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Add 3.0 language changes to CHANGELOG.
Change-Id: Ib5dcc5510b520bc648243801b4dbb435aaf44df2 Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/294128 Reviewed-by: Marya Belanger <mbelanger@google.com> Auto-Submit: Bob Nystrom <rnystrom@google.com> Commit-Queue: Bob Nystrom <rnystrom@google.com>
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CHANGELOG.md
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CHANGELOG.md
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@ -10,17 +10,145 @@
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### Language
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- **Breaking Change** [#50902][]: Report a compile-time error for a `continue` statement
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having an invalid target.
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Dart 3.0 adds the following features. To use them, set your package's [SDK
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constraint][language version] lower bound to 3.0 or greater (`sdk: '^3.0.0'`).
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As per 18.15 of the language spec, it is a compile-time error for a `continue`
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label to have a target which is not a loop (`for`, `do` and `while` statements)
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or a `switch` member.
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[language version]: https://dart.dev/guides/language/evolution
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Breakage is mitigated by changing the code to have the `continue` label properly
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target a valid labeled statement.
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- **[Records]**: Records are anonymous immutable data structures that let you
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aggregate multiple values together, similar to [tuples][] in other languages.
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With records, you can return multiple values from a function, create composite
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map keys, or use them any other place where you want to bundle a couple of
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objects together.
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For example, using a record to return two values:
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```dart
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(double x, double y) geoLocation(String name) {
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if (name == 'Nairobi') {
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return (-1.2921, 36.8219);
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} else {
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...
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}
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}
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```
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- **[Pattern matching]**: Expressions build values out of smaller pieces.
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Conversely, patterns are an expressive tool for decomposing values back into
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their constituent parts. Patterns can call getters on an object, access
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elements from a list, pull fields out of a record, etc. For example, we can
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destructure the record from the previous example like so:
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```dart
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var (lat, long) = geoLocation('Nairobi');
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print('Nairobi is at $lat, $long.');
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```
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Patterns can also be used in [switch cases]. There, you can destructure values
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and also test them to see if they have a certain type or value:
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```dart
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switch (object) {
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case [int a]:
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print('A list with a single integer element $a');
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case ('name', _):
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print('A two-element record whose first field is "name".');
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default: print('Some other object.');
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}
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```
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Also, as you can see, non-empty switch cases no longer need `break;`
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statements.
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**Breaking change**: Dart 3.0 interprets [switch cases] as patterns instead of
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constant expressions. Most constant expressions found in switch cases are
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valid patterns with the same meaning (named constants, literals, etc.). You
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may need to tweak a few constant expressions to make them valid. This only
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affects libraries that have upgraded to language version 3.0.
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- **[Switch expressions]**: Switch expressions allow you to use patterns and
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multi-way branching in contexts where a statement isn't allowed:
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```dart
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return TextButton(
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onPressed: _goPrevious,
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child: Text(switch (page) {
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0 => 'Exit story',
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1 => 'First page',
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_ when page == _lastPage => 'Start over',
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_ => 'Previous page',
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}),
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);
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```
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- **[If-case statements and elements]**: A new if construct that matches a value
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against a pattern and executes the then or else branch depending on whether
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the pattern matches:
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```dart
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if (json case ['user', var name]) {
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print('Got user message for user $name.');
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}
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```
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There is also a corresponding [if-case element] that can be used in collection
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literals.
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- **[Sealed classes]**: When you mark a type `sealed`, the compiler ensures that
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switches on values of that type [exhaustively cover] every subtype. This
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enables you to program in an [algebraic datatype][] style with the
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compile-time safety you expect:
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```dart
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sealed class Amigo {}
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class Lucky extends Amigo {}
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class Dusty extends Amigo {}
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class Ned extends Amigo {}
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String lastName(Amigo amigo) =>
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switch (amigo) {
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case Lucky _ => 'Day';
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case Ned _ => 'Nederlander';
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}
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```
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In this last example, the compiler reports an error that the switch doesn't
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cover the subclass `Dusty`.
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- **[Class modifiers]**: New modifiers `final`, `interface`, `base`, and `mixin`
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on `class` and `mixin` declarations let you control how the type can be used.
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By default, Dart is flexible in that a single class declaration can be used as
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an interface, a superclass, or even a mixin. This flexibility can make it
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harder to evolve an API over time without breaking users. We mostly keep the
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current flexible defaults, but these new modifiers give you finer-grained
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control over how the type can be used.
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**Breaking change:** Class declarations from libraries that have been upgraded
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to Dart 3.0 can no longer be used as mixins by default. If you want the class
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to be usable as both a class and a mixin, mark it [`mixin class`][mixin
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class]. If you want it to be used only as a mixin, make it a `mixin`
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declaration. If you haven't upgraded a class to Dart 3.0, you can still use it
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as a mixin.
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- **Breaking Change** [#50902][]: Dart reports a compile-time error if a
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`continue` statement targets a [label] that is not a loop (`for`, `do` and
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`while` statements) or a `switch` member. Fix this by changing the `continue`
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to target a valid labeled statement.
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[records]: https://dart.dev/language/records
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[tuples]: https://en.wikipedia.org/wiki/Tuple
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[pattern matching]: https://dart.dev/language/patterns
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[switch cases]: https://dart.dev/language/branches#switch
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[switch expressions]: https://dart.dev/language/branches#switch-expressions
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[if-case statements and elements]: https://dart.dev/language/branches#if-case
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[if-case element]: https://dart.dev/language/collections#control-flow-operators
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[sealed classes]: https://dart.dev/language/class-modifiers#sealed
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[exhaustively cover]: https://dart.dev/language/branches#exhaustiveness-checking
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[algebraic datatype]: https://en.wikipedia.org/wiki/Algebraic_data_type
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[class modifiers]: https://dart.dev/language/class-modifiers
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[mixin class]: https://dart.dev/language/mixins#class-mixin-or-mixin-class
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[#50902]: https://github.com/dart-lang/sdk/issues/50902
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[label]: https://dart.dev/language/branches#switch
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### Libraries
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