FlutterKaigi Day1「Dart のこれから」

Dart のこれから 
FlutterKaigi 2022 Day1 

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おかやまん / blendthink 
株式会社ゆめみ 
Android・Flutter テックリード 
将棋・プログラミング 

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Agenda 
01　近年、どのような進化を遂げてきたか  
02　Dart の進化プロセス  
03　仕様化を進めている新機能  
　　・Patterns and related features  
　　・Static Metaprogramming  
　　・Views on an object without a wrapper object  
　　・Sound declaration-site variance  
04　おわりに 

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bool isEmpty(String s) => s.length == 0;
void main() {
// Runtime error
isEmpty(null);
}
Dart 2.12 - Null safety 
bool isEmpty(String s) => s.length == 0;
void main() {
// Compile-time error
isEmpty(null);
}
https://github.com/dart-lang/sdk/blob/main/CHANGELOG.md#2120---2021-03-03  

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class A {
int x;
A(this.x);
A.fromString(String s) : x = int.parse(s);
}
void main() {
final listOfInts = [1, 2, 3];
final listOfStrings = ["1", "2", "3"];
for(final a in listOfInts.map((x) => A(x))) {
print(a.x);
}
for(final a in listOfStrings.map((x) => A.fromString(x))) {
print(a.x);
}
}
Dart 2.15 - Constructor tear-offs 
class A {
int x;
A(this.x);
}
void main() {
for(final a in listOfInts.map(A.new)) {
print(a.x);
}
for(final a in listOfStrings.map(A.fromString)) {
print(a.x);
}
} 

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class A {
int x;
A(this.x);
}
void main() {
for(final a in listOfInts.map((x) => A(x))) {
print(a.x);
}
for(final a in listOfStrings.map((x) => A.fromString(x))) {
print(a.x);
}
}
Dart 2.15 - Constructor tear-offs 
class A {
int x;
A(this.x);
}
void main() {
for(final a in listOfInts.map(A.new)) {
print(a.x);
}
for(final a in listOfStrings.map(A.fromString)) {
print(a.x);
}
}

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enum Status {
success,
failure
}
extension StatusToCode on Status {
int get code {
switch (this) {
case Status.success:
return 0;
case Status.failure:
return 1;
}
}
}
Dart 2.17 - Enhanced enums with members 
enum Status {
success(0),
failure(1);
final int code;
const Status(this.code);
}

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class CupertinoPage extends Page {
const CupertinoPage({
required this.child,
this.maintainState = true,
this.title,
this.fullscreenDialog = false,
LocalKey? key,
String? name,
Object? arguments,
String? restorationId,
}) : super(
key: key,
name: name,
arguments: arguments,
restorationId: restorationId,
);
// ...
}
Dart 2.17 - Super parameters 
class CupertinoPage extends Page {
const CupertinoPage({
required this.child,
this.maintainState = true,
this.title,
this.fullscreenDialog = false,
super.key,
super.name,
super.arguments,
super.restorationId,
});
// ...
}

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main() {
test('A test description', () {
// Very long function body here...
}, skip: true);
}
Dart 2.17 - Named args everywhere 
main() {
test(skip: true, 'A test description', () {
// Very long function body here...
});
}

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● Dart 2.18 
○ Enhanced type inference for generic invocations with function literals
 
● Dart 2.17 
○ Enhanced enums with members 
○ Super parameters 
○ Named args everywhere 
● Dart 2.15 
○ Constructor tear-offs 
○ Generic type literals 
○ Explicit generic method instantiations 
○ Generic instantiation of function objects 
● Dart 2.14 
○ Unsigned shift operator 
○ Removed some restrictions on type arguments 
● Dart 2.13 
○ Non-function type aliases 
● Dart 2.12 
○ Null safety 
https://github.com/dart-lang/sdk/blob/main/CHANGELOG.md  

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Agenda 

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Dart の言語チーム  ※ 2020 年 6 月時点  
leafpetersen   lrhn  eernstg 
munificent 
natebosch  jakemac53  stereotype441   mit-mit 
https://github.com/dart-lang/language/blob/master/README.md#dart-language-team  

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機能が追加されるまでの流れ 
 
https://github.com/dart-lang/language/blob/master/doc/life_of_a_language_feature.md  

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https://github.com/dart-lang/language/issues/2534  

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https://github.com/dart-lang/language/projects/1  

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※ 2022 年 10 月時点  

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https://github.com/dart-lang/language/blob/master/working/macros/feature-specification.md  

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https://github.com/dart-lang/language/blob/master/accepted/2.14/triple-shift-operator/implementation-plan.md  
https://github.com/dart-lang/language/blob/master/accepted/2.14/triple-shift-operator/feature-specification.md  

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Agenda 

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Patterns and related features 
03　仕様化を進めている新機能 - Patterns and related features  

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Patterns and related features 

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コンセプト 
● パターン 
● パターンマッチング 
● 網羅性 
● 分割代入 
● タプル 
● 代数的データ型 
● データクラス 

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Record 機能 - 提案の動機 
Future getPersonName() async => 'John';
Future getUnreadCount() async => 10;
void main() async {
// List
final result = await Future.wait([
getPersonName(),
getUnreadCount(),
]);
final personName = result[0] as String;
final unreadCount = result[1] as int;
final lowOrHigh = unreadCount < 10 ? 'low' : 'high';
// John's unread count is high.
print("$personName’s unread count is $lowOrHigh.");
}

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Record 機能 - 提案の動機 
Future getPersonName() async => 'John';
Future getUnreadCount() async => 10;
void main() async {
// List
final result = await Future.wait([
getPersonName(),
getUnreadCount(),
]);
final personName = result[0] as String;
final unreadCount = result[1] as int;
final lowOrHigh = unreadCount < 10 ? 'low' : 'high';
// John's unread count is high.
print("$personName's unread count is $lowOrHigh.");
}

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Record 機能 - 概要 
var tuple = ("first", 2, true);  var record = (number: 123, name: "Main", type: "Street");
var record = ("first", number: 123, 2, name: "Main", true, type: "Street");

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Record 機能 - 概要 
var record = (1, a: 2, 3, b: 4);
print(record.$0); // Prints "1".
print(record.a); // Prints "2".
print(record.$1); // Prints "3".
print(record.b); // Prints "4".

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Record 機能 - 注釈 
(int, String name, bool) triple;
({int n, String s}) pair;
(bool, num, {int n, String s}) quad;

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@metadata (a, b) function() {} 

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@metadata (a, b) function() {}

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Record 機能 - toString() 
// デバッグビルド
print((1, 2, 3).toString()); // "(1, 2, 3)".
print((a: 'str', 'int').toString()); // "(a: str, int)".

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Record 機能 - 等価性 
var a = (x: 1, 2);
var b = (2, x: 1);
print(a == b); // true.

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Record 機能 - with() 
void main() {
(int, int, {Color color}) x = (0, 1, color: Color.red);
var x2 = x.with(7); // Just changes x[0].
var x3 = x.with(_, 3); // Just changes x[1].
var x4 = x.with(1: 3); // Also just changes x[1].
var x5 = x.with(color: Color.Blue); // Just changes x.color.
}

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Pattern 機能 - 提案の動機 
void main() async {
try {
await fetchData();
} on SocketException {
handleException();
} on TimeoutException {
handleException();
}
}

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Pattern 機能 - 提案の動機 
void main() async {
try {
await fetchData();
} on SocketException, TimeoutException {
handleException();
}
}

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Pattern 機能 - 概要 
パターンに関する基本的な考え方 
● パターンが値と一致するかどうかを判断するために、ある値に対してテストすること
ができる 
● いくつかのパターンは、マッチした場合、マッチした値からデータを取り出すことに
よって、マッチした値を再構築する 
● 変数パターンは、マッチした値または非構造化された値に新しい変数をバインドす
る。変数は、パターンがマッチしたときにのみ到達可能なコード領域内にスコープさ
れる 

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Pattern 機能 - 概要 
種類例
論理和 subpattern1 | subpattern2
論理積 subpattern1 & subpattern2
関連 == expression, < expression
キャスト foo as String
null チェック subpattern?
null アサート subpattern!
定数 123, null, 'string', math.pi, SomeClass.constant, const Thing(1, 2), const (1 + 2)
変数 foo, var bar, String str, _, int _
括弧付き (subpattern)
List [subpattern1, subpattern2]
Map {"key": subpattern1, someConst: subpattern2}
Record (subpattern1, subpattern2), (x: subpattern1, y: subpattern2)
抽出 SomeClass(x: subpattern1, y: subpattern2)
https://github.com/dart-lang/language/blob/master/working/0546-patterns/patterns-feature-specification.md  

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Pattern 機能 - 論理和パターン 
var isPrimary = switch (color) {
case Color.red | Color.yellow | Color.blue => true;
default => false;
};

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switch (shape) {
case Square(size: var s) | Circle(size: var s) when s > 0:
print('Non-empty symmetric shape');
case Square() | Circle():
print('Empty symmetric shape');
default:
print('Asymmetric shape');
}

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switch (list) {
// Matches a two-element list whose first element is 'a' or 'b':
case ['a' | 'b', var c]:
}

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網羅性チェック - 概要 
● 網羅性 
 
 
● 到達可能性 
switch (b) {
case true: print('yes');
case false: print('no');
} 
switch (b) {
case true: print('yes');
case false: print('no');
case bool b: print('unreachable');
} 

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Static Metaprogramming 
03　仕様化を進めている新機能 - Static Metaprogramming  

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Static Metaprogramming 

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マクロ - 提案の動機 
data class User(val name: String, val age: Int) 

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@immutable
class User {
const User({
required this.name,
required this.age,
});
final String name;
final int age;
@override
bool operator ==(Object other) =>
other is User &&
other.runtimeType == runtimeType &&
other.name == name &&
other.age == age;
@override
int get hashCode => Object.hash(
runtimeType,
name,
age,
);
@override
String toString() => 'User('
'name=$name, '
'age=$age'
')';
User copy({
String? name,
int? age,
}) =>
User(
name: name ?? this.name,
age: age ?? this.age,
);
}

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import 'package:freezed_annotation/freezed_annotation.dart';
part 'user.freezed.dart';
@freezed
class User with _$User {
const factory User({
required String name,
required int age,
}) = _User;
}

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マクロ - 概要 
macro class DataClass
implements ClassDeclarationsMacro, ClassDefinitionMacro {
const DataClass();
@override
Future buildDeclarationsForClass(
ClassDeclaration clazz, ClassMemberDeclarationBuilder context) async {
await Future.wait([
const AutoConstructor().buildDeclarationsForClass(clazz, context),
const CopyWith().buildDeclarationsForClass(clazz, context),
const HashCode().buildDeclarationsForClass(clazz, context),
const Equality().buildDeclarationsForClass(clazz, context),
const ToString().buildDeclarationsForClass(clazz, context),
]);
}
@override
Future buildDefinitionForClass(
ClassDeclaration clazz, ClassDefinitionBuilder builder) async {
await Future.wait([
const HashCode().buildDefinitionForClass(clazz, builder),
const Equality().buildDefinitionForClass(clazz, builder),
const ToString().buildDefinitionForClass(clazz, builder),
]);
}
}

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https://github.com/dart-lang/sdk/tree/main/pkg/_fe_analyzer_shared/lib/src/macros  

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https://github.com/dart-lang/language/tree/master/working/macros/example  

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Views on an object without a wrapper object 
03　仕様化を進めている新機能 - Views on an object without a wrapper object  

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Views on an object without a wrapper object 

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View - 提案の動機 
● 拡張メソッドのように、ラッパーオブジェクトなしで、ゼロコストで特定のオブジェクト
を拡張したい 
● ラッピングすることで、不適切な操作を禁止したり、回帰的な処理をより便利かつ安
全に実装したりしたい 

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View - 概要 
view class IdNumber(int i) {
operator <(IdNumber other) => i < other.i;
bool verify(Some parameters) => ...;
}

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View - 概要 
view class IdNumber(int i) {
operator <(IdNumber other) => i < other.i;
bool verify(Some parameters) => ...;
}
void main() {
int myUnsafeId = 42424242;
var safeId = IdNumber(42424242);
safeId + 10; // Compile-time error
10 + safeId; // Compile-time error
myUnsafeId = safeId; // Compile-time error
}

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Sound declaration-site variance 
03　仕様化を進めている新機能 - Sound declaration-site variance  

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Sound declaration-site variance 

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提案の動機 
class Writer {
void write(T x) => print(x);
}
void main() {
Writer objectWriter = Writer();
// Runtime error
objectWriter.write("I'm a string!");
}

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概要 
class Writer {
void write(T x) => print(x);
}
void main() {
// Compile-time error
Writer objectWriter = Writer();
objectWriter.write("I'm a string!");
}

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概要 
https://github.com/dart-lang/language/pull/557  

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Agenda 

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Dart の進化を支えている 
言語チームメンバーに感謝！ 

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ご清聴ありがとうございました 

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FlutterKaigi Day1「Dart のこれから」

FlutterKaigi Day1「Dart のこれから」

おかやまん

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