Happstack is a web framework written in haskell; it provides a rich collection of types and functions for routing requests, generating responses, working with query parameters, form data, and cookies, serving files and more.

this package provides the documentation for a library for the haskell programming language. see http://www.haskell.org/ for more information on haskell. anyway, it is just a stub, depending on the package that actually provides the happstack server.