The State monad allows you to send information forwards to the future, or to receive such information from the past. The Reverse State monad allows you to do the reverse: send information backwards to the past, or receive information from the future.
TardisT is a monad transformer that provides state operations that allow you to send information to both the future and the past, as well as receive information from both directions. It is isomorphic to a StateT on top of a ReverseStateT, or vice-versa.
See test/Example.hs for an example.
This was inspired by Luke Palmer's blog post on the "reverse state monad".
http://lukepalmer.wordpress.com/2008/08/10/mindfuck-the-reverse-state-monad/
See also:
http://panicsonic.blogspot.com/2007/12/backwards-state-or-power-of-laziness.html
(c) 2012 Dan Burton