Warp is a high-performance HTTP server library written in Haskell, a purely functional programming language. Both Yesod, a web application framework, and mighty, an HTTP server, are implemented over Warp. According to our throughput benchmark, mighty provides performance on a par with nginx. This article will explain the architecture of Warp and how we achieved its performance. Warp can run on many platforms, including Linux, BSD variants, Mac OS, and Windows. To simplify our explanation, however, we will only talk about Linux for the remainder of this article.