dart-idiomatic-usage

Writing Idiomatic Dart Collections and Strings

Contents

• String Composition
• Collection Initialization
• Collection Operations
• Type Casting in Collections
• Workflow: Refactoring Legacy Collections

String Composition

Follow these practices to compose strings efficiently and readably.

• Use adjacent strings for concatenation: Do not use the + operator to concatenate string literals. Place them next to each other to form a single string.
• Prefer string interpolation: Use $variable or ${expression} to compose strings and values instead of concatenation (+).
• Omit unnecessary curly braces: Use $identifier instead of ${identifier} when interpolating a simple identifier that is not immediately followed by alphanumeric text.

Examples

Good:

// Adjacent strings
raiseAlarm(
  'ERROR: Parts of the spaceship are on fire. Other '
  'parts are overrun by martians. Unclear which are which.',
);

// Interpolation without unnecessary braces
var greeting = 'Hi, $name! I love your ${decade}s costume.';

Bad:

// Do not use + for literals or variables
raiseAlarm(
  'ERROR: Parts of the spaceship are on fire. Other ' +
  'parts are overrun by martians. Unclear which are which.',
);

var greeting = 'Hi, ' + name + '! I love your ' + decade.toString() + 's costume.';
var badBraces = 'Hi, ${name}!';

Collection Initialization

Leverage Dart's built-in syntax for creating and structuring collections.

• Use collection literals: Instantiate lists, maps, and sets using [], {}, and <Type>{} instead of their unnamed constructors (e.g., Map(), Set()).
• Use spread and control flow operators: Build dynamic collections using the spread operator (..., ...?) and collection if/for rather than imperative add() or addAll() calls.

Examples

Good:

var points = <Point>[];
var addresses = <String, Address>{};

var arguments = [
  ...options,
  command,
  ...?modeFlags,
  for (var path in filePaths)
    if (path.endsWith('.dart')) path.replaceAll('.dart', '.js'),
];

Bad:

var addresses = Map<String, Address>();

var arguments = <String>[];
arguments.addAll(options);
arguments.add(command);
if (modeFlags != null) arguments.addAll(modeFlags);

Collection Operations

Optimize collection querying and iteration.

• Use .isEmpty and .isNotEmpty: Never check if a collection's .length is 0 or > 0. The Iterable contract does not guarantee constant-time length calculations.
• Avoid Iterable.forEach() with function literals: Use standard for-in loops when iterating over sequences. Reserve forEach() for passing existing function tear-offs (e.g., people.forEach(print)).
• Use whereType<T>(): Filter collections by type using the built-in whereType<T>() method rather than where((e) => e is T).

Examples

Good:

if (lunchBox.isEmpty) return 'so hungry...';

for (final person in people) {
  process(person);
}

var ints = objects.whereType<int>();

Bad:

if (lunchBox.length == 0) return 'so hungry...';

people.forEach((person) {
  process(person);
});

var ints = objects.where((e) => e is int).cast<int>();

Type Casting in Collections

Minimize the use of .cast<T>(), as it creates a lazy collection that checks the element type on every operation, degrading performance.

• Create with the correct type: Define the collection with the correct generic type at instantiation.
• Eagerly cast using List.from(): If you must convert a collection and will access most of its elements, use List<T>.from(iterable) or Map<K, V>.from(map) to eagerly cast the elements once.
• Preserve types with .toList(): Use .toList() when you want to copy an iterable while preserving its original type. Use List.from() only when intentionally changing the type.

Examples

Good:

// Eager cast
var stuff = <dynamic>[1, 2];
var ints = List<int>.from(stuff);

// Map with explicit type
var reciprocals = stuff.map<double>((n) => n * 2);

Bad:

// Lazy cast (performance hit on every access)
var stuff = <dynamic>[1, 2];
var ints = stuff.toList().cast<int>();

var reciprocals = stuff.map((n) => n * 2).cast<double>();

Workflow: Refactoring Legacy Collections

When updating legacy Dart code to modern standards, follow this sequential checklist.

Task Progress

• Step 1: Replace Constructors. Scan for Map(), Set(), and List() (if applicable). Replace with <K, V>{}, <T>{}, and <T>[].
• Step 2: Eliminate .length checks. Regex search for .length == 0, .length > 0, .length != 0. Replace with .isEmpty or .isNotEmpty.
• Step 3: Flatten imperative builds. Identify sequential .add() and .addAll() calls on a newly instantiated collection. Rewrite using ..., if, and for inside the collection literal.
• Step 4: Remove .cast<T>(). Search for .cast<.
  • If applied after .toList(), replace with List<T>.from().
  • If applied after .where(), replace with .whereType<T>().
  • If applied after .map(), add the generic type to the map call: .map<T>(...).
• Step 5: Run validator -> review errors -> fix. Run dart analyze to ensure no type promotion or syntax errors were introduced during refactoring.