[maaaats]

Elm has some of this built in. Not a debugger per se (you cannot add breakpoints and step through the code), but since everything is pure and there is only a single state, you have a single source of truth and a function from that truth to your view. One can replay the different transitions, and see the whole state of the program at those times.

[masklinn]

Does it still work? elm-reactor is archived with notes that it’s been merged into elm/compiler and that time travel is not available anymore.

[maaaats]

elm/browser was recently updated. If you compile in dev modus it gives you a small button to press in the webapp, that opens a new window where actions and state can be replayed.