Implementing Dart Patterns

Contents

• Pattern Selection Strategy
• Switch Statements vs. Expressions
• Core Pattern Implementations
• Pragmatic Balance & Anti-Patterns
• 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, List, and Object patterns to validate schema structure and destructure properties in a single step.
• If inspecting URL/path segments (uri.pathSegments, p.split(path)) or String.split() tokens: Use List patterns with rest elements (['api', 'comments', ...], ['assets', ...final rest], [..., final parent, _]) instead of manual .length checks, .first, .skip(1), or length - N index arithmetic.
• If handling polymorphic payloads or responses: Use switch expressions over map discriminant keys to deserialize into sealed class hierarchies.
• 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 within switch arms.
• 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.

Pragmatic Balance & Anti-Patterns

Pattern matching and switch expressions should simplify code, not add syntactic overhead. Observe the following boundaries:

1. Prefer is Type Promotion over if-case for Single Promotable Variables

When checking or promoting a single variable, use standard is checks instead of if-case patterns that introduce shadow aliases.

• Prefer:

  // ✅ Promotes `key` directly in-place without extra variables
  for (final MapEntry(:key, :value) in map.entries) {
    if (key is String && value != null) {
      process(key, value);
    }
  }
  

• Avoid:

  // ❌ Anti-pattern: Introduces unnecessary alias variable `k`
  for (final MapEntry(:key, :value) in map.entries) {
    if (key case final String k when value != null) {
      process(k, value);
    }
  }
  

2. Consolidate Nullable Types in Switch Arms

When mapping or returning values where both null and a type T are valid and handled identically, match the nullable type T? directly rather than creating redundant null arms.

• Prefer:

  // ✅ Clean nullable pattern match
  switch (value) {
    final String? s => s,
    _ => throw FormatException('Invalid value: $value'),
  }
  

• Avoid:

  // ❌ Redundant separate null arm
  switch (value) {
    final String s => s,
    null => null,
    _ => throw FormatException('Invalid value: $value'),
  }
  

3. Preserve Fast-Fail Validation (Do Not Silently Drop Data)

Do not use if-case in loops or deserialization to filter elements if malformed data should trigger an error or diagnostic warning.

• Prefer:

  // ✅ Fast-fail with explicit diagnostic error
  for (final raw in rawTasks) {
    if (raw is! Map<String, dynamic>) {
      throw FormatException('Expected Map item, got ${raw.runtimeType}: $raw');
    }
    _applyTask(raw);
  }
  

• Avoid:

  // ❌ Silently ignores malformed items
  for (final raw in rawTasks) {
    if (raw case final Map<String, dynamic> taskMap) {
      _applyTask(taskMap);
    }
  }
  

4. Avoid Single-Case or Boolean Switches

• Use if (x is T) instead of a switch statement with only 1 case and default: break;.
• Use standard conditional ternary operators (condition ? a : b) instead of switch (condition) { true => a, false => b }.

5. Avoid Gratuitous Object Destructuring

Use standard property access (user.name) rather than object pattern destructuring (final User(:name) = user;) when reading a single property on a known non-null instance.

6. Avoid if-case for Standalone Scalar Comparisons

Use standard boolean operators (if (code >= 200 && code < 300)) instead of if (code case >= 200 && < 300) for standalone conditions. Reserve relational patterns for multi-arm switch tables.

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, List/Segments, 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, ...final rest).
• 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 static analyzer for exhaustiveness and dead code (dart analyze).
• Verify runtime Map/JSON pattern behavior on omitted keys (containsKey semantics) vs explicit null values.

Feedback Loop 1: Exhaustiveness Checking (Static Verification)

When switching over sealed classes or enums, ensure all subtypes are handled at compile time:

1. Run analyzer: Execute dart analyze.
2. Review errors: Look for "The type 'X' is not exhaustively matched by the switch cases" or unreachable pattern arm warnings.
3. Fix: Add the missing Object patterns for unhandled subtypes, or add an explicit wildcard (_) arm if a default fallback or error is acceptable.

Feedback Loop 2: Runtime Map & JSON Pattern Verification

Because dart analyze cannot statically verify dynamic Map<String, dynamic> keys, validate runtime pattern semantics explicitly:

1. Omitted Keys vs. Explicit null: A map pattern {'key': String? val} checks map.containsKey('key'). If 'key' is omitted from the JSON payload, the pattern fails to match at runtime even though String? is nullable. Extract optional keys from the validated map directly (map['key'] as String?).
2. Fast-Fail Fallback: Ensure unmatched or malformed map structures hit an explicit _ => throw FormatException(...) arm rather than silently failing an if-case check.

Examples

Polymorphic JSON Deserialization (Discriminated Unions)

Use Map patterns with switch expressions to validate tagged JSON payloads and construct sealed class hierarchies. See examples/json_patterns.dart for an executable implementation demonstrating tagged ApiResponse parsing into SuccessResponse and ErrorResponse.

Nested JSON Validation and Optional Fields

Use nested Map and List patterns to validate required schema structure and extract collections in a single step. See examples/json_patterns.dart for an executable implementation of processUserPayload.

Map patterns check for key existence (containsKey). If an optional JSON key might be omitted entirely from the payload (rather than explicitly passed as 'key': null), destructure required keys via the pattern and extract optional fields directly from the matched submap.

List and Path Segment Destructuring

Use if-case with list rest elements (...final rest) to validate prefixes and extract remaining elements without a 2-arm _ => null switch or manual .first and .skip(1) indexing.

String? resolveAllowedAssetSubpath(List<String> segments) {
  if (segments case ['assets', ...final rest]
      when rest.isNotEmpty && !rest.contains('..')) {
    return rest.join('/');
  }
  return null;
}

Algebraic Data Types (Sealed Classes)

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

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.

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.

switch (shape) {
  case Square(length: var s) || Circle(radius: var s) when s > 0:
    print('Valid positive shape with dimension $s');
  case Square() || Circle():
    print('Zero or negative dimension shape');
}