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Elm Reactor

by Elm Reactor

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Elm Reactor's own site

What Elm Reactor is

Elm Reactor is a free, open-source interactive tool for developing and debugging Elm programs. It runs on Linux, macOS, Windows, and the web, under a BSD-3-Clause license. From an Elm project’s root directory, running `elm reactor` starts a local server at `http://localhost:8000`. You can open an Elm file through that server to preview its appearance. The Elm guide says `elm make` produces the same compiler messages as Elm Reactor and the online editor, and recommends `elm make` for projects that become too advanced for the reactor workflow. `elm make` compiles Elm code to HTML or JavaScript. Elm packages are hosted at package.elm-lang.org, while `elm install` records dependencies in `elm.json`. The guide links to an online editor, programming lessons, command help, and a GitHub page for reporting unhelpful compiler messages. Time-travel debugging is unavailable, and the repository was archived on August 23, 2018.

Who it suits

Elm developers who want a local preview server and interactive debugging. It also suits learners using Elm’s guide and online editor, while larger projects may need `elm make`.

What it does well

  • Free and open-source
  • Runs on Linux, macOS, Windows, and web
  • Local preview server at localhost:8000
  • Same compiler messages as online editor
  • Elm package installation support

What to know first

  • Time-travel debugging is unavailable
  • Repository archived and read-only
  • Advanced projects should use elm make

Verdict

Elm Reactor provides a straightforward local workflow for viewing and debugging Elm files. Its archived repository and missing time-travel debugging matter when choosing it for newer or more advanced development.

Facts from Elm Reactor's own pages

Compilation
`elm make` compiles Elm code to HTML or JavaScript. · guide.elm-lang.org, 12 Oct 2026
Compiler messages
The guide says `elm make` produces the same messages as the online editor and `elm reactor`. · guide.elm-lang.org, 4 Oct 2026
Debugging limit
The project page says time travel is not available in `elm-reactor` at present. · github.com, 4 Oct 2026
Feedback
The guide invites users to report unhelpful compiler messages through a linked GitHub page. · guide.elm-lang.org, 12 Oct 2026
Founded
2013 · elm-lang.org, 19 Oct 2026
Help
The guide says users can run `elm --help` or command-specific help, such as `elm make --help`, to see available commands and flags. · guide.elm-lang.org, 12 Oct 2026
Installation platforms
The Elm installation guide links to installation options for Mac, Linux, and Windows. · guide.elm-lang.org, 4 Oct 2026
Language guarantees
The guide lists no runtime errors in practice, friendly error messages, reliable refactoring, and automatically enforced semantic versioning for Elm packages as benefits of Elm. · guide.elm-lang.org, 12 Oct 2026
Learning
The Elm guide teaches programming fundamentals and how to build interactive apps with The Elm Architecture. · guide.elm-lang.org, 12 Oct 2026
License
The repository lists a BSD-3-Clause license. · github.com, 4 Oct 2026
Local development
Running `elm reactor` at a project’s root starts a server at localhost:8000 where users can view Elm files. · guide.elm-lang.org, 12 Oct 2026
Local server
Running `elm reactor` at a project’s root starts a server at `http://localhost:8000`. · guide.elm-lang.org, 4 Oct 2026
More advanced projects
The guide recommends `elm make` as the general compilation method when a project becomes too advanced for `elm reactor`. · guide.elm-lang.org, 4 Oct 2026
Online editor
The Elm guide links to an online editor where users can try Elm code. · guide.elm-lang.org, 12 Oct 2026
Packages
Elm packages are hosted at package.elm-lang.org, and `elm install` adds dependencies to a project’s `elm.json` file. · guide.elm-lang.org, 12 Oct 2026
Preview
You can navigate to an Elm file served by the local server and see what it looks like. · guide.elm-lang.org, 4 Oct 2026
Project organization
The Elm Software Foundation describes Elm as connecting web development with academic research on functional programming languages. · foundation.elm-lang.org, 4 Oct 2026
Project status
The `elm-reactor` repository was archived on August 23, 2018 and is read-only. · github.com, 4 Oct 2026
Purpose
Elm is a functional language that compiles to JavaScript and is designed for websites and web apps. · guide.elm-lang.org, 12 Oct 2026
Supported desktop systems
The installation guide provides links for Mac, Linux, and Windows. · guide.elm-lang.org, 12 Oct 2026

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