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An Iterable in Dart is an abstract class representing a sequence of elements that can be accessed sequentially. It serves as the foundational interface for collections like List and Set, defining the contract for traversing elements without exposing the underlying data structure’s implementation details.

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The Iterator Mechanism

An Iterable itself does not maintain iteration state. Instead, it provides an iterator getter that returns an Iterator object. The Iterator is responsible for maintaining the cursor position and traversing the sequence using moveNext() and current. 
Iterable<int> numbers = [1, 2, 3];
Iterator<int> iterator = numbers.iterator;

while (iterator.moveNext()) {
  print(iterator.current);
}

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Lazy Evaluation

A defining characteristic of Dart’s Iterable methods (such as map, where, expand, and take) is lazy evaluation. Invoking these methods does not immediately process the elements or allocate memory for a new collection. Instead, they return a new Iterable that wraps the original one. The computation is deferred until the elements are explicitly consumed (e.g., via a for-in loop, .toList(), or .length). 
Iterable<int> baseIterable = [1, 2, 3, 4];

// No computation happens here. Returns a MappedIterable.
Iterable<int> lazyMapped = baseIterable.map((n) {
  print('Processing $n');
  return n * 2;
});

// Computation is triggered here during consumption.
List<int> materializedList = lazyMapped.toList();

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Synchronous Generators

Dart provides language-level support for constructing custom Iterable sequences using synchronous generator functions. By marking a function with sync* and using the yield keyword, the Dart compiler automatically generates the underlying Iterable and Iterator boilerplate. 
Iterable<int> generateSequence(int max) sync* {
  for (int i = 0; i < max; i++) {
    yield i; // Emits a single value to the Iterable
  }
}

Iterable<int> recursiveSequence(int max) sync* {
  if (max > 0) {
    yield max;
    yield* recursiveSequence(max - 1); // Delegates to another Iterable
  }
}

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Structural Characteristics

  1. Immutability of Interface: The base Iterable<E> class exposes a strictly read-only view. It defines accessors (first, last, elementAt) and transformation methods, but lacks mutator methods (add, remove). Mutation is strictly the domain of concrete subclasses like List<E>.
  2. Iteration Behavior: The Dart Iterable contract explicitly requires that iterables can be iterated multiple times. Each time the .iterator getter is accessed, it must return a new, independent Iterator that starts from the beginning of the sequence. However, while multiple passes are guaranteed to be possible, the elements yielded—or their order—are not strictly required to be identical across iterations if the Iterable is backed by impure functions, randomized logic, or changing external state (such as those created via Iterable.generate or sync*).
  3. Length Computation: Because iterables can be lazily generated or infinite, accessing the .length property on an unmaterialized Iterable operates in O(n)O(n) time, requiring a full traversal of the sequence to count the elements, unless overridden by a concrete subclass (like List) to operate in O(1)O(1) time.
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