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 Control/Monad/Tardis/Example.hs for an example.
You can decide how evil you would like this library to be. By default, it will not include typeclass instances that require "scary" language extensions.
cabal install tardis
You can instead install it with the additional instances by indicating that it is OK:
cabal install -f use-undecidable-instances
You can tell whether you have the extra instances with this ghci session:
ghci> :m +Control.Monad.Tardis ghci> :m +Control.Monad.Trans.Identity ghci> :t IdentityT (return ()) >> getPast
If you don't have the additional instances, it will say the type is:
(Monad m, MonadTardis bw b (IdentityT m)) => IdentityT m b
If you do have the additional instances, it will say the type is:
MonadTardis bw b m => IdentityT m b
See Control/Monad/Tardis.hs for details.
This was inspired by Luke Palmer's blog post on the "reverse state monad".
(c) 2012 Dan Burton