Merged into elm/compiler
Historical Note
In 2013, Laszlo Pandy figured out how to make a reliable Time Traveling Debugger for Elm. It became part of elm-reactor in 2014 thanks to Michael James who adapted it for how we thought people would work with it in practice. Since then (1) the internal implementation of Elm has changed a decent amount and (2) we have learned a lot about what production users need to use this tool seriously. So time travel is not available in elm-reactor right now, but it will be coming back with a new design focused on real workflows. I think this will lead us to a tool that is both more reliable and more broadly useful whether you are making games or web apps.
For now, the Debug module still makes it easy to peek at values in your program.