Bidirectional state monad transformer.
A Tardis is a combination of both a forwards and a backwards state transformer, providing two state values that "travel" in opposite directions.
A detailed description of what a Tardis is and how to use it can be found in the documentation for Control.Monad.Tardis.
TardisT
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.