wai-session (hackage): Provides a generic, cookie-based middleware for sessions that is parameterised over the session store, the cookie name, and the cookie parameters (such as path, expiry, etc).  Passes a pair of functions (lookup key, set key) for the current session through the Vault in the Request.  Also provides a simple example session store based on threadsafe IORefs and Data.Map.  See example/Main.hs in git for example usage.
Keywords: library|web