Elm loader

Webpack loader for the Elm programming language.

It is aware of Elm dependencies and tracks them. This means that in --watch mode, if you require an Elm module from a Webpack entry point, not only will that .elm file be watched for changes, but any other Elm modules it imports will be watched for changes as well.

Installation

$ npm install --save elm-webpack-loader

Usage

Webpack 2

Documentation: rules

webpack.config.js:

module.exports = {
  module: {
    rules: [{
      test: /\.elm$/,
      exclude: [/elm-stuff/, /node_modules/],
      use: {
        loader: 'elm-webpack-loader',
        options: {}
      }
    }]
  }
};

Webpack 1

Documentation: loaders

webpack.config.js:

module.exports = {
  module: {
    rules: [{
      test: /\.elm$/,
      exclude: [/elm-stuff/, /node_modules/],
      use: 'elm-webpack-loader'
    }]
  }
};

See the examples section below for the complete webpack configuration.

Options

cwd (default null) Recommended

You can add cwd=elmSource to the loader:

var elmSource = __dirname + '/elm/path/in/project'
  ...
  use: {
    loader: 'elm-webpack-loader',
    options: {
      cwd: elmSource
    }
  }
  ...

cwd should be set to the same directory as your elm-package.json file. You can use this to specify a custom location within your project for your elm files. Note, this will cause the compiler to look for all elm source files in the specified directory. This approach is recommended as it allows the compile to watch elm-package.json as well as every file in the source directories.

maxInstances (default 1)

You can add maxInstances=8 to the loader:

  ...
  use: {
    loader: 'elm-webpack-loader',
    options: {
      maxInstances: 8
    }
  }
  ...

Set a limit to the number of maxInstances of elm that can spawned. This should be set to a number less than the number of cores your machine has. The ideal number is 1, as it will prevent Elm instances causing deadlocks.

Cache (default false)

You can add cache=true to the loader:

  ...
  use: {
    loader: 'elm-webpack-loader',
    options: {
      cache: true
    }
  }
  ...

If you add this, when using npm run watch, the loader will only load the dependencies at startup. This could be performance improvement, but know that new files won't be picked up and so won't be watched until you restart webpack.

This flag doesn't matter if you don't use watch mode.

ForceWatch (default false)

This loader will infer if you are running webpack in watch mode by checking the webpack arguments. If you are running webpack programmatically and wants to force this behaviour you can add forceWatch=true to the loader:

  ...
  use: {
    loader: 'elm-webpack-loader',
    options: {
      forceWatch: true
    }
  }
  ...

RuntimeOptions (default undefined)

This allows you to control aspects of how elm-make runs with GHC Runtime Options.

The 0.18 version of elm-make supports a limited set of those options, the most useful of which is for profiling a build. To profile a build use the settings runtimeOptions: '-s', which will print out information on how much time is spent in mutations, in the garbage collector, etc.

Note: Using the flags below requires building a new elm-make binary with -rtsopts enabled!

If you notice your build spending a lot of time in the garbage collector, you can likely optimize it with some additional flags to give it more memory, e.g. -A128M -H128M -n8m.

  ...
  use: {
    loader: 'elm-webpack-loader',
    options: {
      runtimeOptions: '-A128M -H128M -n8m'
    }
  }
  ...

Files (default - path to 'required' file)

elm-make allows you to specify multiple modules to be combined into a single bundle

elm-make Main.elm Path/To/OtherModule.elm --output=combined.js

The files option allows you to do the same within webpack

module: {
  loaders: [
    {
      test: /\.elm$/,
      exclude: [/elm-stuff/, /node_modules/],
      loader: "elm-webpack",
      options: {
        files: [
          path.resolve(__dirname, "path/to/Main.elm"),
          path.resolve(__dirname, "Path/To/OtherModule.elm")
        ]
      }
    }
  ]
}

(Note: It's only possible to pass array options when using the object style of loader configuration.)

You're then able to use this with

import Elm from "./elm/Main";

Elm.Main.embed(document.getElementById("main"));
Elm.Path.To.OtherModule.embed(document.getElementById("other"));

Modules with elm-hot

Because you must use the object style configuration it isn't possible to use the chained loader syntax (loader: 'elm-hot!elm-webpack'). Instead you may use webpack-combine-loaders

var combineLoaders = require("webpack-combine-loaders");

module: {
  loaders: [
    {
      test: /\.elm$/,
      exclude: [/elm-stuff/, /node_modules/],
      loader: combineLoaders([
        {
          loader: "elm-hot"
        },
        {
          loader: "elm-webpack",
          options: {
            files: [
              path.resolve(__dirname, "path/to/Main.elm"),
              path.resolve(__dirname, "Path/To/OtherModule.elm")
            ]
          }
        }
      ])
    }
  ]
}

Upstream options

All options are sent down as an options object to node-elm-compiler. For example, you can explicitly pick the local elm-make binary by setting the option pathToMake:

  ...
  use: {
    loader: 'elm-webpack-loader',
    options: {
      pathToMake: 'node_modules/.bin/elm-make'
    }
  }
  ...

For a list all possible options, consult the source.

Notes

Example

You can find an example in the example folder. To run:

npm install
npm run build

You can have webpack watch for changes with: npm run watch

You can run the webpack dev server with: npm run dev

For a full featured example project that uses elm-webpack-loader see pmdesgn/elm-webpack-starter .

noParse

Webpack can complain about precompiled files (files compiled by elm-make). You can silence this warning with noParse. You can see it in use in the example.

  module: {
    rules: [...],
    noParse: [/.elm$/]
  }

Revisions

4.5.0

• Support for Webpack 3+
• Upgrade node-elm-compiler to 4.5.0+

4.4.0

• Support for Webpack 2+
• Support for multiple main modules
• Support for --watch-stdin
• Add an example for Webpack 2+

4.3.1

• Fix a bug where maxInstances might end up being higher than expected

4.3.0

• Set maxInstances to 1
• Patch watching behaviour
• Add forceWatch to force watch mode

4.2.0

Make live reloading work more reliably

4.1.0

Added maxInstances for limiting of instances

4.0.0

Watching is now done based on elm-package.json, faster startup time via @eeue56

3.1.0

Add support for --debug via node-elm-compiler

3.0.6

Allow version bumps of node-elm-compiler.

3.0.5

Upgrade to latest node-elm-compiler, which fixes some dependency tracking issues.

3.0.4

Fix potential race condition between dependency checking and compilation.

3.0.3

Use node-elm-compiler 4.0.1+ for important bugfix.

3.0.2

Use node-elm-compiler 4.0.0+

3.0.1

Pass a real error object to webpack on failures.

3.0.0

Support Elm 0.17, and remove obsolete appendExport option.

2.0.0

Change warn to be a pass-through compiler flag rather than a way to specify logging behavior.

1.0.0

Initial stable release.