Add a basic YAML processor. Much of the language is still unimplemented.

utils/yaml/composer.dart

Index: utils/yaml/composer.dart
diff --git a/utils/yaml/composer.dart b/utils/yaml/composer.dart
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+// Copyright (c) 2012, 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.
+
+/**
+ * Takes a parsed YAML document (what the spec calls the "serialization tree")
+ * and resolves aliases, resolves tags, and parses scalars to produce the
+ * "representation graph".
+ */
+class _Composer extends _Visitor {
+  /** The root node of the serialization tree. */
+  _Node root;
+
+  /**
+   * Map from anchor names to the most recent representation graph node with
+   * that anchor.
+   */
+  Map<String, _Node> anchors;
+
+  /**
+   * The next id to use for the represenation graph's anchors. The spec doesn't
+   * use anchors in the representation graph, but we do so that the constructor
+   * can ensure that the same node in the representation graph produces the same
+   * native object.
+   */
+  int idCounter;
+
+  _Composer(this.root) : this.anchors = <_Node>{}, this.idCounter = 0;
+
+  /** Runs the Composer to produce a representation graph. */
+  _Node compose() => root.visit(this);
+
+  /** Returns the anchor to which an alias node refers. */
+  _Node visitAlias(_AliasNode alias) {
+    if (!anchors.containsKey(alias.anchor)) {
+      throw new Error("no anchor for alias ${alias.anchor}");
+    }
+    return anchors[alias.anchor];
+  }
+
+  /**
+   * Parses a scalar node according to its tag, or auto-detects the type if no
+   * tag exists. Currently this only supports the YAML core type schema.
+   */
+  _Node visitScalar(_ScalarNode scalar) {
+    if (scalar.tag.name == "!") {
+      return setAnchor(scalar, parseString(scalar.content));
+    } else if (scalar.tag.name == "?") {
+      for (var fn in [parseNull, parseBool, parseInt, parseFloat]) {
+        var result = fn(scalar.content);
+        if (result != null) return result;
+      }
+      return setAnchor(scalar, parseString(scalar.content));
+    }
+
+    // TODO(nweiz): support the full YAML type repository
+    var tagParsers = {
+      'null': parseNull, 'bool': parseBool, 'int': parseInt,
+      'float': parseFloat, 'str': parseString
+    };
+
+    for (var key in tagParsers.getKeys()) {
+      if (scalar.tag.name != _Tag.yaml(key)) continue;
+      var result = tagParsers[key](scalar.content);
+      if (result != null) return setAnchor(scalar, result);
+      throw new Error('invalid literal for $key: "${scalar.content}"');
+    }
+
+    throw new Error('undefined tag: "${scalar.tag.name}"');
+  }
+
+  /** Assigns a tag to the sequence and recursively composes its contents. */
+  _Node visitSequence(_SequenceNode seq) {
+    var tagName = seq.tag.name;
+    if (tagName != "!" && tagName != "?" && tagName != _Tag.yaml("seq")) {
+      throw new Error("invalid tag for sequence: ${tagName}");
+    }
+
+    var result = setAnchor(seq, new _SequenceNode(_Tag.yaml("seq"), null));
+    result.content = super.visitSequence(seq);
+    return result;
+  }
+
+  /** Assigns a tag to the mapping and recursively composes its contents. */
+  _Node visitMapping(_MappingNode map) {
+    var tagName = map.tag.name;
+    if (tagName != "!" && tagName != "?" && tagName != _Tag.yaml("map")) {
+      throw new Error("invalid tag for mapping: ${tagName}");
+    }
+
+    var result = setAnchor(map, new _MappingNode(_Tag.yaml("map"), null));
+    result.content = super.visitMapping(map);
+    return result;
+  }
+
+  /**
+   * If the serialization tree node [anchored] has an anchor, records that
+   * that anchor is pointing to the representation graph node [result].
+   */
+  _Node setAnchor(_Node anchored, _Node result) {
+    if (anchored.anchor == null) return result;
+    result.anchor = '${idCounter++}';
+    anchors[anchored.anchor] = result;
+    return result;
+  }
+
+  /** Parses a null scalar. */
+  _ScalarNode parseNull(String content) {
+    if (!const RegExp("^(null|Null|NULL|~|)\$").hasMatch(content)) return null;
+    return new _ScalarNode(_Tag.yaml("null"), value: null);
+  }
+
+  /** Parses a boolean scalar. */
+  _ScalarNode parseBool(String content) {
+    var match = const RegExp("^(?:(true|True|TRUE)|(false|False|FALSE))\$").
+      firstMatch(content);
+    if (match == null) return null;
+    return new _ScalarNode(_Tag.yaml("bool"), value: match.group(1) != null);
+  }
+
+  /** Parses an integer scalar. */
+  _ScalarNode parseInt(String content) {
+    var match = const RegExp("^[-+]?[0-9]+\$").firstMatch(content);
+    if (match != null) {
+      return new _ScalarNode(_Tag.yaml("int"),
+          value: Math.parseInt(match.group(0)));
+    }
+
+    match = const RegExp("^0o([0-7]+)\$").firstMatch(content);
+    if (match != null) {
+      // TODO(nweiz): clean this up when Dart can parse an octal string
+      var n = 0;
+      for (var c in match.group(1).charCodes()) {
+        n *= 8;
+        n += c - 48;
+      }
+      return new _ScalarNode(_Tag.yaml("int"), value: n);
+    }
+
+    match = const RegExp("^0x[0-9a-fA-F]+\$").firstMatch(content);
+    if (match != null) {
+      return new _ScalarNode(_Tag.yaml("int"),
+          value: Math.parseInt(match.group(0)));
+    }
+
+    return null;
+  }
+
+  /** Parses a floating-point scalar. */
+  _ScalarNode parseFloat(String content) {
+    var match = const RegExp(
+        "^[-+]?(\.[0-9]+|[0-9]+(\.[0-9]*)?)([eE][-+]?[0-9]+)?\$").
+      firstMatch(content);
+    if (match != null) {
+      return new _ScalarNode(_Tag.yaml("float"),
+          value: Math.parseDouble(match.group(0)));
+    }
+
+    match = const RegExp("^([+-]?)\.(inf|Inf|INF)\$").firstMatch(content);
+    if (match != null) {
+      return new _ScalarNode(_Tag.yaml("float"),
+          value: Math.parseDouble("${match.group(1)}Infinity"));
+    }
+
+    match = const RegExp("^\.(nan|NaN|NAN)\$").firstMatch(content);
+    if (match != null) {
+      return new _ScalarNode(_Tag.yaml("float"),
+          value: Math.parseDouble("NaN"));
+    }
+
+    return null;
+  }
+
+  /** Parses a string scalar. */
+  _ScalarNode parseString(String content) =>
+    new _ScalarNode(_Tag.yaml("str"), value: content);
+}