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Include import prefixes on type error messages - DDC #33695

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srawlins opened this issue Jun 28, 2018 · 0 comments
Open

Include import prefixes on type error messages - DDC #33695

srawlins opened this issue Jun 28, 2018 · 0 comments
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area-web-js Issues related to JavaScript support for Dart Web, including DDC, dart2js, and JS interop. web-dev-compiler

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@srawlins
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Similar to #33667 (CFE) and #33693 (VM).

Reproduction for DDC is simple. It's like 3 steps.

  1. First, create a new directory, for a package. You'll need a whole package.

  2. Next create a pubspec.yaml with lots of goodies:

    name: examples.hello_world

environment:
  sdk: ">=2.0.0-dev.60.0 <2.0.0"

dev_dependencies:
  build_runner: ^0.9.0
  build_test: ^0.10.2+5
  build_web_compilers: ^0.4.0+4

  3. Of course, pub get that jazz.

  4. Make a web app in two easy steps. So easy.

    1. Write this into web/index.html:
    <!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Hello World</title>
  <link rel="icon" type="image/png" href="favicon.png">
  <script defer src="main.dart.js"></script>
</head>
<body>
</body>
</html>
    1. Write this into web/main.dart:
    import 'dart:math' as math;

// Poorly named wrapper for math.Random
class Random {
  math.Random inner;

  Random(this.inner);
}

void main() {
  List<Random> list = new List<Random>()..add(new Random(new math.Random(7)));
  addRandom(list.map((e) => e.inner));
}

addRandom(List<math.Random> list) => list..add(new math.Random());

  5. Install webdev: pub global active webdev.

  6. Run webdev serve.

  7. We're almost there, home stretch.

  8. Load up http://localhost:8080 in a browser.

  9. Look at the JavaScript console. Click the expand buttons until you see:

    "Type 'MappedListIterable<Random, Random>' is not a subtype of expected type 'List'."

OK, the issue is that that error message is a great clue if we're game for a puzzle. But for the more casual developer, I think we should show the prefix that they wanted to use:

"Type 'MappedListIterable<Random, math.Random>' is not a subtype of expected type 'List<math.Random>'."

This case I think illuminates how this would be useful: the Randoms are a red herring. The issue is that Iterable<blah-blah> is not List<blah-blah>. (I'll open a separate issue for the mysterious MappedListIterable showing up in errors, a class that I neither wrote, nor can find in SDK docs 😮 .)
@srawlins srawlins mentioned this issue Jun 28, 2018
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@vsmenon vsmenon added the area-web-js Issues related to JavaScript support for Dart Web, including DDC, dart2js, and JS interop. label Jul 20, 2019
@FMorschel FMorschel mentioned this issue Feb 11, 2025
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