What is "happstack"

Happstack (Haskell Application Stack) is a free application server for websites written in the functional programming language Haskell. It integrates handling Web, persistence, XML/ XSLT, and templating functionality.

It makes use of a number of Haskell libraries and systems such as Parsec, Cabal, Template Haskell, and monad transformers ("Your app then is just a set of state transformer functions (in the M ACID Monad) that take an event and state as input and that evaluate to a new state, a response, and a (possibly null) set of sideeffects.") among others.