What is a .elm file and how do I open it?

An .elm file is Elm source code, a purely functional language that compiles to JavaScript for building web front ends. Its compiler is famous for friendly error messages and for ruling out runtime exceptions.

What is a .elm file used for?

Elm takes the Haskell family of ideas and aims them squarely at the browser: no null, no undefined, and if an Elm program compiles it essentially cannot throw a runtime exception in practice.

The Elm Architecture (model, update, view) that structures every Elm app inspired Redux and much of modern front-end state management. The files are plain text.

What opens a .elm file?

The .elm format opens in the following apps, grouped by operating system. If one app does not work, try another from the same list.

Windows

VS Code (Elm extension)
any text editor

macOS

VS Code (Elm extension)
any text editor

Linux

VS Code (Elm extension)
any text editor

Open a .elm file step by step

Confirm the file really is a .elm. Drop it into the WhatFileType identifier, which reads its signature in your browser without uploading anything.
On Windows, open it with VS Code (Elm extension) or any text editor.
On Mac, use VS Code (Elm extension) or any text editor.
On Linux, try VS Code (Elm extension) or any text editor.
Still stuck? Convert the .elm to js (compiled), which opens more widely.

Why can't I open my .elm file?

Almost always because no installed app understands Elm. Install one of the apps above, or convert the file to a format your existing software already reads. If double-clicking opens the wrong program instead of nothing at all, the file association is set to the wrong app rather than the file being broken.

The other possibility is that the file is not really a .elm at all. Drop it into the file identifier to check it by its bytes instead of its name. Nothing is uploaded.

How to convert a .elm file

You can convert a .elm file to js (compiled) using a conversion tool or the export menu of an app that opens it.

Elm has no fixed magic number, so there is no signature table on this page. Its bytes alone do not prove the format: identification relies on the extension, the structure of the content and the program that created it. The magic-numbers reference lists the formats that do carry one.

Frequently asked questions

What is a .elm file?

How do I run an .elm file?

Install Elm and run elm reactor for a quick dev server, or elm make src/Main.elm to compile it to JavaScript or HTML you open in a browser.

Does Elm really have no runtime exceptions?

In practice, yes. The compiler forces every case (no null or undefined), so the classic JavaScript crashes cannot occur in pure Elm code.

What is the Elm Architecture?

A pattern of model, update and view that structures every Elm app. It predates and inspired Redux-style state management in JavaScript.

Related formats

Sources

Details on this page were checked against authoritative references. How sources are ranked and what is deliberately left out is set out in the methodology.

Elm guide