deepak / guide.elm-lang.org

An Introduction to Elm.

An Introduction to Elm

Elm is a functional language that compiles to JavaScript. It competes with projects like React as a tool for creating websites and web apps. Elm has a very strong emphasis on simplicity, ease-of-use, and quality tooling.

This guide will:

• Teach you the fundamentals of programming in Elm.
• Show you how to make interactive apps with The Elm Architecture.
• Emphasize the principles and patterns that generalize to programming in any language.

By the end I hope you will not only be able to create great web apps in Elm, but also understand the core ideas and patterns that make Elm nice to use.

If you are on the fence, I can safely guarantee that if you give Elm a shot and actually make a project in it, you will end up writing better JavaScript and React code. The ideas transfer pretty easily!

A Quick Sample

Of course I think Elm is good, so look for yourself.

Here is a simple counter. If you look at the code, it just lets you increment and decrement the counter:

import Html exposing (Html, button, div, text)
import Html.App as App
import Html.Events exposing (onClick)

main =
  App.beginnerProgram { model = 0, view = view, update = update }

type Msg = Increment | Decrement

update msg model =
  case msg of
    Increment ->
      model + 1

    Decrement ->
      model - 1

view model =
  div []
    [ button [ onClick Decrement ] [ text "-" ]
    , div [] [ text (toString model) ]
    , button [ onClick Increment ] [ text "+" ]
    ]

Notice that the update and view are entirely decoupled. You describe your HTML in a declarative way and Elm takes care of messing with the DOM.

Why a functional language?

Forget what you have heard about functional programming. Fancy words, weird ideas, bad tooling. Barf. Elm is about:

• No runtime errors in practice. No null. No undefined is not a function.
• Friendly error messages that help you add features more quickly.
• Well-architected code that stays well-architected as your app grows.
• Automatically enforced semantic versioning for all Elm packages.

No combination of JS libraries can ever give you this, yet it is all free and easy in Elm. Now these nice things are only possible because Elm builds upon 40+ years of work on typed functional languages. So Elm is a functional language because the practical benefits are worth the couple hours you'll spend reading this guide.

I have put a huge emphasis on making Elm easy to learn and use, so all I ask is that you give Elm a shot and see what you think. I hope you will be pleasantly surprised!