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Using Elm inside create-react-app

Portrait von Max Strübing

Max Strübing, Softwareentwicklung


In this Blog-post we will use create-react-app to demonstrate how you could use Elm inside and while doing that how to patch your node_modules in a proper, reusable way by using patch-package .

Create Application

We will create a react-app with

npx create-react-app elm-react-app

This gives us a basic react-app with everything out of the box.

Note that we are using react-scripts at version 3.1.1 here, future versions might have different line numbers we are using later on.

Install Elmish-dependencies

Switch into your newly created react-app with cd elm-react-app. We will use elm so that you don't need to install Elm globally on your system. We will also use react-elm-components to use Elm inside our react components.

#!/usr/bin/env bash
yarn add --dev elm && \
yarn add react-elm-components

Initialize Elm

We now want to initialize the Elm part of our project. Think of it as a package.json for Elm. We can do this by running: ./node_modules/.bin/elm init.

The initializer will ask if you are already familiar with Elm and provides you with a link in case you want to read how you can use Elm. Let's assume we

already know and press Y. :)

This gives us an elm.json file which lists our dependencies, source directory, Elm version and type, which is in this case "application". There is also "package" if you want to write a library, but that's nothing we want to do right now.

We need to add elm-stuff to our .gitignore which will contain some build files you don't want to touch and should not be in your vcs:

#!/usr/bin/env bash
echo "elm-stuff" >> .gitignore

Add an Elm component

We will add a simple Hello-World as an Elm-component but keep in mind that this could be anything like real complex Elm applications.

Let's create an Elm file:

#!/usr/bin/env bash
touch src/Hello.elm

Open that file with your editor of choice and add this code:

#!/usr/bin/env elm
-- src/Hello.elm
module Hello exposing (main)

import Html exposing (Html, text)

main : Html msg
main =
    text "Hello!"

Now we need to use that component inside our react application. We use our prior added dependency react-elm-components. Let's add our Elm-component right after the logo:

#!/usr/bin/env javascript
// src/App.js
/* imports */

import Elm from 'react-elm-components';
import MyElmComponent from './Hello.elm';

/* other create-react-app-code */
  <header className="App-header">
    <img src={logo} className="App-logo" alt="logo" />
    <Elm src={MyElmComponent.Elm.Hello} />
/* other create-react-app-code */

If we now start the server with yarn start we can see that our "Hello" is not


Patching the webpack configuration to load Elm files correctly

That is because the default webpack configuration of create-react-app doesn't know how to read Elm files.

We have different options now. We could either:

  • Eject our create-react-app with yarn eject
    but that would remove our updating capabilities from create-react-app

  • Fork create-react-app and modify the configuration there
    that would allow us to update with rebasing our fork, but in my experience, you will never really update because you are not forced to and you

  • We use a cool package which makes patching node_modules very easy

As you've might guess we use the third option. We edit the webpack configuration of create-react-app directly inside the node_modules for now.

For that open node_modules/react-scripts/config/webpack.config.js in your editor of choice and jump to line 372. There you can see the different loaders which are provided by create-react-app by default. We want to add our Elm loader which we

at first need to install:

#!/usr/bin/env bash
yarn add --dev elm-webpack-loader

You can add the loader directly as the first item in the array:

#!/usr/bin/env javascript
// node_modules/react-scripts/config/webpack.config.js
// load our elm files with the correct loader
  test: /\.elm$/,
  exclude: [/elm-stuff/, /node_modules/],
  use: 'elm-webpack-loader'
}, // trailing comma in case there are other items in the array ;)

If you now start your application with yarn start you should see the "Hello" from your Elm component. That is working fine for now, but as soon as we install or uninstall a different dependency or a coworker wants to set up the project our changes are lost. To make this work we use the patch-package tool:

#!/usr/bin/env javascript
npx patch-package react-scripts

We call patch-package with the node_modules dependency where we've made changes to.

The output of this shows:

#!/usr/bin/env bash
npx patch-package react-scripts
npx: installed 211 in 7.952s
patch-package 6.1.4
• Creating temporary folder
• Installing react-scripts@3.1.1 with yarn
• Diffing your files with clean files
✔ Created file patches/react-scripts+3.1.1.patch

At the bottom you can see that npx created a file

patches/react-scripts+3.1.1.patch. If we look at it it is exactly a normal patch file like

patch would give us.

#!/usr/bin/env bash
cat patches/react-scripts+3.1.1.patch
diff --git a/node_modules/react-scripts/config/webpack.config.js b/node_modules/react-scripts/config/webpack.config.js
index 12157a3..3d3d2d2 100644
--- a/node_modules/react-scripts/config/webpack.config.js
+++ b/node_modules/react-scripts/config/webpack.config.js
@@ -370,6 +370,12 @@ module.exports = function(webpackEnv) {
           // match the requirements. When no loader matches it will fall
           // back to the "file" loader at the end of the loader list.
           oneOf: [
+            // load our elm files with the correct loader
+            {
+              test: /\.elm$/,
+              exclude: [/elm-stuff/, /node_modules/],
+              use: 'elm-webpack-loader'
+            },
             // "url" loader works like "file" loader except that it embeds assets
             // smaller than specified limit in bytes as data URLs to avoid requests.
             // A missing `test` is equivalent to a match.

This file should be added to your version control as you will need it to run your project. Now, in order to automate some things we add a postinstall script to our package.json:

#!/usr/bin/env javascript
/* package.json */
  /* ... */
  "scripts": {
    "start": "react-scripts start",
    "build": "react-scripts build",
    "test": "react-scripts test",
    "eject": "react-scripts eject",
    "postinstall": "patch-package"
  /* ... */

This will automatically call patch-package when we install a dependency. Of course we need to add the patch-package dependency at first to our project:

#!/usr/bin/env bash
yarn add --dev patch-package

As we are using yarn we need to use the postinstall-postinstall package because yarn only runs the postinstall hook after a yarn add or yarn install but not after a yarn remove.

#!/usr/bin/env bash
yarn add --dev postinstall-postinstall

Now, if you run yarn start you should still see the "Hello" from our Elm component. If you now remove the node_modules and reinstall them you should still be able to see it because patch-package takes care of patching our node_modules.

#!/usr/bin/env bash
rm -rf node_modules && \
yarn install && \
yarn start


That's all and I think that this is a good way to patch your node_modules if you really have to. Of course, if it would make sense for the upstream package to have this change it would be better way to fork and make a pull request and use your fork until the pull request is merged and released. But sometimes it doesn't make sense to have it upstream, like in this example, it wouldn't make sense for create-react-app to have an Elm loader configured.

You can see the full example on GitHub .


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