Revert "Revert "PEG Parser: under review at http://codereview.chromium.org/8399029""

utils/peg/pegparser.dart

+// Copyright (c) 2011, 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('Peg Parser');
+
+/*
+ * The following functions are combinators for building Rules.
+ *
+ * A rule is one of the following
+ * - A String which matches the string literally.
+ * - A Symbol which matches the symbol's definition.
+ * - A list of rules with an optional reducing function, which matches a sequence.
+ * - The result of calling one of the combinators.
+ *
+ * Some rules are 'value-generating' rules, they return an 'abstract syntax
+ * tree' with the match.  If a rule is not value-generating [:null:] is the
+ * value.
+ *
+ * A Symbol is always a value-generating rule. If the value is not required, use
+ * [:SKIP(aSymbol):] in place of [:aSymbol:].
+ *
+ * A String is not a value-generating rule but can be converted into one by
+ * using [:TEXT('string'):] in place of [:'string':].
+ *
+ * A list or sequence is value-generating depending on the subrules.  The
+ * sequence is value-generating if any of the subrules are value-generating or
+ * if there is a reducing function.  If no reducing function is given, the value
+ * returned depends on the number of value-generating subrules.  If there is
+ * only one value generating subrule, that provideds the value for the sequence.
+ * If there are more, then the value is a list of the values of the
+ * value-generating subrules.
+ */
+
+/**
+ * Matches one character by a predicate on the character code.
+ * If [spec] is an int, that character is matched.
+ * If [spec] is a function it is used
+ *
+ * Example [: CHARCODE((code) => 48 <= code && code <= 57) :] recognizes an
+ * ASCII digit.
+ *
+ * CHARCODE does not generate a value.
+ */
+_Rule CHARCODE(spec, [name]) {
+  if (spec is int)
+    return new _CharCodeRule((code) => code == spec, name);
+  else
+    return new _CharCodeRule(spec, name);
+}
+
+/**
+ * Matches one of the [characters].
+ *
+ * CHAR does not generate a value.
+ */
+_Rule CHAR([characters]) {
+  if (characters == null)
+    return const _AnyCharRule();
+  if (characters is int)
+    return CHARCODE(characters);
+
+  // Find the range of character codes and construct an array of flags for codes
+  // within the range.
+  List<int> codes = characters.charCodes();
+  codes.sort((a, b) => a < b ? -1 : a > b ? 1 : 0);
+  int lo = codes[0];
+  int hi = codes[codes.length - 1];
+  if (lo == hi)
+    return CHARCODE(lo);
+  int len = hi - lo + 1;
+  var flags = new List<bool>(len);
+  for (int i = 0; i < len; ++i)
+    flags[i] = false;
+  for (int code in codes)
+    flags[code - lo] = true;
+
+  return CHARCODE((code) => code >= lo && code <= hi && flags[code - lo]);
+}
+
+/**
+ * Matches the end of the input.
+ *
+ * END does not generate a value.
+ */
+_Rule get END() => new _EndOfInputRule();
+
+/**
+ * Throws an exception.
+ */
+_Rule ERROR(String message) => new _ErrorRule(message);
+
+/**
+ * Matches [rule] but does not consume the input.  Useful for matching a right
+ * context.
+ *
+ * AT does not generate a value.
+ */
+_Rule AT(rule) => new _ContextRule(_compile(rule));
+
+/**
+ * Matches when [rule] does not match.  No input is consumed.
+ *
+ * NOT does not generate a value.
+ */
+_Rule NOT(rule) => new _NegativeContextRule(_compile(rule));
+
+/**
+ * Matches [rule] but generates no value even if [rule] generates a value.
+ *
+ * SKIP never generates a value.
+ */
+_Rule SKIP(rule) => new _SkipRule(_compile(rule));
+
+/**
+ * Matches [rule] in a lexical context where whitespace is not automatically
+ * skipped.  Useful for matching what would normally be considered to be tokens.
+ * [name] is a user-friendly description of what is being matched and is used in
+ * error messages.
+ *
+ * LEX(rule)
+ * LEX(name, rule)
+ *
+ * LEX does not generate a value.  If a value is required, wrap LEX with TEXT.
+ */
+_Rule LEX(arg1, [arg2]) {
+  if (arg2 == null)
+    return new _LexicalRule(arg1 is String ? arg1 : null, _compile(arg1));
+  else
+    return new _LexicalRule(arg1, _compile(arg2));
+}
+
+/**
+ * Matches [rule] and generates a value from the matched text. If the [rule]
+ * matches, then TEXT(rule) matches and has a value derived from the string
+ * fragment that was matched.  The default derived value is the string fragment.
+ *
+ * TEXT always generates a value.
+ */
+_Rule TEXT(rule, [extractor]) =>
+  new _TextValueRule(_compile(rule),
+                     extractor == null
+                     ? (string, start, end) => string.substring(start, end)
+                     : extractor);
+
+/**
+ * Matches an optional rule.
+ *
+ * MAYBE is a value generating matcher.
+ *
+ * If [rule] is value generating then the value is the value generated by [rule]
+ * if it matches, and [:null:] if it does not.
+ *
+ * If [rule] is not value generatinge then the value is [:true:] if [rule]
+ * matches and [:false:] if it does not.
+ */
+_Rule MAYBE(rule) => new _OptionalRule(_compile(rule));
+
+/**
+ * MANY(rule) matches [rule] [min] or more times.
+ * [min] must be 0 or 1.
+ * If [separator] is provided it is used to match a separator between matches of
+ * [rule].
+ *
+ * MANY is a value generating matcher.  The value is a list of the matches of
+ * [rule].  The list may be empty if [:min == 0:].
+ */
+_Rule MANY(rule, [separator = null, int min = 1]) {
+  assert(0 <= min && min <= 1);
+  return new _RepeatRule(_compile(rule), _compileOptional(separator), min);
+}
+
+/**
+ * Matches [rule] zero or more times.  Shorthand for [:MANY(rule, min:0):]
+ * TODO: retire min: parameter?
+ *
+ * MANY0 is a value generating matcher.
+ */
+_Rule MANY0(rule, [separator = null]) {
+  return new _RepeatRule(_compile(rule), _compileOptional(separator), 0);
+}
+
+/**
+ * Matches [rules] in order until one succeeds.
+ *
+ * OR is value-generating.
+ */
+_Rule OR([a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z]) =>
+    _compileMultiRule(
+        (a is List && b == null)  // Backward compat. OR([a, b]) => OR(a, b).
+          ? a
+          : _unspread(a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z),
+        false,
+        (compiledRules, valueCount, reducer) =>
+            new _ChoiceRule(compiledRules));
+
+
+
+_Rule SEQ([a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z]) =>
+  _compile(_unspread(a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z));
+
+/**
+ * Matches [rule]
+ */
+_Rule MEMO(rule) => new _MemoRule(_compile(rule));
+
+_Rule TAG(tag, rule) => _compile([rule, (ast) => [tag, ast]]);
+
+
+class ParseError implements Exception {
+  const ParseError(String this._message);
+  String toString() => _message;
+  final String _message;
+}
+
+/**
+ * A grammar is a collection of symbols and rules that may be used to parse an
+ * input.
+ */
+class Grammar {
+  Map<String, Symbol> _symbols;
+
+  /** This rule may be set by the user to define whitespace. */
+  _Rule _whitespace;
+
+  _Rule get whitespace() => _whitespace;
+  void set whitespace(rule) { _whitespace = _compile(rule); }
+
+  Grammar() {
+    _symbols = new Map<String, Symbol>();
+    whitespace = CHAR(' \t\r\n');
+  }
+
+  /**
+   * operator [] is used to find or create symbols. Symbols may appear in rules
+   * to define recursive rules.
+   */
+  Symbol operator [](String name) {
+    if (_symbols.containsKey(name))
+      return _symbols[name];
+    Symbol s = new Symbol(name, this);
+    _symbols[name] = s;
+    return s;
+  }
+
+  /**
+   * Parses the input string and returns the parsed AST, or throws an exception
+   * if the input can't be parsed.
+   */
+  parse(root, String text) {
+    for (var symbol in _symbols.getValues())
+      if (symbol._rule == null)
+        print('${symbol.name} is undefined');
+
+    var state = new _ParserState(text, whitespace: whitespace);
+    var match = _compile(root).match(state, 0);
+    if (match == null)
+      return diagnose(state);
+    var pos = match[0];
+    pos = _skip_whitespace(state, pos);
+    if (pos == state._end)
+      return match[1];
+    // TODO: Make this complain about expecting end of file.
+    return diagnose(state);
+  }
+
+  diagnose(state) {
+    var message = 'unexpected error';
+    if (!state.max_rule.isEmpty()) {
+      var s = new Set();
+      for (var rule in state.max_rule)
+        s.add(rule.description());
+      var tokens = new List<String>.from(s);
+      tokens.sort((a, b) =>
+                  a.startsWith("'") == b.startsWith("'")
+                      ? a.compareTo(b)
+                      : a.startsWith("'") ? +1 : -1);
+      var expected = Strings.join(tokens, ' or ');
+      var found = state.max_pos == state._end ? 'end of file'
+          : "'${state._text[state.max_pos]}'";
+      message = 'Expected $expected but found $found';
+    }
+    int start = state.max_pos;
+    int end = start;
+    while (start >= 1 && state._text[start - 1] != '\n') --start;
+    while (end < state._text.length && state._text[end] != '\n') ++end;
+    var line = state._text.substring(start, end);
+    var indicator = '';
+    for (var i = 0; i < line.length && start + i < state.max_pos; i++)
+      indicator = indicator + ' ';
+    indicator = indicator + '^';
+    // TODO: Convert to an exception.
+    print(message);
+    print(line);
+    print(indicator);
+    return null;
+  }
+}
+
+class Symbol {
+  final String name;
+  final Grammar grammar;
+  _Rule _rule;
+
+  Symbol(this.name, this.grammar);
+
+  void set def(rule) {
+    assert(_rule == null);  // Assign once.
+    _rule = _compile(rule);
+  }
+
+  toString() => _rule == null ? '<$name>' : '<$name = $_rule>';
+}
+
+
+class _ParserState {
+  _ParserState(this._text, [_Rule whitespace = null]) {
+    _end = this._text.length;
+    whitespaceRule = whitespace;
+    max_rule = [];
+  }
+
+  String _text;
+  int _end;
+
+  //
+  bool inWhitespaceMode = false;
+  _Rule whitespaceRule = null;
+
+  // Used for constructing an error message.
+  int inhibitExpectedTrackingDepth = 0;
+  int max_pos = 0;
+  var max_rule;
+}
+
+/**
+ * An interface tag for rules. If this tag is on a rule, then the description()
+ * of the rule is something sensible to put in a message.
+ */
+interface _Expectable {
+  String description();
+}
+
+class _Rule {
+  const _Rule();
+  // Returns null for a match failure or [pos, ast] for success.
+  match(_ParserState state, int pos) {
+    if (! state.inWhitespaceMode) {
+      pos = _skip_whitespace(state, pos);
+    }
+    return matchAfterWS(state, pos);
+  }
+
+  // Faster entry point for matching a sub-rule that is matched to the start
+  // position of the super-rule.  Whitespace has already been skipped so no need
+  // to try to skip it again.
+  matchAfterWS(_ParserState state, int pos) {
+    if (state.inhibitExpectedTrackingDepth == 0) {
+      // Track position for possible error messaging
+      if (pos > state.max_pos) {
+        // Store position and the rule.
+        state.max_pos = pos;
+        if (this is _Expectable) {
+          state.max_rule = [this];
+        } else {
+          state.max_rule = [];
+        }
+      } else if (pos == state.max_pos) {
+        if (this is _Expectable) {
+          state.max_rule.add(this);
+        }
+      }
+    }
+    // Delegate the matching logic to the the specialized function.
+    return _match(state, pos);
+  }
+
+  // Overridden in subclasses to match the rule.
+  _match(_ParserState state, int pos) => null;
+
+  // Does the rule generate a value (AST) with the match?
+  bool get generatesValue() => false;
+
+  get defaultValue() => null;
+}
+
+int _skip_whitespace(state, pos) {
+  // Returns the next non-whitespace position.
+  // This is done by matching the optional whitespaceRule with the current text.
+  if (state.whitespaceRule == null)
+    return pos;
+  state.inWhitespaceMode = true;
+  state.inhibitExpectedTrackingDepth++;
+  while (true) {
+    var match = state.whitespaceRule.match(state, pos);
+    if (match == null)
+      break;
+    pos = match[0];
+  }
+  state.inWhitespaceMode = false;
+  state.inhibitExpectedTrackingDepth--;
+  return pos;
+}
+
+
+_Rule _compileOptional(rule) {
+  return rule == null ? null : _compile(rule);
+}
+
+_Rule _compile(rule) {
+  if (rule is _Rule)
+    return rule;
+  if (rule is String)
+    return new _StringRule(rule);
+  if (rule is Symbol)
+    return new _SymbolRule(rule);
+  if (rule is RegExp)
+    return new _RegExpRule(rule);
+  if (rule is List) {
+    return _compileMultiRule(
+        rule, true,
+        (compiledRules, valueCount, reducer) =>
+            new _SequenceRule(compiledRules, valueCount, reducer));
+  }
+  throw new Exception('Cannot compile rule: $rule');
+}
+
+class _EndOfInputRule extends _Rule {
+  _match(_ParserState state, int pos) {
+    if (pos == state._end)
+      return [pos, null];
+    return null;
+  }
+
+  toString() => 'END';
+}
+
+class _ErrorRule extends _Rule {
+  String message;
+  _ErrorRule(String this.message);
+  _match(_ParserState state, int pos) {
+    throw new ParseError(message);
+  }
+
+  toString() => 'ERROR($message)';
+}
+
+class _CharCodeRule extends _Rule {
+  Function _predicate;
+  var _name;
+  _CharCodeRule(this._predicate, this._name);
+  _match(_ParserState state, int pos) {
+    if (pos == state._end)
+      return null;
+    int code = state._text.charCodeAt(pos);
+    if (_predicate(code))
+      return [pos + 1, null];
+    return null;
+  }
+
+  toString() => _name == null ? 'CHARCODE($_predicate)' : 'CHARCODE($_name)';
+}
+
+class _AnyCharRule extends _Rule {
+  const _AnyCharRule();
+  _match(_ParserState state, int pos) {
+    if (pos == state._end)
+      return null;
+    return [pos + 1, null];
+  }
+
+  toString() => 'CHAR()';
+}
+
+class _SymbolRule extends _Rule {
+  final Symbol _symbol;
+  _SymbolRule(Symbol this._symbol);
+  _match(_ParserState state, int pos) {
+    if (_symbol._rule == null)
+      throw new Exception("Symbol '${_symbol.name}' is undefined");
+    return _symbol._rule.match(state, pos);
+  }
+
+  bool get generatesValue() => true;
+
+  toString() => '<${_symbol.name}>';
+}
+
+class _SkipRule extends _Rule {
+  // A rule that has no value.
+  _Rule _rule;
+  _SkipRule(_Rule this._rule);
+  _match(_ParserState state, int pos) {
+    var match = _rule.matchAfterWS(state, pos);
+    if (match == null)
+      return null;
+    return [match[0], null];
+  }
+
+  toString() => 'TOKEN($_rule)';
+}
+
+class _StringRule extends _Rule implements _Expectable {
+  final String _string;
+  int _len;
+  _StringRule(this._string) {
+    _len = _string.length;
+  }
+
+  _match(_ParserState state, int pos) {
+    if (pos + _len > state._end)
+      return null;
+    for (int i = 0; i < _len; i++) {
+      if (state._text.charCodeAt(pos + i) != _string.charCodeAt(i))
+        return null;
+    }
+    return [pos + _len, null];
+  }
+
+  //get defaultValue() => _string;
+
+  toString() => '"$_string"';
+
+  description() => "'$_string'";
+}
+
+class _RegExpRule extends _Rule {
+  RegExp _re;
+  _RegExpRule(this._re) {
+    // There is no convenient way to match an anchored substring.
+    throw new Exception('RegExp matching not supported');
+  }
+
+  toString() => '"$_re"';
+}
+
+class _LexicalRule extends _Rule implements _Expectable {
+  final String _name;
+  final _Rule _rule;
+
+  _LexicalRule(String this._name, _Rule this._rule);
+
+  _match(_ParserState state, int pos) {
+    state.inWhitespaceMode = true;
+    state.inhibitExpectedTrackingDepth++;
+    var match = _rule.matchAfterWS(state, pos);
+    state.inhibitExpectedTrackingDepth--;
+    state.inWhitespaceMode = false;
+    return match;
+  }
+
+  toString() => _name;
+
+  description() => _name == null ? '?' : _name;
+}
+
+class _TextValueRule extends _Rule {
+  final _Rule _rule;
+  final _extract;  // Function
+
+  _TextValueRule(_Rule this._rule, Function this._extract);
+
+  _match(_ParserState state, int pos) {
+    var match = _rule.matchAfterWS(state, pos);
+    if (match == null) {
+      return null;
+    }
+    var endPos = match[0];
+    return [endPos, _extract(state._text, pos, endPos)];
+  }
+
+  bool get generatesValue() => true;
+
+  toString() => 'TEXT($_rule)';
+}
+
+_Rule _compileMultiRule(List rules,
+                        bool allowReducer,
+                        finish(compiledRules, valueCount, reducer)) {
+  int valueCount = 0;
+  List compiledRules = new List<_Rule>();
+  Function reducer;
+  for (var rule in rules) {
+    if (reducer != null)
+      throw new Exception('Reducer must be last in sequence: $rule');
+    if (rule is Function) {
+      if (allowReducer)
+        reducer = rule;
+      else
+        throw new Exception('Bad rule: "$rule"');
+    } else {
+      _Rule compiledRule = _compile(rule);
+      if (compiledRule.generatesValue)
+        ++valueCount;
+      compiledRules.add(compiledRule);
+    }
+  }
+  return finish(compiledRules, valueCount, reducer);
+}
+
+String _formatMultiRule(String functor, List rules) {
+  var sb = new StringBuffer(functor);
+  sb.add('(');
+  var separator = '';
+  for (var rule in rules) {
+    sb.add(separator);
+    sb.add(rule);
+    separator = ',';
+  }
+  sb.add(')');
+  return sb.toString();
+}
+
+class _SequenceRule extends _Rule {
+  // This rule matches the component rules in order.
+  final List<_Rule> _rules;
+  final int _generatingSubRules = 0;
+  final Function _reducer;
+  bool _generatesValue;
+  _SequenceRule(List<_Rule> this._rules,
+                int this._generatingSubRules,
+                Function this._reducer) {
+    _generatesValue = _generatingSubRules > 0 || _reducer != null;
+  }
+
+  _match(state, pos) {
+    var sequence = [];
+    for (var rule in _rules) {
+      var match = rule.match(state, pos);
+      if (match == null)
+        return null;
+      if (rule.generatesValue) {
+        var ast = match[1];
+        sequence.add(ast);
+      }
+      pos = match[0];
+    }
+    if (_reducer == null) {
+      if (_generatingSubRules == 0)
+        return [pos, null];
+      if (_generatingSubRules == 1)
+        return [pos, sequence[0]];
+      return [pos, sequence];
+    } else {
+      return [pos, _apply(_reducer, sequence)];
+    }
+  }
+
+  bool get generatesValue() => _generatesValue;
+
+  toString() => _formatMultiRule('SEQ', _rules);
+}
+
+class _ChoiceRule extends _Rule {
+  // This rule matches the first component rule that matches.
+  List<_Rule> _rules;
+  _ChoiceRule(List<_Rule> this._rules);
+
+  _match(state, pos) {
+    for (var rule in _rules) {
+      var match = rule.match(state, pos);
+      if (match != null) {
+        /*
+        if (!rule.generatesValue) {
+          var value = rule.defaultValue;
+          if (value != null)
+            return [match[0], value];
+        }
+        */
+        return match;
+      }
+    }
+    return null;
+  }
+
+  bool get generatesValue() => true;
+
+  toString() => _formatMultiRule('OR', _rules);
+}
+
+class _OptionalRule extends _Rule {
+  _Rule _rule;
+  _OptionalRule(_Rule this._rule);
+  _match(_ParserState state, int pos) {
+    var match = _rule.match(state, pos);
+    if (_rule.generatesValue)
+      return match == null
+          ? [pos, null]
+          : match;
+    return match == null
+        ? [pos, false]
+        : [match[0], true];
+  }
+
+  bool get generatesValue() => true;
+
+  toString() => 'MAYBE($_rule)';
+}
+
+class _ContextRule extends _Rule {
+  _Rule _rule;
+  _ContextRule(_Rule this._rule);
+  _match(_ParserState state, int pos) {
+    // TODO: protect error state.
+    var match = _rule._match(state, pos);
+    if (match == null)
+      return null;
+    return [pos, null];
+  }
+
+  toString() => 'AT($_rule)';
+}
+
+class _NegativeContextRule extends _Rule {
+  _Rule _rule;
+  _NegativeContextRule(_Rule this._rule);
+  _match(_ParserState state, int pos) {
+    // TODO: protect error state.
+    var match = _rule._match(state, pos);
+    if (match == null)
+      return [pos, null];
+    return null;
+  }
+
+  toString() => 'NOT($_rule)';
+}
+
+class _RepeatRule extends _Rule {
+  // Matches zero, one or more items.
+  _Rule _rule;
+  _Rule _separator;
+  int _min;
+
+  _RepeatRule(this._rule, this._separator, this._min);
+
+  _match(state, pos) {
+    // First match.
+    var match = _rule.match(state, pos);
+    if (match == null)
+      if (_min == 0)
+        return [pos, []];
+      else
+        return null;
+    pos = match[0];
+    var result = [match[1]];
+
+    // Subsequent matches:
+    while (true)  {
+      var newPos = pos;
+      if (_separator != null) {
+        match = _separator.match(state, pos);
+        if (match == null)
+          return [pos, result];
+        newPos = match[0];
+      }
+      match = _rule.match(state, newPos);
+      if (match == null)
+        return [pos, result];
+      pos = match[0];
+      result.add(match[1]);
+    }
+  }
+
+  bool get generatesValue() => true;
+
+  toString() => 'MANY(min:$_min, $_rule${_separator==null?'':", sep: $_separator"})';
+}
+
+class _MemoRule extends _Rule {
+  final _Rule _rule;
+
+  var parseInstance;
+
+  // A map from position to result.  Can this be replaced with something
+  // smaller?
+  // TODO: figure out how to discard the map and parseInstance after parsing.
+  Map<int,Object> map;
+
+  _MemoRule(this._rule);
+
+  _match(state, pos) {
+    // See if we are still parsing the same input.  Relies on the fact that the
+    // input is a string and strings are immutable.
+    if (parseInstance !== state._text) {
+      map = new Map<int,Object>();
+      parseInstance = state._text;
+    }
+    // TODO: does this have to check or preserve parse state (like
+    // inWhitespaceMode, error position info etc?)
+    // Stored result can be null (memoized failure).
+    if (map.containsKey(pos)) {
+      return map[pos];
+    }
+    var match = _rule.match(state, pos);
+    map[pos] = match;
+    return match;
+  }
+
+  bool get generatesValue() => _rule.generatesValue;
+
+  toString() => 'MEMO($_rule)';
+}
+
+_apply(fn, List args) {
+  switch (args.length) {
+    case 0: return fn();
+    case 1: return fn(args[0]);
+    case 2: return fn(args[0], args[1]);
+    case 3: return fn(args[0], args[1], args[2]);
+    case 4: return fn(args[0], args[1], args[2], args[3]);
+    case 5: return fn(args[0], args[1], args[2], args[3], args[4]);
+    case 6: return fn(args[0], args[1], args[2], args[3], args[4],
+                      args[5]);
+    case 7: return fn(args[0], args[1], args[2], args[3], args[4],
+                      args[5], args[6]);
+    case 8: return fn(args[0], args[1], args[2], args[3], args[4],
+                      args[5], args[6], args[7]);
+    case 9: return fn(args[0], args[1], args[2], args[3], args[4],
+                      args[5], args[6], args[7], args[8]);
+    case 10: return fn(args[0], args[1], args[2], args[3], args[4],
+                       args[5], args[6], args[7], args[8], args[9]);
+
+    default:
+      throw new Exception('Too many arguments in _apply: $args');
+  }
+}
+
+List _unspread(a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z) {
+  List list = new List();
+  add(element) { if (element != null) list.add(element); }
+  add(a);
+  add(b);
+  add(c);
+  add(d);
+  add(e);
+  add(f);
+  add(g);
+  add(h);
+  add(i);
+  add(j);
+  add(k);
+  add(l);
+  add(m);
+  add(n);
+  add(o);
+  add(p);
+  add(q);
+  add(r);
+  add(s);
+  add(t);
+  add(u);
+  add(v);
+  add(w);
+  add(x);
+  add(y);
+  add(z);
+  return list;
+}