Pattern Matching in Dart

What is Pattern Matching?

Pattern matching (Dart 3.0+) lets you match a value against a shape or structure and extract parts of it in one expression. Dart patterns work in switch statements and expressions, variable declarations, and for-loop variables.

Switch Expressions

// Switch expression (returns a value)
String describe(Object value) => switch (value) {
  int n when n < 0  => 'negative int',
  int n when n == 0 => 'zero',
  int _             => 'positive int',
  double _          => 'a double',
  String s          => 'a string: "$s"',
  List l            => 'a list of $${l.length} items',
  _                 => 'unknown',
};

void main() {
  print(describe(-5));       // negative int
  print(describe(0));        // zero
  print(describe(42));       // positive int
  print(describe(3.14));     // a double
  print(describe('hello')); // a string: "hello"
  print(describe([1,2,3])); // a list of 3 items
}

Record Patterns

String describePoint((int, int) point) => switch (point) {
  (0, 0) => 'Origin',
  (int x, 0) => 'On x-axis at $x',
  (0, int y) => 'On y-axis at $y',
  (int x, int y) when x == y => 'On diagonal at ($x, $y)',
  (int x, int y) => 'Point at ($x, $y)',
};

void main() {
  print(describePoint((0, 0)));    // Origin
  print(describePoint((5, 0)));    // On x-axis at 5
  print(describePoint((3, 3)));    // On diagonal at (3, 3)
  print(describePoint((2, 7)));    // Point at (2, 7)
}

List Patterns

String describeList(List<int> list) => switch (list) {
  []       => 'empty',
  [var x]  => 'single: $x',
  [var x, var y] => 'pair: $x, $y',
  [var first, ..., var last] => 'starts $first, ends $last',
};

print(describeList([]));         // empty
print(describeList([42]));       // single: 42
print(describeList([1, 2]));     // pair: 1, 2
print(describeList([1,2,3,4,5]));// starts 1, ends 5

Guard Clauses

// 'when' adds a boolean guard condition
String classify(int n) => switch (n) {
  0                 => 'zero',
  int x when x < 0 => 'negative',
  int x when x % 2 == 0 => 'positive even',
  _                 => 'positive odd',
};

Practical Example

sealed class Shape {}
class Circle extends Shape { final double radius; Circle(this.radius); }
class Rectangle extends Shape { final double w, h; Rectangle(this.w, this.h); }
class Triangle extends Shape { final double b, height; Triangle(this.b, this.height); }

double area(Shape shape) => switch (shape) {
  Circle(radius: var r)         => 3.14159 * r * r,
  Rectangle(w: var w, h: var h) => w * h,
  Triangle(b: var b, height: var h) => 0.5 * b * h,
};

void main() {
  print(area(Circle(5)).toStringAsFixed(2));         // 78.54
  print(area(Rectangle(4, 6)).toStringAsFixed(2));   // 24.00
  print(area(Triangle(3, 8)).toStringAsFixed(2));    // 12.00
}

🧠 Quiz

Summary

Dart 3.0 pattern matching enables switch expressions that return values, record/list patterns for structural matching, and when guards for conditional cases. Use sealed classes with pattern matching for exhaustive type checking. Patterns can appear in switch, variable declarations, and for loops.