Lambda the Ultimate

André Pang, Don Stewart, Sean Seefried, and Manuel M. T. Chakravarty. Plugging Haskell In (pdf). Proceedings of the ACM SIGPLAN Workshop on Haskell, 2004, pp. 10-21.

Extension languages enable users to expand the functionality of an application without touching its source code. Commonly, these languages are dynamically typed languages, such as Lisp, Python, or domain-specific languages, which support runtime plugins via dynamic loading of components. We show that Haskell can be comfortably used as a statically typed extension language, and that it can support type-safe dynamic loading of plugins using dynamic types.

Two of the authors also have a more recent paper (pdf) describing applications they've built using hs-plugins.