Jane Street Software Developer interview questions

I applied through their website, receiving a letter from them a week later. We scheduled an initial phone interview. They don't do interviews via Skype or Hangouts, and their interviewers use some form of an interactive codepad like collabedit. I'd recommend plugging a headset to your phone — I didn't so I had to constantly switch between holding my phone close enough and typing code. It was very frustrating. The interview lasted for an hour, the engineer I talked to was nice and pleasant. He presented the problem to solve by giving a type signature for a function using Ocaml, but you can use any programming language during the interview (their blog post about their interviewing process specifically recommends candidates to «know their tools»). Upon discussing the last question I think he said something like «time's up», which I guess was the sign that my performance was subpar. In a week after the interview I received a short sorry letter.