Massive open online courses (MOOCs) have launched a scale shift in higher education, with several individual MOOCs now boasting tens or hundreds of thousands of participants worldwide. Our MOOC on the principles of functional programming has more than 100,000 registered students to date, and boasts one of the highest rates of completion (19.2%) for its size. In this paper, we describe our experience organizing this popular MOOC, and demonstrate how providing innovative supporting tools (IDE plugins, testing frameworks, interactive build tools, automated cloud-based graders, style checkers) and considering key human-computer interaction factors potentially contributed to this markedly high completion rate. We collect an unprecedented volume of course statistics and survey results and have made them available, along with scripts for generating interactive web-based visualizations, as an open-source project.