Interactive development tool that makes it easy to develop and debug Elm programs.
Haskell Elm Python

Elm Reactor

Interactive development tool that makes it easier to develop Elm programs. Key features include:

  • Automatically compile any Elm program
  • Compatible with any editor

This means you can get a great development experience whether you are using Sublime Text, Emacs, vim, or whatever else to edit Elm code.


Install Elm Platform. This will install Elm Reactor and everything else it needs.


Navigate to the Elm project you want to work on. A great project to get started with is elm-architecture-tutorial which contains some simple programs that should be fun to debug.

In the root of your Elm project start the reactor with:


Now open http://localhost:8000 in your browser. You should see a navigation page for your project.

Click on any file to see what it looks like. For example, you can navigate to an Elm file like examples/1-button.elm and try it out. If you modify the file, you can just refresh that page and see the new version!

Note About Time Travel

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.