Add constant parser.

Author: renggli

lib/parser.dart

@@ -4,6 +4,7 @@ library;
 export 'src/core/parser.dart';
 export 'src/parser/action/cast.dart';
 export 'src/parser/action/cast_list.dart';
+export 'src/parser/action/constant.dart';
 export 'src/parser/action/continuation.dart';
 export 'src/parser/action/flatten.dart';
 export 'src/parser/action/map.dart';

lib/src/parser/action/constant.dart

@@ -0,0 +1,40 @@
+import 'package:meta/meta.dart';
+
+import '../../core/context.dart';
+import '../../core/parser.dart';
+import '../../core/result.dart';
+import '../combinator/delegate.dart';
+
+extension ConstantParserExtension<R> on Parser<R> {
+  /// Returns a parser that returns a constant value.
+  ///
+  /// For example, the parser `digit().star().constant(42)` returns the number
+  /// `42` for whatever input digits it consumes.
+  @useResult
+  Parser<S> constant<S>(S value) => ConstantParser<R, S>(this, value);
+}
+
+/// A parser that returns a constant value.
+class ConstantParser<R, S> extends DelegateParser<R, S> {
+  ConstantParser(super.delegate, this.value);
+
+  final S value;
+
+  @override
+  Result<S> parseOn(Context context) {
+    final result = delegate.parseOn(context);
+    if (result is Failure) return result;
+    return result.success(value);
+  }
+
+  @override
+  int fastParseOn(String buffer, int position) =>
+      delegate.fastParseOn(buffer, position);
+
+  @override
+  bool hasEqualProperties(ConstantParser<R, S> other) =>
+      super.hasEqualProperties(other) && value == other.value;
+
+  @override
+  ConstantParser<R, S> copy() => ConstantParser<R, S>(delegate, value);
+}

lib/src/reflection/internal/linter_rules.dart

@@ -1,6 +1,7 @@
 import '../../core/parser.dart';
 import '../../parser/action/cast.dart';
 import '../../parser/action/cast_list.dart';
+import '../../parser/action/constant.dart';
 import '../../parser/action/flatten.dart';
 import '../../parser/action/map.dart';
 import '../../parser/action/permute.dart';
@@ -334,6 +335,7 @@ class UnusedResult extends LinterRule {
   bool isResultProducing(Parser parser) =>
       parser is CastParser ||
       parser is CastListParser ||
+      parser is ConstantParser ||
       parser is FlattenParser ||
       (parser is MapParser && !parser.hasSideEffects) ||
       parser is PermuteParser ||

test/parser_action_test.dart

@@ -24,6 +24,14 @@ void main() {
       );
     });
   });
+  group('constant', () {
+    final parser = digit().constant(42);
+    expectParserInvariants(parser);
+    test('default', () {
+      expect(parser, isParseSuccess('1', result: 42));
+      expect(parser, isParseFailure('a', message: 'digit expected'));
+    });
+  });
   group('continuation', () {
     expectParserInvariants(
       any().callCC<String>((continuation, context) => continuation(context)),