Walk Backwards to Happiness -- Debugging by Time Travel

In circumstances when a variable in a program has an incorrect value the process of debugging it is often a process about discovering the history of that variable, or rather the ancestry of the value that it contains. In this paper we propose a new technique for debugging that revolves around being able to trace back through the history of a particular variable or variables that it depends upon. Our development is in the domain of functional programming as the proposal has particular significance in this domain due to the fact that so many standard debugging techniques cannot be used at all