurser, Michelle Steigerwalt, had done a couple of years before. I found this out because I read everyone's self-posted bio ahead of time and browsed the interesting-sounding links.

4. Aditya Mukerjee was implementing Git in Go. He wanted help deciphering the delta format. Later I paired with Aditya again and we debugged his implementation of the code that expanded the deltas back into complete files. I hadn't known any Go but it's easy to pick up.

5. Geoffrey Gilmore had read my ancient article on how to write a regex matcher. He had written his own implementation in Scala and wanted to show it to me. I didn't know any Scala but the code was very clear. Geoffrey had worked out a clever way to visualize the resulting finite automaton: his automaton object had a method that would dump out its graph in the "dot" language, and he could feed that to Graphviz to get it to draw the graph.

6. I had a conference with Ahmed Abdalla and Joel Burget about SML. The main question they wanted me to answer: Why might they want to look at SML instead of Haskell? This was a stroke of luck: I was unusually well-prepared to answer this question, having written many thousands of lines of SML since about 1993. My answer was unequivocally that there was no reason, SML was obsolete, because it had big problems which had never been solved, and Haskell had been introduced in part to solve, avoid, or finesse these problems.

   For example, nobody knows how to incorporate references into a Hindley-Milner type system. SML has tried at least three methods for doing this over the years. They all suck, and none of them work right. Haskell avoids the whole issue: no references. If you want something like references, you can build a monad that simulates it locally.

   I could probably write a whole blog article about this, so maybe another time.

   [ Addendum 20220425: The article is written! ]

7. Libby wanted to pair with me again. She offered me a choice: she was building an e-reader, controlled by a Raspberry Pi, and mounted inside an antique book that she had hollowed out. I would have been willing to try this, although I didn't know anything about Raspberry Pi. But my other choice was very attractive: she was reviving KiSS, an ancient Windows paper-doll app that had been current in the 1990s. people had designed hundreds or thousands of dolls and costumes, which were all languishing because nobody wanted to run the app any more. She wanted to reimplement the dress-up program in Javascript, and port the doll and clothing cels to PNG files. Here I had another stroke of luck. I was already familiar with the program, and I think I have even been into its source code at some point.

   Libby had found that Gimp could load a KiSS cel, so we looked at the Gimp source code to figure out the file format. She had been planning to use libpng to turn the cel into a PNG, but I showed her a better way: convert it into a PPM file and feed to to ppmtopng. This saved a lot of trouble! (I have written a little bit about this approach in the past.) Libby hacked in the Gimp code, grafting her PPM file writing code into the Gimp cel reading code in place of Gimp's internal pixmap operations. It worked!

8. I talked to Chris Ball about his GitTorrent project. Chris wants to make a decentralized github that doesn't depend on the GitHub company or on their technical infrastructure. He spent a long time trying to make me understand why he wanted to do the project at all and what it was for. I think I eventually got it. It also transpired that Chris knows way more about BitTorrent than I do. I don't think I was much help to Chris.

9. Jesse Chen paired with me to fix the layout problems that have been troubling my blog for years. We redid the ancient table-based layout that I had inherited from Blosxom ten years ago, converting it mostly to CSS, and fixed a bunch of scrolling problems. We also fixed it to be legible on a phone display, which it previously wasn't. Thanks Jesse!

10. I had a discussion with Michelle Steigerwalt about big-O notation and how you figure out what an algorithm's big-O-ness is, either from counting lines in the source code or the pseudocode, or from running the algorithm on different-size inputs and timing it. It's fun that you can do the static analysis and then run the program and see it produce the results you predicted.

There was a lot of other stuff. I met or at least spoke with around 90% of the seventy or so Recursers who were there with me. I attended the daily stand-up status meetings with a different group each time. I ate lunch and dinner with many people and had many conversations. I went out drinking with Recursers at least once. The RC principals kindly rescheduled the usual Thursday lightning talks to Monday so I could attend. I met Erik Osheim for lunch one day. And I baked cookies for our cookie-decorating party!

It was a great time, definitely a high point in my life. A thousand thanks to RC, to Rachel Vincent and Dave Albert for essential support while I was there, and to the facilitators, principals, and especially to the other Recursers.

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