tardis

A Tardis is a combination of both a forwards and a backwards state transformer, providing two state values that "travel" in opposite directions. You can install this library with the use-undecidable-instances flag, but this feature is only provided for toying around. If you depend on this library, or install any libraries that depend on this library, then you should install this library without that flag. A detailed description of what a Tardis is and how to use it can be found in the documentation for Control.Monad.Tardis.