ezyang’s blog

I'm hoping that this will be the beginning of a series of posts describing all of the visit days/open houses that I attended over the past month. Most of the information is being sucked out of the notes I took during the visits, so it's very stream of consciousness style. It's kind of personal, and I won't be offended if you decide not to read. You've been warned!

I arrive at the Inn at Penn shortly before midnight, and check in. Well, attempt it; they appear to have no reservation on hand. It appears that I hadn't actually registered for the visit weekend. Oops.

Sam tells me later that first impressions stick, and that her first impression of me was a thoroughly discombobulated PhD admit. Ruffle up my hair! Ask if they're a fellow CS PhD admit, whether or not they have a room, oh, and by the way, what is your name? (I didn't realize that was the real sticking point until she told me about it at the end of the CMU visit.) But Brent is on IRC and I ping him and he knows a guy named Antal who is also visiting UPenn so I give him a call and he lends me his room for a night and things are good. (Mike, the graduate studies coordinator, is kind of enough to fit me into schedules the next day. Thank you Mike!)

I've been to UPenn before. I visited with my Dad; Roy Vagelos was former CEO of Merck and a large influence on UPenn, and at one point I had private aspirations of doing my undergraduate degree in something non-CS before hopping to computer science graduate school. But when the MIT and Princeton admissions came UPenn got soundly bumped off the table, and this experience was wrapped up and stowed away. But I recognized small pieces of the campus from that visit, stitched together with the more recent experience of Hac Phi and POPL. Our tour of Philadelphia lead us to the Bed and Breakfast I stayed at POPL, and I was Surprised™.

Benjamin Pierce is flying off to Sweden for WG 2.8 (a magical place of fairies, skiing and functional programming), so he's only around for the morning presentations, but during breakfast he talks a bit about differential privacy to a few of the candidates, and then heads up the presentations. I hear from his students that he’s gotten quite involved with the CRASH project, and is looking a lot more at the hardware side of things. One of the remarkable things that I heard in the morning talks was how programming languages has sort of leaked into all of the CS research that was presented (or maybe that was just selection bias?) Well, not everyone: the machine learning researchers just “want to do a lot of math” (but maybe we still get a little worried about privacy.)

UPenn is well known for its PL group, where “we write a little bit of Greek every day.” It’s really quite impressive walking into the PL lunch, with a battalion of grad students lined up against the back wall cracking jokes about Coq and Haskell and the presentation is about F*.

Here are some "facts". At UPenn you spend two semesters TAing, there’s an office lottery but people in the same group tend to cluster together, you don’t have to work on grants, the professors are super understanding of graduate student's life situations, the living costs are around $500/mo, and that it is a very laid back place. Your advisor is very much in charge of your degree, except once a year when there's a department wide review to check no one falls through the cracks. The drop-out rate is low. UPenn has a nice gym which is $400/yr. Biking Philadelphia is great, but you shouldn't live in grad housing. Because there is a power trio of Pierce, Zdancewic and Weirich, when it comes time for you to form your thesis committee, the other two profs who aren't your advisor serve on your committee. The PL group is in a slightly unusual situation where there aren't any 2nd or 3rd year students (this was due to a double-sabbatical one year, and bad luck of the draw another.) But there are infinitely many 1st year students. You have to take three laid back exams. Philadelphia is a nice and large city (5th biggest!) UPenn and CMU are perhaps the biggest pure programming languages departments out of the places I visited.

Stephanie Weirich is super-interested in dependently typed languages. She wants to find out how to get functional programmers to use dependent types, and she's attacking it from two sides: you can take a language like Haskell for which we are adding more and more features moving it towards dependent types, and you can build a language from scratch like TRELLYS which attempts to integrate dependent types in a usable way. She’s not shy about giving 1st years really hard projects, but she's happy to do random stuff with students. She reflects on Dimitrios Vytiniotis' trajectory: where he discovered his true calling with polymorphism and Haskell, and is now off designing crazy type systems with Simon Peyton Jones at Microsoft Research. “I can’t make you go crazy about a topic,” (in the good sense) but she’s interested in helping you find out what lights you on fire. I ask her about the technical skills involved in developing metatheory for type systems (after all, it’s very rare for an undergraduate to have any experience in this sort of thing): she tells me it’s all about seeing lots of examples, figuring out what’s going to get you in trouble. (In some sense, it’s not “deep”, although maybe any poor computer scientist who has tried to puzzle out the paper of a type theorist might disagree.)

Steve Zdancewic has a bit more spread of research interests, but one of the exciting things coming out is the fact that they have Coq infrastructure for LLVM that they've been working on over the course of the year, and now it's time to use the infrastructure for cool things, such as do big proofs and figure out what's going on. There have been a lot of cases where doing just this has lead to lots of interesting insights (including realization that many proofs were instances of a general technique): mechanized verification is a good thing. He also has some side projects, including compilers for programming languages that will execute on quantum computers (all the computations have to be reversible! There is in fact a whole conference on this; it turns out you can do things like reduce heat emissions.) There's also a bit of interest in program synthesis. Steve echoes Stephanie when he says, “I am enthusastic about anything that my students are interested in enough to convince me to be enthusiastic about.” They don’t do the chemistry model of getting a PhD, where you “use this pipette a hundred million times.”

In the words of one grad student, these are professors that are “friendly and much smarter than you!” They’re happy to play around and have fun with programming languages concepts (as opposed to the very hard core type theory research happening at CMU; but more on that later.)

We'll close with a picture, seen in the neighborhoods of Philadelphia.