jdeveloper / Elm

The Elm programming language aims to make web development more pleasant. Elm is a type-safe, functional reactive language that compiles to HTML, CSS, and JavaScript.

Elm

This is the Elm compiler and server, allowing you to develop Elm applications that run in any modern browser.

If you intend to serve Elm code with a Haskell backend, be sure to read all the way to the "Installation for Haskell-Users" section.

Installation for General Use

Download the Haskell Platform. This will give you access to the Haskell compiler (needed to build Elm) and Haskell's package distribution system (to make installation of Elm easier). Once installed (even if it already was), you must update your listing of Haskell packages with:

cabal update

This will ensure that the elm package is available. Then install Elm with:

cabal install elm-server

Assuming everything goes correctly (potential problems are discussed later), this will build two executables on your machine:

• elm is a standard compiler that takes .elm files and produces .html and/or .js files. You can then use these files with your favorite web-framework.

• elm-server is both a compiler and server, allowing you to develop without designing and setting up a server yourself. Running elm-server starts a server in the current directory. It will compile and serve any .elm files in the current directory and its sub-directories. This is how I prefer to develop Elm programs.

To use these executables you need to add a new directory to your PATH. For me, the executables were placed in /home/evan/.cabal/bin which I appended to the end of my PATH variable in my .bashrc file. Cabal should tell you where your executables are located upon successful installation, so you can make a similar addition (see this tutorial if you are new to changing your PATH in Unix/Linux).

That is almost everything. Now, we will create a simple Elm project. The following commands will set-up a very basic project and start the Elm server.

mkdir helloElm
cd helloElm
echo main = lift asText Mouse.position > main.elm
elm-server

The first two commands create a new directory and navigate into it. The echo command places a simple program into main.elm (do this manually if you do not have echo). The final command starts the Elm server at localhost, allowing you to navigate to main.elm and see your first program in action.

Installation for Haskell-users

Elm as described in the previous section is actually split into two packages: elm which contains the guts of the compiler and elm-server which provides a simple HAppStack-based server to simplify development. Those of you planning to write your own server in Haskell only need the elm package:

cabal install elm

The elm package provides support for compilation of Elm code directly in Haskell and QuasiQuoting. See the Examples/ directory for information and examples on how to get started with Elm+Haskell.

Yesod users should also install the elm-yesod package which provides functions for idiomatically embedding Elm in Yesod:

cabal install elm-yesod

Some extra tips on Elm+Yesod can be found here.

An important note: When you install the elm compiler, it automatically downloads Elm's JavaScript runtime system to ~/.cabal/share/Elm-x.y.z/. The runtime system will follow the name scheme elm-runtime-x.y.z.js where x.y.z matches the version number of the compiler. If you want to serve this file from a different location, copy it from its home and always be sure that code compiled with version x.y.z of the compiler is served with version x.y.z of the runtime system.

Potential problems and their solutions

• Try cabal install elm. This will give you access to the elm executable (but not elm-server).

These problems all appeared before Elm version 0.1.1.4:

• Install errors having to do with happstack-server-7.0.2. This version of happstack-server has stricter dependency restrictions that conflict with other libraries required by Elm. Try installing with an earlier version of happstack-server with the following command: cabal install elm --constrain="happstack-server<7.0.2"
• When installing on Debian, blaze-html-0.4.3.2 fails to compile. You must install blaze-html-0.4.3.1 instead.
• Elm does not appear to work with the latest versions of containers (i.e. 0.4.2.*). I know it works with earlier versions of containers, so to avoid this problem, you can try: cabal install elm --constrain="containers==0.4.1.0" --force-reinstall
• On Windows, HAppStack has trouble installing because of issues with the "network" package. I struggled with this problem on Windows 7 until I found the suggestion at the bottom of this page.
• Likely more to come...

Areas for further work:

Error messages need work in general. Syntax, Parsing, and Type errors are reported, but the messages are not very specific. I hope to fix this as soon as possible.

If you are interested in making a large contribution, please contact me at info (at) elm-lang (dot) org so that we do not duplicate any work!