Implementing Dart Patterns

Contents

• Pattern Selection Strategy
• Switch Statements vs. Expressions
• Core Pattern Implementations
• Workflows
• Examples

Pattern Selection Strategy

Apply specific pattern types based on the data structure and desired outcome. Follow these conditional guidelines:

• If validating and extracting from deserialized data (e.g., JSON): Use Map and List patterns to simultaneously check structure and destructure key-value pairs.
• If handling multiple return values: Use Record patterns to destructure fields directly into local variables.
• If executing type-specific behavior (Algebraic Data Types): Use Object patterns combined with sealed classes to ensure exhaustiveness.
• If matching numeric ranges or conditions: Use Relational (>=, <=) and Logical-and (&&) patterns.
• If multiple cases share logic: Use Logical-or (||) patterns to share a single case body or guard clause.
• If ignoring specific values: Use the Wildcard pattern (_) or a non-matching Rest element (...) in collections.

Switch Statements vs. Expressions

Select the appropriate switch construct based on the execution context:

• If producing a value: Use a switch expression.
  • Syntax: switch (value) { pattern => expression, }
  • Rule: Each case must be a single expression. No implicit fallthrough. Must be exhaustive.
• If executing statements or side effects: Use a switch statement.
  • Syntax: switch (value) { case pattern: statements; }
  • Rule: Empty cases fall through to the next case. Non-empty cases implicitly break (no break keyword required).

Core Pattern Implementations

Implement patterns using the following syntax and rules:

• Logical-or (||): pattern1 || pattern2. Both branches must define the exact same set of variables.
• Logical-and (&&): pattern1 && pattern2. Branches must not define overlapping variables.
• Relational: ==, !=, <, >, <=, >= followed by a constant expression.
• Cast (as): pattern as Type. Throws if the value does not match the type. Use to forcibly assert types during destructuring.
• Null-check (?): pattern?. Fails the match if the value is null. Binds the variable to the non-nullable base type.
• Null-assert (!): pattern!. Throws if the value is null.
• Variable: var name or Type name. Binds the matched value to a new local variable.
• Wildcard (_): Matches any value and discards it.
• List: [pattern1, pattern2]. Matches lists of exact length unless a Rest element (... or ...var rest) is used.
• Map: {"key": pattern}. Matches maps containing the specified keys. Ignores unmatched keys.
• Record: (pattern1, named: pattern2). Matches records of the exact shape. Use :var name to infer the getter name.
• Object: ClassName(field: pattern). Matches instances of ClassName. Use :var field to infer the getter name.

Workflows

Task Progress: Implementing Pattern Matching

Copy this checklist to track progress when implementing complex pattern matching logic:

• Identify the data structure being evaluated (JSON, Record, Class, Enum).
• Select the appropriate switch construct (Expression for values, Statement for side-effects).
• Define the required patterns (Object, Map, List, Record).
• Extract required data using Variable patterns (var x, :var y).
• Apply Guard clauses (when condition) for logic that cannot be expressed via patterns.
• Handle unmatched cases using a Wildcard (_) or default clause (if not using a sealed class).
• Run exhaustiveness validator.

Feedback Loop: Exhaustiveness Checking

When switching over sealed classes or enums, you must ensure all subtypes are handled.

1. Run validator: Execute dart analyze.
2. Review errors: Look for "The type 'X' is not exhaustively matched by the switch cases" errors.
3. Fix: Add the missing Object patterns for the unhandled subtypes, or add a Wildcard (_) case if a default fallback is acceptable.

Examples

JSON Validation and Destructuring

Use Map and List patterns to validate structure and extract data in a single step.

Input:

var data = {
  'user': ['Lily', 13],
};

Implementation:

if (data case {'user': [String name, int age]}) {
  print('User $name is $age years old.');
} else {
  print('Invalid JSON structure.');
}

Algebraic Data Types (Sealed Classes)

Use Object patterns with switch expressions to handle family types exhaustively.

Implementation:

sealed class Shape {}

class Square implements Shape {
  final double length;
  Square(this.length);
}

class Circle implements Shape {
  final double radius;
  Circle(this.radius);
}

// Switch expression guarantees exhaustiveness due to `sealed` modifier.
double calculateArea(Shape shape) => switch (shape) {
  Square(length: var l) => l * l,
  Circle(:var radius)   => math.pi * radius * radius,
};

Variable Swapping and Destructuring

Use variable assignment patterns to swap values or extract record fields without temporary variables.

Implementation:

var (a, b) = ('left', 'right');
(b, a) = (a, b); // Swap values

// Destructuring a function return
var (name, age) = getUserInfo();

Guard Clauses and Logical-or

Use when to evaluate arbitrary conditions after a pattern matches.

Implementation:

switch (shape) {
  case Square(size: var s) || Circle(size: var s) when s > 0:
    print('Valid symmetric shape with size $s');
  case Square() || Circle():
    print('Invalid or empty shape');
  default:
    print('Unknown shape');
}