Defer throwing asset errors until after dependencies are handled.

packages/core/integration-tests/test/hmr.js

@@ -529,4 +529,48 @@ describe('hmr', function() {
 
     await buildEnd;
   });
+
+  it('should watch new dependencies that cause errors', async function() {
+    await ncp(
+      path.join(__dirname, '/integration/elm'),
+      path.join(__dirname, '/input')
+    );
+
+    b = bundler(path.join(__dirname, '/input/index.js'), {
+      watch: true,
+      hmr: true
+    });
+    await b.bundle();
+
+    ws = new WebSocket('ws://localhost:' + b.options.hmrPort);
+
+    const buildEnd = nextEvent(b, 'buildEnd');
+
+    await sleep(100);
+    ncp(
+      path.join(__dirname, '/input/src/MainWithBrokenDep.elm'),
+      path.join(__dirname, '/input/src/Main.elm')
+    );
+
+    let msg = JSON.parse(await nextEvent(ws, 'message'));
+    assert.equal(msg.type, 'error');
+
+    await sleep(100);
+    fs.writeFile(
+      path.join(__dirname, '/input/src/BrokenDep.elm'),
+      `
+module BrokenDep exposing (anError)
+
+
+anError : String
+anError =
+    "2"
+      `
+    );
+
+    msg = JSON.parse(await nextEvent(ws, 'message'));
+    assert.equal(msg.type, 'error-resolved');
+
+    await buildEnd;
+  });
 });

packages/core/integration-tests/test/integration/elm/src/BrokenDep.elm

@@ -0,0 +1,8 @@
+module BrokenDep exposing (anError)
+
+{- This module causes a compiler error -}
+
+
+anError : String
+anError =
+    2

packages/core/integration-tests/test/integration/elm/src/MainWithBrokenDep.elm

@@ -0,0 +1,48 @@
+module Main exposing (main)
+
+import BrokenDep
+import Browser
+import Html exposing (Html, button, div, text)
+import Html.Events exposing (onClick)
+
+
+type alias Model =
+    { count : Int }
+
+
+initialModel : Model
+initialModel =
+    { count = 0 }
+
+
+type Msg
+    = Increment
+    | Decrement
+
+
+update : Msg -> Model -> Model
+update msg model =
+    case msg of
+        Increment ->
+            { model | count = model.count + 1 }
+
+        Decrement ->
+            { model | count = model.count - 1 }
+
+
+view : Model -> Html Msg
+view model =
+    div []
+        [ button [ onClick Increment ] [ text "+1" ]
+        , div [] [ text <| String.fromInt model.count ]
+        , button [ onClick Decrement ] [ text "-1" ]
+        ]
+
+
+main : Program () Model Msg
+main =
+    Browser.sandbox
+        { init = initialModel
+        , view = view
+        , update = update
+        }

packages/core/parcel-bundler/src/Bundler.js

@@ -593,6 +593,13 @@ class Bundler extends EventEmitter {
       })
     );
 
+    // If there was a processing error, re-throw now that we've set up
+    // depdenency watchers. This keeps reloading working if there is an
+    // error in a dependency not directly handled by Parcel.
+    if (processed.error !== null) {
+      throw processed.error;
+    }
+
     // Store resolved assets in their original order
     dependencies.forEach((dep, i) => {
       asset.dependencies.set(dep.name, dep);

packages/core/parcel-bundler/src/Pipeline.js

@@ -17,16 +17,23 @@ class Pipeline {
     }
 
     let asset = this.parser.getAsset(path, options);
-    let generated = await this.processAsset(asset);
+    let error = null;
     let generatedMap = {};
-    for (let rendition of generated) {
-      generatedMap[rendition.type] = rendition.value;
+    try {
+      let generated = await this.processAsset(asset);
+      for (let rendition of generated) {
+        generatedMap[rendition.type] = rendition.value;
+      }
+    } catch (err) {
+      error = err;
+      error.fileName = path;
     }
 
     return {
       id: asset.id,
       dependencies: Array.from(asset.dependencies.values()),
       generated: generatedMap,
+      error: error,
       hash: asset.hash,
       cacheData: asset.cacheData
     };

packages/core/parcel-bundler/src/assets/ElmAsset.js

@@ -44,12 +44,7 @@ class ElmAsset extends Asset {
       options.optimize = true;
     }
 
-    let compiled = await this.elm.compileToString(this.name, options);
-    this.contents = compiled.toString();
-    if (this.options.hmr) {
-      let {inject} = await localRequire('elm-hot', this.name);
-      this.contents = inject(this.contents);
-    }
+    this.elmOpts = options;
   }
 
   async collectDependencies() {
@@ -76,6 +71,13 @@ class ElmAsset extends Asset {
   }
 
   async generate() {
+    let compiled = await this.elm.compileToString(this.name, this.elmOpts);
+    this.contents = compiled.toString();
+    if (this.options.hmr) {
+      let {inject} = await localRequire('elm-hot', this.name);
+      this.contents = inject(this.contents);
+    }
+
     let output = this.contents;
 
     if (this.options.minify) {
@@ -129,6 +131,19 @@ class ElmAsset extends Asset {
       return result.code;
     }
   }
+
+  generateErrorMessage(err) {
+    // For some reason, if not converted to a plain object,
+    // the error message is lost somewhere between Pipeline.js
+    // and Bundler.js.
+
+    // The stack is not particularly useful, but other code may
+    // expect it and try to print it.
+    return {
+      message: err.message,
+      stack: ''
+    };
+  }
 }
 
 module.exports = ElmAsset;