Elm used to have a crazy time-traveling debugger in elm-reactor[0], but that's been replaced by session-event-export[1]. This is really, really easy to do well

Elm used to have a crazy time-traveling debugger in elm-reactor[0], but that's been replaced by session-event-export[1]. This is really, really easy to do well when every function is pure and every data structure is immutable, and you use an MVC/Event-sourced/FRP architecture for (more-or-less) all control flow. Of course, Elm enforces all of these things. I've written / worked on other systems (e.g. accounting / transaction pipelines) in other, less safe languages and implementing a shoddy version of this is really straightforward and immensely useful. That said, with pure functions and immutable data structures, you don't really need a debugger for the same reasons, or indeed nearly as often, as in less safe languages. [0]: http://debug.elm-lang.org/ http://debug.elm-lang.org/ [1]: http://elm-lang.org/blog/the-perfect-bug-report http://elm-lang.org/blog/the-perfect-bug-report