Anonymous Functions and Closures

In Dart, functions are values: you can store them in variables, pass them to other functions, and return them. An anonymous function is a function without a name, written inline.

Real-world analogy: an anonymous function is an instruction slip you hand to someone along with a job — "for each parcel, check the weight like this". They decide when to follow it.

Syntax

void main() {
  final double = (int n) => n * 2;
  final add = (int a, int b) {
    return a + b;
  };

  print(double(21));
  print(add(2, 3));
}

42
5

Passing functions to functions

A function that takes or returns another function is a higher-order function. The parameter's type describes the function it expects:

void repeatTask(int times, void Function(int pass) task) {
  for (var i = 1; i <= times; i++) {
    task(i);
  }
}

void main() {
  repeatTask(3, (pass) {
    print('Pass $pass');
  });

  final names = ['Linus', 'ada', 'Grace'];
  names.sort((a, b) => a.toLowerCase().compareTo(b.toLowerCase()));
  print(names);
}

Pass 1
Pass 2
Pass 3
[ada, Grace, Linus]

void Function(int pass) is a function type: "takes an int, returns nothing". Flutter uses this everywhere — onPressed: () { ... } passes a function to a button.

Tear-offs

You can pass an existing function by name, without calling it — a tear-off:

bool isLong(String word) => word.length > 4;

void main() {
  final words = ['fig', 'banana', 'kiwi', 'cherry'];
  print(words.where(isLong).toList());
}

[banana, cherry]

Closures

A function remembers the variables around it when it was created — it's a closure:

int Function() makeCounter() {
  var count = 0;
  return () => ++count;
}

void main() {
  final next = makeCounter();
  next();
  next();
  print(next());
}

3

count would normally disappear when makeCounter returns, but the closure keeps it alive.

Practice

Sort ['banana', 'fig', 'apple'] by word length, shortest first.

void main() {
  final words = ['banana', 'fig', 'apple'];
  words.sort((a, b) => a.length.compareTo(b.length));
  print(words);
}

[fig, apple, banana]