Absolutely working

example/basic.dart

@@ -8,10 +8,14 @@ Stream<String> input() async* {
     tagline
   }
 }
-  '''.trim();
+  '''
+      .trim();
 }
 
 main() async {
   var lexer = new Lexer(), parser = new Parser();
-  await input().transform(lexer).forEach(print);
+  var stream = input().transform(lexer).asBroadcastStream();
+  await stream.forEach(print);
+  stream.pipe(parser);
+  await parser.onNode.forEach(print);
 }

example/boolean.dart

@@ -0,0 +1,12 @@
+import 'dart:async';
+import 'package:graphql_parser/src/language/language.dart';
+
+Stream<String> input() async* {
+  yield 'true';
+}
+
+main() async {
+  var lexer = new Lexer(), parser = new Parser();
+  input().transform(lexer).pipe(parser);
+  await parser.onBooleanValue.forEach(print);
+}

lib/src/language/ast/boolean_value.dart

@@ -5,7 +5,11 @@ import 'value.dart';
 class BooleanValueContext extends ValueContext {
   final Token BOOLEAN;
 
-  BooleanValueContext(this.BOOLEAN);
+  BooleanValueContext(this.BOOLEAN) {
+    assert(BOOLEAN?.text == 'true' || BOOLEAN?.text == 'false');
+  }
+
+  bool get booleanValue => BOOLEAN.text == 'true';
 
   @override
   SourceSpan get span => BOOLEAN.span;

lib/src/language/parser.dart

@@ -1,26 +1,96 @@
 import 'dart:async';
 import 'ast/ast.dart';
 import 'stream_reader.dart';
+import 'syntax_error.dart';
 import 'token.dart';
+import 'token_type.dart';
 
 class Parser implements StreamConsumer<Token> {
   bool _closed = false;
+  final Completer _closer = new Completer();
+  final List<SyntaxError> _errors = [];
   final StreamReader<Token> _reader = new StreamReader();
 
+  final StreamController<BooleanValueContext> _onBooleanValue =
+      new StreamController<BooleanValueContext>();
+  final StreamController<DocumentContext> _onDocument =
+      new StreamController<DocumentContext>();
   final StreamController<Node> _onNode = new StreamController<Node>();
 
+  List<SyntaxError> get errors => new List<SyntaxError>.unmodifiable(_errors);
+
+  Stream<Node> get onBooleanValue => _onBooleanValue.stream;
+  Stream<Node> get onDocument => _onDocument.stream;
   Stream<Node> get onNode => _onNode.stream;
 
-  @override
-  Future addStream(Stream<Token> stream) async {
-    if (_closed) throw new StateError('Parser is already closed.');
-    stream.pipe(_reader);
+  Future _waterfall(List<Function> futures) async {
+    for (var f in futures) {
+      var r = await f();
+      if (r != null) return r;
+    }
   }
 
   @override
-  Future close() async {
+  Future addStream(Stream<Token> stream) {
+    if (_closed) throw new StateError('Parser is already closed.');
+
     _closed = true;
 
-    await _onNode.close();
+    _reader.onData.listen((data) => _waterfall([document, booleanValue]))
+      ..onDone(() => Future.wait([
+            _onBooleanValue.close(),
+            _onDocument.close(),
+            _onNode.close(),
+          ]))
+      ..onError(_closer.completeError);
+
+    return stream.pipe(_reader);
+  }
+
+  @override
+  Future close() {
+    return _closer.future;
+  }
+
+  Future<bool> expect(TokenType type) async {
+    var peek = await _reader.peek();
+
+    if (peek?.type != type) {
+      _errors.add(new SyntaxError.fromSourceLocation(
+          "Expected $type, found '${peek?.text ?? 'empty text'}' instead.",
+          peek?.span?.start));
+      return false;
+    } else {
+      await _reader.consume();
+      return true;
+    }
+  }
+
+  Future<bool> maybe(TokenType type) async {
+    var peek = await _reader.peek();
+
+    if (peek?.type == type) {
+      await _reader.consume();
+      return true;
+    }
+
+    return false;
+  }
+
+  Future<bool> nextIs(TokenType type) =>
+      _reader.peek().then((t) => t?.type == type);
+
+  Future<DocumentContext> document() async {
+    return null;
+  }
+
+  Future<BooleanValueContext> booleanValue() async {
+    if (await nextIs(TokenType.BOOLEAN)) {
+      var result = new BooleanValueContext(await _reader.consume());
+      _onBooleanValue.add(result);
+      return result;
+    }
+
+    return null;
   }
 }

lib/src/language/stream_reader.dart

@@ -4,10 +4,40 @@ import 'dart:collection';
 class StreamReader<T> implements StreamConsumer<T> {
   final Queue<T> _buffer = new Queue();
   bool _closed = false;
+  T _current;
+  bool _onDataListening = false;
   final Queue<Completer<T>> _nextQueue = new Queue();
   final Queue<Completer<T>> _peekQueue = new Queue();
 
+  StreamController<T> _onData;
+
   bool get isDone => _closed;
+  Stream<T> get onData => _onData.stream;
+
+  _onListen() {
+    _onDataListening = true;
+  }
+
+  _onPause() {
+    _onDataListening = false;
+  }
+
+  StreamReader() {
+    _onData = new StreamController<T>(
+        onListen: _onListen,
+        onResume: _onListen,
+        onPause: _onPause,
+        onCancel: _onPause);
+  }
+
+  Future<T> current() {
+    if (_current == null) {
+      if (_nextQueue.isNotEmpty) return _nextQueue.first.future;
+      return consume();
+    }
+
+    return new Future.value(_current);
+  }
 
   Future<T> peek() {
     if (isDone) throw new StateError('Cannot read from closed stream.');
@@ -18,9 +48,13 @@ class StreamReader<T> implements StreamConsumer<T> {
     return c.future;
   }
 
-  Future<T> next() {
+  Future<T> consume() {
     if (isDone) throw new StateError('Cannot read from closed stream.');
-    if (_buffer.isNotEmpty) return new Future.value(_buffer.removeFirst());
+
+    if (_buffer.isNotEmpty) {
+      _current = _buffer.removeFirst();
+      return close().then((_) => new Future.value(_current));
+    }
 
     var c = new Completer<T>();
     _nextQueue.addLast(c);
@@ -34,19 +68,21 @@ class StreamReader<T> implements StreamConsumer<T> {
     var c = new Completer();
 
     stream.listen((data) {
+      if (_onDataListening) _onData.add(data);
+
       if (_peekQueue.isNotEmpty || _nextQueue.isNotEmpty) {
         if (_peekQueue.isNotEmpty) {
           _peekQueue.removeFirst().complete(data);
         }
 
         if (_nextQueue.isNotEmpty) {
-          _nextQueue.removeFirst().complete(data);
+          _nextQueue.removeFirst().complete(_current = data);
         }
       } else {
         _buffer.add(data);
       }
     })
-      ..onDone(c.complete)
+      ..onDone(() => close().then(c.complete))
       ..onError(c.completeError);
 
     return c.future;
@@ -54,7 +90,17 @@ class StreamReader<T> implements StreamConsumer<T> {
 
   @override
   Future close() async {
-    _closed = true;
+    if (_buffer.isEmpty && _nextQueue.isEmpty && _peekQueue.isEmpty) {
+      _closed = true;
+
+      kill(Completer c) {
+        c.completeError(new StateError(
+            'Reached end of stream, although more input was expected.'));
+      }
+
+      _peekQueue.forEach(kill);
+      _nextQueue.forEach(kill);
+    }
   }
 }