Rewrite the pkg/yaml parser.

pkg/yaml/lib/src/loader.dart

+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library yaml.loader;
+
+import 'package:source_span/source_span.dart';
+
+import 'equality.dart';
+import 'event.dart';
+import 'parser.dart';
+import 'yaml_document.dart';
+import 'yaml_exception.dart';
+import 'yaml_node.dart';
+
+/// A loader that reads [Event]s emitted by a [Parser] and emits
+/// [YamlDocument]s.
+///
+/// This is based on the libyaml loader, available at
+/// https://github.com/yaml/libyaml/blob/master/src/loader.c. The license for
+/// that is available in ../../libyaml-license.txt.
+class Loader {

[Bob Nystrom, 2014/10/31 20:03:27]

The separation between Loader and Parser is interesting. I can see it making
sense in C maybe because you don't have exceptions? Is there another rationale
behind it?

[nweiz, 2014/11/04 22:19:36]

It might make sense to expose the parser on its own at some point to provide
access to the raw tags as they exist in the document. It's also a separation of
concerns, so that the parser can focus entirely on making sense of the scanner's
tokens and not on constructing aliases. Mostly it just makes the port more
direct, though.

+  /// The underlying [Parser] that generates [Event]s.
+  final Parser _parser;
+
+  /// Aliases by the alias name.
+  final _aliases = new Map<String, YamlNode>();
+
+  /// The span of the entire stream emitted so far.
+  FileSpan get span => _span;
+  FileSpan _span;
+
+  /// Creates a parser that loads [source].

[Bob Nystrom, 2014/10/31 20:03:27]

"loader".

[nweiz, 2014/11/04 22:19:36]

Done.

+  ///
+  /// [sourceUrl] can be a String or a [Uri].
+  Loader(String source, {sourceUrl})
+      : _parser = new Parser(source, sourceUrl: sourceUrl) {
+    var event = _parser.parse();
+    _span = event.span;
+    assert(event.type == EventType.STREAM_START);
+  }
+
+  /// Loads the next document from the stream.
+  YamlDocument load() {
+    if (_parser.isDone) return null;

[Bob Nystrom, 2014/10/31 20:03:27]

Document this.

[nweiz, 2014/11/04 22:19:36]

Done.

+
+    var event = _parser.parse();
+    if (event.type == EventType.STREAM_END) {
+      _span = _span.expand(event.span);
+      return null;
+    }
+
+    var document = _loadDocument(event);
+    _span = _span.expand(document.span);
+    _aliases.clear();
+    return document;
+  }
+
+  /// Composes a document object.
+  YamlDocument _loadDocument(DocumentStartEvent firstEvent) {
+    var contents = _loadNode(_parser.parse());
+
+    var lastEvent = _parser.parse();
+    assert(lastEvent.type == EventType.DOCUMENT_END);
+
+    return new YamlDocument.internal(
+        contents,
+        firstEvent.span.expand(lastEvent.span),
+        firstEvent.versionDirective,
+        firstEvent.tagDirectives,
+        startImplicit: firstEvent.implicit,
+        endImplicit: lastEvent.implicit);
+  }
+
+  /// Composes a node.
+  YamlNode _loadNode(Event firstEvent) {
+    switch (firstEvent.type) {
+      case EventType.ALIAS: return _loadAlias(firstEvent);
+      case EventType.SCALAR: return _loadScalar(firstEvent);
+      case EventType.SEQUENCE_START: return _loadSequence(firstEvent);
+      case EventType.MAPPING_START: return _loadMapping(firstEvent);
+      default: throw "Unreachable";
+    }
+  }
+
+  /// Registers an anchor.
+  void _registerAnchor(String anchor, YamlNode node) {
+    if (anchor == null) return;
+
+    // libyaml throws an error for duplicate anchors, but example 7.1 makes it
+    // clear that they should be overridden:
+    // http://yaml.org/spec/1.2/spec.html#id2786448.
+
+    _aliases[anchor] = node;
+  }
+
+  /// Composes a node corresponding to an alias.
+  YamlNode _loadAlias(AliasEvent event) {
+    var alias = _aliases[event.name];
+    if (alias != null) return alias;
+
+    throw new YamlException("Undefined alias.", event.span);
+  }
+
+  /// Composes a scalar node.
+  YamlNode _loadScalar(ScalarEvent scalar) {
+    var node;
+    if (scalar.tag == "!") {
+      node = _parseString(scalar);
+    } else if (scalar.tag == null) {
+      node = _parseNull(scalar);
+      if (node == null) node = _parseBool(scalar);
+      if (node == null) node = _parseInt(scalar);
+      if (node == null) node = _parseFloat(scalar);
+      if (node == null) node = _parseString(scalar);
+    } else {
+      node = _parseByTag(scalar);

[Bob Nystrom, 2014/10/31 20:03:27]

I think it would read a little clearer to do this case first:

if (scalar.tag != null) {
  node = _parseByTag(scalar);
} ...

[nweiz, 2014/11/04 22:19:36]

Done.

+    }
+
+    _registerAnchor(scalar.anchor, node);
+    return node;
+  }
+
+  /// Composes a sequence node.
+  YamlNode _loadSequence(SequenceStartEvent firstEvent) {
+    if (firstEvent.tag != "!" && firstEvent.tag != null &&
+        firstEvent.tag != "tag:yaml.org,2002:seq") {
+      throw new YamlException("Invalid tag for sequence.", firstEvent.span);
+    }
+
+    var children = [];
+    var node = new YamlList.internal(
+        children, firstEvent.span, firstEvent.style);
+    _registerAnchor(firstEvent.anchor, node);
+
+    var event = _parser.parse();
+    while (event.type != EventType.SEQUENCE_END) {
+      children.add(_loadNode(event));
+      event = _parser.parse();
+    }
+
+    setSpan(node, firstEvent.span.expand(event.span));
+    return node;
+  }
+
+  /// Composes a mapping node.
+  YamlNode _loadMapping(MappingStartEvent firstEvent) {
+    if (firstEvent.tag != "!" && firstEvent.tag != null &&
+        firstEvent.tag != "tag:yaml.org,2002:map") {
+      throw new YamlException("Invalid tag for mapping.", firstEvent.span);
+    }
+
+    var children = deepEqualsMap();
+    var node = new YamlMap.internal(
+        children, firstEvent.span, firstEvent.style);
+    _registerAnchor(firstEvent.anchor, node);
+
+    var event = _parser.parse();
+    while (event.type != EventType.MAPPING_END) {
+      var key = _loadNode(event);
+      var value = _loadNode(_parser.parse());
+      children[key] = value;
+      event = _parser.parse();
+    }
+
+    setSpan(node, firstEvent.span.expand(event.span));
+    return node;
+  }
+
+  /// Parses a scalar according to its tag name.
+  YamlScalar _parseByTag(ScalarEvent scalar) {
+    switch (scalar.tag) {
+      case "tag:yaml.org,2002:null": return _parseNull(scalar);
+      case "tag:yaml.org,2002:bool": return _parseBool(scalar);
+      case "tag:yaml.org,2002:int": return _parseInt(scalar);
+      case "tag:yaml.org,2002:float": return _parseFloat(scalar);
+      case "tag:yaml.org,2002:str": return _parseString(scalar);
+    }
+    throw new YamlException('Undefined tag: ${scalar.tag}.', scalar.span);
+  }
+
+  /// Parses a null scalar.
+  YamlScalar _parseNull(ScalarEvent scalar) {
+    // TODO(nweiz): stop using regexps.
+    // TODO(nweiz): add ScalarStyle and implicit metadata to the scalars.
+    if (new RegExp(r"^(null|Null|NULL|~|)$").hasMatch(scalar.value)) {
+      return new YamlScalar.internal(null, scalar.span, scalar.style);
+    } else {
+      return null;
+    }
+  }
+
+  /// Parses a boolean scalar.
+  YamlScalar _parseBool(ScalarEvent scalar) {
+    var match = new RegExp(r"^(?:(true|True|TRUE)|(false|False|FALSE))$").
+        firstMatch(scalar.value);
+    if (match == null) return null;
+    return new YamlScalar.internal(
+        match.group(1) != null, scalar.span, scalar.style);
+  }
+
+  /// Parses an integer scalar.
+  YamlScalar _parseInt(ScalarEvent scalar) {
+    var match = new RegExp(r"^[-+]?[0-9]+$").firstMatch(scalar.value);

[Bob Nystrom, 2014/10/31 20:03:27]

Is "0000123" allowed?

I wonder if it's better to just do:

try {
  int.parse...
} on FormatException catch (error) { ... }

[nweiz, 2014/11/04 22:19:36]

Yes.
I imagine the Dart parsing rules aren't identical to the Yaml parsing rules.

+    if (match != null) {
+      return new YamlScalar.internal(
+          int.parse(match.group(0)), scalar.span, scalar.style);
+    }
+
+    match = new RegExp(r"^0o([0-7]+)$").firstMatch(scalar.value);
+    if (match != null) {
+      var n = int.parse(match.group(1), radix: 8);
+      return new YamlScalar.internal(n, scalar.span, scalar.style);
+    }
+
+    match = new RegExp(r"^0x[0-9a-fA-F]+$").firstMatch(scalar.value);
+    if (match != null) {
+      return new YamlScalar.internal(
+          int.parse(match.group(0)), scalar.span, scalar.style);
+    }
+
+    return null;
+  }
+
+  /// Parses a floating-point scalar.
+  YamlScalar _parseFloat(ScalarEvent scalar) {
+    var match = new RegExp(
+          r"^[-+]?(\.[0-9]+|[0-9]+(\.[0-9]*)?)([eE][-+]?[0-9]+)?$").
+        firstMatch(scalar.value);
+    if (match != null) {
+      // YAML allows floats of the form "0.", but Dart does not. Fix up those
+      // floats by removing the trailing dot.
+      var matchStr = match.group(0).replaceAll(new RegExp(r"\.$"), "");
+      return new YamlScalar.internal(
+          double.parse(matchStr), scalar.span, scalar.style);
+    }
+
+    match = new RegExp(r"^([+-]?)\.(inf|Inf|INF)$").firstMatch(scalar.value);
+    if (match != null) {
+      var value = match.group(1) == "-" ? -double.INFINITY : double.INFINITY;
+      return new YamlScalar.internal(value, scalar.span, scalar.style);
+    }
+
+    match = new RegExp(r"^\.(nan|NaN|NAN)$").firstMatch(scalar.value);
+    if (match != null) {
+      return new YamlScalar.internal(double.NAN, scalar.span, scalar.style);
+    }
+
+    return null;
+  }
+
+  /// Parses a string scalar.
+  YamlScalar _parseString(ScalarEvent scalar) =>
+      new YamlScalar.internal(scalar.value, scalar.span, scalar.style);
+}