Understanding Brackets in Dart

Question

I have many confusion about Brackets in Dart(Flutter). Which bracket "(), {}, []" is used for what?

Answer 1

Parentheses: ()

• Group expressions:

  var x = (1 + 2) * 3;
  

  or can designate parameter lists for functions:

  var getAnswer = () => 42;
  
  int square(int x) => x * x;
  

• Or can designate function calls (including calls to a .call method on callable classes):

  var answer = getAnswer();
  var squared = square(4);
  

• Or is part of the syntax to some keywords. This includes (but is not limited to) if, assert, for, while, switch, catch:

  if (condition) {
    ...
  }
  
  assert(condition);
  
  for (var item in collection) {
    ...
  }
  
  while (condition) {
    ...
  }
  
  switch (value) {
    ...
  }
  
  try {
    ...
  } on Exception catch (e) {
    ...
  }
  

• In Dart 3, specifies a record:

  var emptyRecord = ();
  var singleField = (123,);
  var namedFields = (123, foo: 456);
  

Square brackets: []

• By itself, creates List literals:

  var list = [1, 2, 3];
  var emptyList = []; // Creates a List<dynamic>.
  

• When used on an object, calls operator [] or operator []=, which usually accesses or sets an element of a collection respectively:

  var element = list[index];
  var value = map[key];
  
  list[index] = element;
  map[key] = value;
  

• In a parameter list, specifies optional positional parameters:

  int function(int x, [int y, int z]) {
    return x + y ?? 0 + z ?? 0;
  }
  
  function(1, 2);
  

• In dartdoc comments, creates linked references to other symbols:

  /// Creates a [Bar] from a [Foo].
  Bar fooToBar(Foo foo) {
    // ...
  }
  

Curly braces: {}

• Can create a block of code, grouping lines together and limiting variable scope. This includes (but is not limited to) function bodies, class declarations, if-else blocks, try-catch blocks, for blocks, while blocks, switch blocks, etc.:

  class Class {
    int member;
  }
  
  void doNothing() {}
  
  void function(bool condition) {
    {
      String x = 'Hello world!';
      print(x);
    }
  
    if (condition) {
      int x = 42; // No name collision due to separate scopes.
      print(x);
    }
  }
  

• By itself, can create Set or Map literals:

  var set = {1, 2, 3};
  var emptySet = <int>{};
  
  var map = {'one': 1, 'two': 2, 'three': 3};
  var emptyMap = {}; // Creates a Map<dynamic, dynamic>
  

• In a parameter list, specifies named parameters:

  int function(int x, {required int y, int? z}) {
    return x + y + z ?? 0;
  }
  
  function(1, y: 2);
  

• Creates enumerations:

  enum SomeEnumeration {
    foo,
    bar,
    baz,
  }
  

• In Strings can be used to disambiguate interpolated expressions:

  var foo = 'foo';
  var foobar = '${foo}bar';
  
  var s = '${function(1, 2)}';
  

• Or in Strings is used to specify Unicode code points with more than 4 hexadecimal digits:

  var bullseye = '\u{1F3AF}'; // 🎯
  

Angle brackets: <>

• When used as a pair in function or class declarations, creates generics:

  class GenericClass<T> {
    T member;
  }
  
  T function<T>(T x) {
    // ...
  }
  

• Or specifies explicit types when using generics:

  var map = <String, int>{};
  

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Note that the above does not include the meaning of (), [], {}, and <> when used in RegExp patterns. Regular expression syntax is its own language and isn't Dart-specific.