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Adapt implicit cast example for type system page #6478

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Adapt implicit cast example for type system page
parlough Mar 7, 2025
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Highlight relevant line and add highlighted-line style
parlough Mar 7, 2025
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Merge branch 'main' into feat/type-system-implicit-casts
parlough Mar 7, 2025
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20 changes: 17 additions & 3 deletions examples/type_system/lib/strong_analysis.dart
View file
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Original file line number Diff line number Diff line change
Expand Up @@ -38,18 +38,32 @@ void _miscDeclAnalyzedButNotTested() {
}
}

{
// #docregion downcast-check
void assumeStrings(dynamic objects) {
// ignore: stable, beta, dev, invalid_assignment
List<String> strings = objects; // Runtime downcast check.
String string = strings[0];
}
// #enddocregion downcast-check

// #docregion fail-downcast-check
assumeStrings(<int>[1, 2, 3]);
// #enddocregion fail-downcast-check
}

{
// #docregion type-inference-1-orig
Map<String, dynamic> arguments = {'argA': 'hello', 'argB': 42};
Map<String, Object?> arguments = {'argA': 'hello', 'argB': 42};
// #enddocregion type-inference-1-orig

// ignore: stable, beta, dev, argument_type_not_assignable
arguments[1] = null;

// #docregion type-inference-2-orig
Map<String, dynamic> message = {
Map<String, Object?> message = {
'method': 'someMethod',
'args': <Map<String, dynamic>>[arguments],
'args': <Map<String, Object?>>[arguments],
};
// #enddocregion type-inference-2-orig
}
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11 changes: 11 additions & 0 deletions src/_sass/components/_code.scss
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Original file line number Diff line number Diff line change
Expand Up @@ -50,9 +50,14 @@ pre {
padding-left: 1.25rem;
padding-right: 1.25rem;
width: 100%;
display: inline-block;
min-width: 100%;
border-left: 2px solid rgba(0, 0, 0, 0);

&.highlighted-line {
background: color.change($brand-primary, $alpha: 0.1);
min-width: 100%;
border-left-color: $brand-primary;
}
}

Expand Down Expand Up @@ -199,6 +204,12 @@ pre {
&:not([lang="console"]) {
line-height: 1.8;
}

code {
display: block;
min-width: fit-content;
width: 100%;
}
}
}

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41 changes: 39 additions & 2 deletions src/content/language/type-system.md
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Expand Up @@ -277,6 +277,43 @@ void main() {
}
```

### Implicit casts

Expressions with a static type of `dynamic` can be
implicitly cast to a more specific type.
If the actual type doesn't match, the cast throws an error at runtime.
Consider the following `assumeStrings` method:

<?code-excerpt "lib/strong_analysis.dart (downcast-check)" replace="/strings = objects/[!$&!]/g"?>
```dart tag=passes-sa
void assumeStrings(dynamic objects) {
List<String> [!strings = objects!]; // Runtime downcast check.
String string = strings[0];
}
```
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I'd recommend using a simpler example: The downcast in the current example takes two steps, but we would already have a perfectly useful example if it is adjusted to take just one step.

The first step in the current example is concerned with the class: Starting from dynamic, it is tested whether the given object is a list. We could say that we're checking for the type List, which is just an abbreviated notation for List<dynamic>. Next, when we know that it is a list, we check for the type argument String (which is a cast from List<dynamic> to List<String>). This second step is based on a subtype relation known as covariance.

I think the topic of the example should just be "implicit downcasts from dynamic", and there's no need to include anything that requires the reader to think about a two-step subtype relationship that also involves variance.

Here's one possible way to do this:

int assumeString(dynamic object) {
  String string = object; // Check at run time that it is a `String`.
  return string.length;
}

I'd suggest using 'run time' rather than 'runtime', because the latter commonly refers to the support system which is running along with a program (providing external functions like print, services like garbage collection, etc.) In contrast, 'run time' is a noun phrase that denotes a specific kind of point in time, namely: One that occurs during an execution of the program. If you want to use the corresponding adjective then it would be 'run-time', as in 'a run-time error occurs if it is not a string'.


In this example, if `objects` is a `List<String>`, the cast succeeds.
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Suggested change
In this example, if `objects` is a `List<String>`, the cast succeeds.
In this example, if `object` is a `String`, the cast succeeds.

If it's not a subtype of `List<String>`, such as `List<int>`,
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Suggested change
If it's not a subtype of `List<String>`, such as `List<int>`,
If it's not a subtype of `String`, such as `int`,

a `TypeError` is thrown:

<?code-excerpt "lib/strong_analysis.dart (fail-downcast-check)" replace="/\[.*\]/[!$&!]/g"?>
```dart tag=runtime-fail
assumeStrings(<int>[![1, 2, 3]!]);
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The notation [!1!] should be a highlighted version of 1 (I'm not 100% sure about the syntax):

Suggested change
assumeStrings(<int>[![1, 2, 3]!]);
final length = assumeString([!1!]);

I introduced a local variable to store the returned result. That's still a bit artificial because it isn't used anywhere, but I thought it was one step more natural than just having a local variable inside assumeStrings and not returning anything.

```

:::tip
To prevent implicit downcasts from `dynamic` and avoid this issue,
consider enabling the analyzer's _strict casts_ mode.

```yaml title="analysis_options.yaml" highlightLines=3
analyzer:
language:
strict-casts: true
```

To learn more about customizing the analyzer's behavior,
check out [Customizing static analysis](/tools/analysis).
:::

## Type inference

Expand All @@ -291,9 +328,9 @@ pairs string keys with values of various types.

If you explicitly type the variable, you might write this:

<?code-excerpt "lib/strong_analysis.dart (type-inference-1-orig)" replace="/Map<String, dynamic\x3E/[!$&!]/g"?>
<?code-excerpt "lib/strong_analysis.dart (type-inference-1-orig)" replace="/Map<String, Object\?\x3E/[!$&!]/g"?>
```dart
[!Map<String, dynamic>!] arguments = {'argA': 'hello', 'argB': 42};
[!Map<String, Object?>!] arguments = {'argA': 'hello', 'argB': 42};
```

Alternatively, you can use `var` or `final` and let Dart infer the type:
Expand Down