Higher Order Functions & Closures

One of Dart's most powerful features is treating functions as first-class citizens. This means you can pass functions as arguments, return them from other functions, and assign them to variables — just like you do with integers or strings.

---

1. Functions as First-Class Citizens

dart
void greet(String name) => print('Hello, $name!');

void main() {
  // Assign a function to a variable
  var myFunction = greet;
  myFunction('Rahul'); // Hello, Rahul!

  // Store functions in a list
  List<Function> actions = [
    () => print('Action 1'),
    () => print('Action 2'),
    () => print('Action 3'),
  ];

  actions.forEach((action) => action());
}

---

2. Higher Order Functions

A higher-order function is any function that:

• Takes one or more functions as parameters, OR
• Returns a function as its result

dart
// Takes a function as a parameter
void doTwice(Function action) {
  action();
  action();
}

// Returns a function
Function adder(int x) {
  return (int y) => x + y;
}

void main() {
  doTwice(() => print('Hello!')); 
  // Hello!
  // Hello!

  var add5 = adder(5);
  print(add5(3));  // 8
  print(add5(10)); // 15
}

---

3. Collection Higher-Order Functions

Dart's List, Set, and Iterable classes come with powerful built-in higher-order functions. These are the backbone of functional-style Dart programming.

Let's explore each one with a real-world example — a student grade system.

dart
void main() {
  List<int> grades = [85, 92, 56, 73, 90, 47, 68, 95];
  List<String> students = ['Amit', 'Priya', 'Rahul', 'Zara', 'Dev', 'Nisha', 'Karan', 'Meera'];
}

---

3.1 map() — Transform Each Element

map() applies a function to every element and returns a new Iterable.

dart
void main() {
  List<int> grades = [85, 92, 56, 73, 90];

  // Add 5 bonus marks to each student
  var bonusGrades = grades.map((grade) => grade + 5).toList();
  print(bonusGrades); // [90, 97, 61, 78, 95]

  // Convert grades to letter grades
  var letterGrades = grades.map((grade) {
    if (grade >= 90) return 'A';
    if (grade >= 75) return 'B';
    if (grade >= 60) return 'C';
    return 'F';
  }).toList();
  print(letterGrades); // [B, A, F, B, A]

  // String transformation example
  List<String> names = ['amit', 'priya', 'rahul'];
  var uppercased = names.map((name) => name.toUpperCase()).toList();
  print(uppercased); // [AMIT, PRIYA, RAHUL]
}

---

3.2 where() — Filter Elements (like filter in JavaScript)

where() returns only the elements that satisfy a condition.

dart
void main() {
  List<int> grades = [85, 92, 56, 73, 90, 47, 68, 95];

  // Students who passed (>= 60)
  var passed = grades.where((grade) => grade >= 60).toList();
  print(passed); // [85, 92, 73, 90, 68, 95]

  // Students who failed
  var failed = grades.where((grade) => grade < 60).toList();
  print(failed); // [56, 47]

  // Distinction holders (>= 90)
  var distinction = grades.where((grade) => grade >= 90).toList();
  print(distinction); // [92, 90, 95]
}

---

3.3 reduce() — Collapse to a Single Value

reduce() combines all elements using a function, starting from the first element. It does not accept an initial value.

dart
void main() {
  List<int> grades = [85, 92, 56, 73, 90];

  // Find total marks
  int total = grades.reduce((sum, grade) => sum + grade);
  print('Total: $total'); // Total: 396

  // Find maximum grade
  int highest = grades.reduce((max, grade) => grade > max ? grade : max);
  print('Highest: $highest'); // Highest: 92

  // Find minimum grade
  int lowest = grades.reduce((min, grade) => grade < min ? grade : min);
  print('Lowest: $lowest'); // Lowest: 56
}

---

3.4 fold() — Like reduce() with an Initial Value

fold() is like reduce() but you provide a starting/initial value. Safe on empty lists.

dart
void main() {
  List<int> grades = [85, 92, 56, 73, 90];

  // Calculate total with initial value of 0
  int total = grades.fold(0, (sum, grade) => sum + grade);
  print('Total: $total'); // Total: 396

  // Average calculation
  double average = grades.fold(0, (sum, grade) => sum + grade) / grades.length;
  print('Average: $average'); // Average: 79.2

  // Build a summary string using fold
  String summary = grades.fold('Grades: ', (str, grade) => '$str$grade ');
  print(summary); // Grades: 85 92 56 73 90

  // Works on empty list too
  List<int> empty = [];
  int result = empty.fold(0, (sum, x) => sum + x);
  print(result); // 0 (no crash!)
}

---

3.5 any() — Returns true if Any Element Matches

dart
void main() {
  List<int> grades = [85, 92, 56, 73, 90, 47];

  bool anyFailed = grades.any((grade) => grade < 60);
  print('Anyone failed? $anyFailed'); // Anyone failed? true

  bool anyDistinction = grades.any((grade) => grade >= 95);
  print('Anyone got distinction? $anyDistinction'); // Anyone got distinction? false

  List<String> fruits = ['apple', 'mango', 'banana'];
  bool hasMango = fruits.any((fruit) => fruit == 'mango');
  print('Has mango? $hasMango'); // Has mango? true
}

---

3.6 every() — Returns true if All Elements Match

dart
void main() {
  List<int> grades = [85, 92, 73, 90];

  bool allPassed = grades.every((grade) => grade >= 60);
  print('All passed? $allPassed'); // All passed? true

  bool allDistinction = grades.every((grade) => grade >= 90);
  print('All distinction? $allDistinction'); // All distinction? false

  List<int> evenNumbers = [2, 4, 6, 8, 10];
  bool allEven = evenNumbers.every((n) => n % 2 == 0);
  print('All even? $allEven'); // All even? true
}

---

3.7 forEach() — Iterate with Side Effects

forEach() runs a function on each element. It returns void — use it when you want to perform actions (print, update UI, etc.) rather than transform data.

dart
void main() {
  List<String> students = ['Amit', 'Priya', 'Rahul'];
  List<int> grades = [85, 92, 56];

  // Print each student's info
  for (int i = 0; i < students.length; i++) {
    print('${students[i]}: ${grades[i]}');
  }

  // Using forEach
  students.forEach((student) {
    print('Student: $student');
  });

  // Arrow style
  grades.forEach((grade) => print('Grade: $grade'));
}

---

3.8 toList() and toSet() — Convert Back to Collection

Most HOFs return an Iterable, not a List. Use toList() or toSet() to convert:

dart
void main() {
  List<int> numbers = [1, 2, 2, 3, 3, 4, 5];

  // map/where return Iterable — call .toList()
  List<int> doubled = numbers.map((n) => n * 2).toList();
  print(doubled); // [2, 4, 4, 6, 6, 8, 10]

  // .toSet() removes duplicates
  Set<int> uniqueDoubled = numbers.map((n) => n * 2).toSet();
  print(uniqueDoubled); // {2, 4, 6, 8, 10}

  // Filter then convert
  List<int> evens = numbers.where((n) => n % 2 == 0).toList();
  print(evens); // [2, 2, 4]
}

---

4. Method Chaining

The real power comes from chaining multiple HOFs together to build expressive data pipelines:

dart
void main() {
  List<int> grades = [85, 92, 56, 73, 90, 47, 68, 95];

  // Find the average of passing grades only
  var passingGrades = grades
      .where((g) => g >= 60)      // filter: keep passing grades
      .map((g) => g + 2)          // transform: add 2 grace marks
      .toList();

  double average = passingGrades.fold(0, (sum, g) => sum + g) / passingGrades.length;
  print('Average of passing grades (with grace): $average');
  // Average of passing grades (with grace): 85.83...

  // Count failed students
  int failCount = grades.where((g) => g < 60).length;
  print('Failed count: $failCount'); // Failed count: 2

  // Get top 3 scorers
  List<int> sorted = [...grades]..sort((a, b) => b.compareTo(a));
  List<int> top3 = sorted.take(3).toList();
  print('Top 3: $top3'); // Top 3: [95, 92, 90]
}

---

5. Real Project Example — Product Filtering & Price Calculation

dart
class Product {
  final String name;
  final String category;
  final double price;
  final bool inStock;

  Product(this.name, this.category, this.price, this.inStock);
}

void main() {
  List<Product> products = [
    Product('Laptop', 'Electronics', 75000, true),
    Product('Phone', 'Electronics', 25000, false),
    Product('Shirt', 'Clothing', 999, true),
    Product('Headphones', 'Electronics', 5000, true),
    Product('Jeans', 'Clothing', 1999, true),
    Product('Tablet', 'Electronics', 35000, false),
  ];

  // 1. Get all in-stock electronics
  var availableElectronics = products
      .where((p) => p.category == 'Electronics' && p.inStock)
      .toList();

  print('Available Electronics:');
  availableElectronics.forEach((p) => print('  ${p.name}: Rs.${p.price}'));

  // 2. Total price of in-stock items
  double total = products
      .where((p) => p.inStock)
      .fold(0.0, (sum, p) => sum + p.price);
  print('\nTotal in-stock value: Rs.$total');

  // 3. Get product names sorted by price
  var sortedNames = products
      .where((p) => p.inStock)
      .toList()
    ..sort((a, b) => a.price.compareTo(b.price));

  print('\nProducts by price (low to high):');
  sortedNames.forEach((p) => print('  ${p.name}: Rs.${p.price}'));

  // 4. Check if any item is out of stock
  bool anyOutOfStock = products.any((p) => !p.inStock);
  print('\nAny out of stock? $anyOutOfStock'); // true

  // 5. All clothing available?
  bool allClothingAvailable = products
      .where((p) => p.category == 'Clothing')
      .every((p) => p.inStock);
  print('All clothing in stock? $allClothingAvailable'); // true
}

---

Summary

Function	Purpose	Returns
map()	Transform each element	Iterable
where()	Filter elements by condition	Iterable
reduce()	Collapse to one value (no initial)	Single value
fold()	Collapse with initial value	Single value
any()	True if at least one matches	bool
every()	True if all elements match	bool
forEach()	Side effects on each element	void
toList()	Convert Iterable to List	List
toSet()	Convert Iterable to Set (unique)	Set

Higher-order functions are at the heart of modern Dart and Flutter development. They let you write declarative, readable, and chainable data processing code — replacing complex loops with expressive one-liners.