Elm - delightful language for reliable web applications

No Runtime Exceptions

Elm uses type inference to detect corner cases and give friendly hints. NoRedInk switched to Elm about four years ago, and 300k+ lines later, they still have not had to scramble to fix a confusing runtime exception in production. Learn more.

-- TYPE MISMATCH ---------------------------- Main.elm The 1st argument to `drop` is not what I expect: 8| List.drop (String.toInt userInput) [1,2,3,4,5,6] ^^^^^^^^^^^^^^^^^^^^^^ This `toInt` call produces: Maybe Int But `drop` needs the 1st argument to be: Int Hint: Use Maybe.withDefault to handle possible errors.

Fearless refactoring

The compiler guides you safely through your changes, ensuring confidence even through the most widereaching refactorings in unfamiliar codebases.

Whether it's renaming a function or a type, or making a drastic change in a core data type, you just follow the compiler errors and come out the other end with a working app.

James Carlson, Software engineer, Brilliant

Understand anyone's code

Including your own, six months later. All Elm programs are written in the same pattern, eliminating doubt and lengthy discussions when deciding how to build new projects and making it easy to navigate old or foreign codebases. Learn more.

-- THE ELM ARCHITECTURE init : ( Model, Cmd Msg ) update : Msg -> Model -> ( Model, Cmd Msg ) subscriptions : Model -> Sub Msg view : Model -> Html Msg

Fast and friendly feedback

Enjoy Elm's famously helpful error messages. Even on codebases with hundreds of thousands lines of code, compilation is done in a blink.

I love how fast Elm is. I make a change and I get an immediate response. It’s like I’m having a conversation with the compiler about how best to build things.

Wolfgang Schuster, Software engineer

Great Performance

Elm has its own virtual DOM implementation, designed for simplicity and speed. All values are immutable in Elm, and the benchmarks show that this helps us generate particularly fast JavaScript code. Learn more.

Enforced Semantic Versioning

Elm detects all API changes automatically thanks to its type system. We use that information to guarantee that every single Elm package follows semantic versioning precisely. No surprises in PATCH releases. Learn more.

$ elm diff Microsoft/elm-json-tree-view 1.0.0 2.0.0 This is a MAJOR change. ---- JsonTree - MAJOR ---- Changed: - parseString : String -> Result String Node + parseString : String -> Result Error Node - parseValue : Value -> Result String Node + parseValue : Value -> Result Error Node

Small Assets

Smaller assets means faster downloads and faster page loads, so Elm does a bunch of optimizations to make small assets the default. Just compile with the --optimize flag and let the compiler do the rest. No complicated set up. Learn more.

JavaScript Interop

Elm can take over a single node, so you can try it out on a small part of an existing project. Try it for something small. See if you like it. Learn more.

var Elm = require('./dist/elm/main.js'); var app = Elm.Main.init({ node: document.getElementById('elm-app') }); // set up ports here

A delightful language for reliable web applications.