Auto install elm and create elm.json if needed

src/assets/ElmAsset.js

@@ -1,7 +1,10 @@
 const process = require('process');
 const Asset = require('../Asset');
+const commandExists = require('command-exists');
 const localRequire = require('../utils/localRequire');
 const {minify} = require('terser');
+const path = require('path');
+const spawn = require('cross-spawn');
 
 class ElmAsset extends Asset {
   constructor(name, options) {
@@ -10,34 +13,65 @@ class ElmAsset extends Asset {
     this.hmrPageReload = true;
   }
 
-  async collectDependencies() {
-    await this.getConfig(['elm.json'], {load: false});
-    const dependencies = await this.elm.findAllDependencies(this.name);
+  async parse() {
+    let options = {
+      cwd: path.dirname(this.name)
+    };
 
-    dependencies.forEach(dependency => {
-      this.addDependency(dependency, {includedInParent: true});
-    });
-  }
+    // If elm is not installed globally, install it locally.
+    try {
+      await commandExists('elm');
+    } catch (err) {
+      await localRequire('elm', this.name);
+      options.pathToElm = path.join(
+        path.dirname(require.resolve('elm')),
+        'bin',
+        'elm'
+      );
+    }
 
-  async parse() {
     this.elm = await localRequire('node-elm-compiler', this.name);
 
-    const options = {
-      cwd: process.cwd()
-    };
+    // Ensure that an elm.json file exists, and initialize one if not.
+    let elmConfig = await this.getConfig(['elm.json'], {load: false});
+    if (!elmConfig) {
+      await this.createElmConfig(options);
 
-    if (process.env.NODE_ENV === 'test') {
-      options.cwd = this.options.rootDir;
+      // Ensure we are watching elm.json for changes
+      await this.getConfig(['elm.json'], {load: false});
     }
 
     if (this.options.minify) {
       options.optimize = true;
     }
 
-    const compiled = await this.elm.compileToString(this.name, options);
+    let compiled = await this.elm.compileToString(this.name, options);
     this.contents = compiled.toString();
   }
 
+  async collectDependencies() {
+    let dependencies = await this.elm.findAllDependencies(this.name);
+    for (let dependency of dependencies) {
+      this.addDependency(dependency, {includedInParent: true});
+    }
+  }
+
+  async createElmConfig(options) {
+    let cp = spawn(options.pathToElm || 'elm', ['init']);
+    cp.stdin.write('y\n');
+
+    return new Promise((resolve, reject) => {
+      cp.on('error', reject);
+      cp.on('close', function(code) {
+        if (code !== 0) {
+          return reject(new Error('elm init failed.'));
+        }
+
+        return resolve();
+      });
+    });
+  }
+
   async generate() {
     let output = this.contents;
 
@@ -53,7 +87,7 @@ class ElmAsset extends Asset {
     // Based on:
     // - http://elm-lang.org/0.19.0/optimize
     function pack(source) {
-      const options = {
+      let options = {
         compress: {
           keep_fargs: false,
           passes: 2,
@@ -83,7 +117,7 @@ class ElmAsset extends Asset {
         rename: false
       };
 
-      const result = minify(source, options);
+      let result = minify(source, options);
 
       if (result.error) {
         throw result.error;