Elm's time-traveling debugger

"[Historically] debuggers typically only go forward because each step may destroy past state. In short, low-level languages lead to low-level debuggers.
..."Laszlo implemented a debugger that lets you travel backwards and forwards in time. It lets you change history. On a deeper level, it lets you visualize and interact with a program’s meaning. It lets you see how a program changes over time."

 Lazlo Pandy on Elm's time-traveling debugger