Make Optional implement Iterable

lib/optional.dart

@@ -14,12 +14,14 @@
 
 library quiver.optional;
 
+import 'dart:collection';
+
 /// A value that might be absent.
 ///
 /// Use Optional as an alternative to allowing fields, parameters or return
 /// values to be null. It signals that a value is not required and provides
 /// convenience methods for dealing with the absent case.
-class Optional<T> {
+class Optional<T> extends IterableBase<T> {
   final T _value;
 
   /// Constructs an empty Optional.
@@ -90,6 +92,10 @@ class Optional<T> {
         : new Optional.of(transformer(_value));
   }
 
+  @override
+  Iterator<T> get iterator =>
+      isPresent ? <T>[_value].iterator : new Iterable<T>.empty().iterator;
+
   /// Delegates to the underlying [value] hashCode.
   int get hashCode => _value.hashCode;
 

test/optional_test.dart

@@ -110,5 +110,22 @@ main() {
       expect(new Optional<int>.fromNullable(null).toString(),
           equals('Optional { absent }'));
     });
+
+    test('length when absent should return 0', () {
+      expect(const Optional.absent().length, equals(0));
+    });
+
+    test('length when present should return 1', () {
+      expect(new Optional<int>.of(1).length, equals(1));
+    });
+
+    test('expand should behave as equivalent iterable', () {
+      final optionals = <Optional<int>>[
+        new Optional<int>.of(1),
+        const Optional.absent(),
+        new Optional<int>.of(2)
+      ].expand((i) => i);
+      expect(optionals, orderedEquals([1, 2]));
+    });
   });
 }