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| 1 part of petitparser; | |
| 2 | |
| 3 /// A builder that allows the simple definition of expression grammars with | |
| 4 /// prefix, postfix, and left- and right-associative infix operators. | |
| 5 /// | |
| 6 /// The following code creates the empty expression builder: | |
| 7 /// | |
| 8 /// var builder = new ExpressionBuilder(); | |
| 9 /// | |
| 10 /// Then we define the operator-groups in descending precedence. The highest | |
| 11 /// precedence have the literal numbers themselves: | |
| 12 /// | |
| 13 /// builder.group() | |
| 14 /// ..primitive(digit().plus() | |
| 15 /// .seq(char('.').seq(digit().plus()).optional()) | |
| 16 /// .flatten().trim().map((a) => double.parse(a))); | |
| 17 /// | |
| 18 /// Then come the normal arithmetic operators. Note, that the action blocks rece
ive | |
| 19 /// both, the terms and the parsed operator in the order they appear in the pars
ed | |
| 20 /// input. | |
| 21 /// | |
| 22 /// // negation is a prefix operator | |
| 23 /// builder.group() | |
| 24 /// ..prefix(char('-').trim(), (op, a) => -a); | |
| 25 /// | |
| 26 /// // power is right-associative | |
| 27 /// builder.group() | |
| 28 /// ..right(char('^').trim(), (a, op, b) => math.pow(a, b)); | |
| 29 /// | |
| 30 /// // multiplication and addition is left-associative | |
| 31 /// builder.group() | |
| 32 /// ..left(char('*').trim(), (a, op, b) => a * b) | |
| 33 /// ..left(char('/').trim(), (a, op, b) => a / b); | |
| 34 /// builder.group() | |
| 35 /// ..left(char('+').trim(), (a, op, b) => a + b) | |
| 36 /// ..left(char('-').trim(), (a, op, b) => a - b); | |
| 37 /// | |
| 38 /// Finally we can build the parser: | |
| 39 /// | |
| 40 /// var parser = builder.build(); | |
| 41 /// | |
| 42 /// After executing the above code we get an efficient parser that correctly | |
| 43 /// evaluates expressions like: | |
| 44 /// | |
| 45 /// parser.parse('-8'); // -8 | |
| 46 /// parser.parse('1+2*3'); // 7 | |
| 47 /// parser.parse('1*2+3'); // 5 | |
| 48 /// parser.parse('8/4/2'); // 2 | |
| 49 /// parser.parse('2^2^3'); // 256 | |
| 50 class ExpressionBuilder { | |
| 51 final List<ExpressionGroup> _groups = new List(); | |
| 52 | |
| 53 /// Creates a new group of operators that share the same priority. | |
| 54 ExpressionGroup group() { | |
| 55 var group = new ExpressionGroup(); | |
| 56 _groups.add(group); | |
| 57 return group; | |
| 58 } | |
| 59 | |
| 60 /// Builds the expression parser. | |
| 61 Parser build() => _groups.fold( | |
| 62 failure('Highest priority group should define a primitive parser.'), | |
| 63 (a, b) => b._build(a)); | |
| 64 } | |
| 65 | |
| 66 /// Models a group of operators of the same precedence. | |
| 67 class ExpressionGroup { | |
| 68 | |
| 69 /// Defines a new primitive or literal [parser]. | |
| 70 void primitive(Parser parser) { | |
| 71 _primitives.add(parser); | |
| 72 } | |
| 73 | |
| 74 Parser _buildPrimitive(Parser inner) { | |
| 75 return _buildChoice(_primitives, inner); | |
| 76 } | |
| 77 | |
| 78 final List<Parser> _primitives = new List(); | |
| 79 | |
| 80 /// Adds a prefix operator [parser]. Evaluates the optional [action] with the | |
| 81 /// parsed `operator` and `value`. | |
| 82 void prefix(Parser parser, [action(operator, value)]) { | |
| 83 if (action == null) { | |
| 84 action = (operator, value) => [operator, value]; | |
| 85 } | |
| 86 _prefix.add(parser.map((operator) => new _ExpressionResult(operator, action)
)); | |
| 87 } | |
| 88 | |
| 89 Parser _buildPrefix(Parser inner) { | |
| 90 if (_prefix.isEmpty) { | |
| 91 return inner; | |
| 92 } else { | |
| 93 return new SequenceParser([_buildChoice(_prefix).star(), inner]).map( | |
| 94 (tuple) { | |
| 95 return tuple.first.reversed.fold(tuple.last, (value, result) { | |
| 96 return result.action(result.operator, value); | |
| 97 }); | |
| 98 }); | |
| 99 } | |
| 100 } | |
| 101 | |
| 102 final List<Parser> _prefix = new List(); | |
| 103 | |
| 104 /// Adds a postfix operator [parser]. Evaluates the optional [action] with the | |
| 105 /// parsed `value` and `operator`. | |
| 106 void postfix(Parser parser, [action(value, operator)]) { | |
| 107 if (action == null) { | |
| 108 action = (value, operator) => [value, operator]; | |
| 109 } | |
| 110 _postfix.add(parser.map((operator) => new _ExpressionResult(operator, action
))); | |
| 111 } | |
| 112 | |
| 113 Parser _buildPostfix(Parser inner) { | |
| 114 if (_postfix.isEmpty) { | |
| 115 return inner; | |
| 116 } else { | |
| 117 return new SequenceParser([inner, _buildChoice(_postfix).star()]).map( | |
| 118 (tuple) { | |
| 119 return tuple.last.fold(tuple.first, (value, result) { | |
| 120 return result.action(value, result.operator); | |
| 121 }); | |
| 122 }); | |
| 123 } | |
| 124 } | |
| 125 | |
| 126 final List<Parser> _postfix = new List(); | |
| 127 | |
| 128 /// Adds a right-associative operator [parser]. Evaluates the optional [action
] with | |
| 129 /// the parsed `left` term, `operator`, and `right` term. | |
| 130 void right(Parser parser, [action(left, operator, right)]) { | |
| 131 if (action == null) { | |
| 132 action = (left, operator, right) => [left, operator, right]; | |
| 133 } | |
| 134 _right.add(parser.map((operator) => new _ExpressionResult(operator, action))
); | |
| 135 } | |
| 136 | |
| 137 Parser _buildRight(Parser inner) { | |
| 138 if (_right.isEmpty) { | |
| 139 return inner; | |
| 140 } else { | |
| 141 return inner.separatedBy(_buildChoice(_right)).map((sequence) { | |
| 142 var result = sequence.last; | |
| 143 for (var i = sequence.length - 2; i > 0; i -= 2) { | |
| 144 result = | |
| 145 sequence[i].action(sequence[i - 1], sequence[i].operator, result); | |
| 146 } | |
| 147 return result; | |
| 148 }); | |
| 149 } | |
| 150 } | |
| 151 | |
| 152 final List<Parser> _right = new List(); | |
| 153 | |
| 154 /// Adds a left-associative operator [parser]. Evaluates the optional [action]
with | |
| 155 /// the parsed `left` term, `operator`, and `right` term. | |
| 156 void left(Parser parser, [action(left, operator, right)]) { | |
| 157 if (action == null) { | |
| 158 action = (left, operator, right) => [left, operator, right]; | |
| 159 } | |
| 160 _left.add(parser.map((operator) => new _ExpressionResult(operator, action)))
; | |
| 161 } | |
| 162 | |
| 163 Parser _buildLeft(Parser inner) { | |
| 164 if (_left.isEmpty) { | |
| 165 return inner; | |
| 166 } else { | |
| 167 return inner.separatedBy(_buildChoice(_left)).map((sequence) { | |
| 168 var result = sequence.first; | |
| 169 for (var i = 1; i < sequence.length; i += 2) { | |
| 170 result = | |
| 171 sequence[i].action(result, sequence[i].operator, sequence[i + 1]); | |
| 172 } | |
| 173 return result; | |
| 174 }); | |
| 175 } | |
| 176 } | |
| 177 | |
| 178 final List<Parser> _left = new List(); | |
| 179 | |
| 180 // helper to build an optimal choice parser | |
| 181 Parser _buildChoice(List<Parser> parsers, [Parser otherwise]) { | |
| 182 if (parsers.isEmpty) { | |
| 183 return otherwise; | |
| 184 } else if (parsers.length == 1) { | |
| 185 return parsers.first; | |
| 186 } else { | |
| 187 return new ChoiceParser(parsers); | |
| 188 } | |
| 189 } | |
| 190 | |
| 191 // helper to build the group of parsers | |
| 192 Parser _build(Parser inner) { | |
| 193 return _buildLeft(_buildRight(_buildPostfix(_buildPrefix(_buildPrimitive(inn
er))))); | |
| 194 } | |
| 195 } | |
| 196 | |
| 197 // helper class to associate operators and actions | |
| 198 class _ExpressionResult { | |
| 199 final operator; | |
| 200 final Function action; | |
| 201 _ExpressionResult(this.operator, this.action); | |
| 202 } | |
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Chromium Code Reviews
Issue 2989763002:
Update charted to 0.4.8 and roll (Closed)
