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petitparser/lib/src/core/expression.dart

-part of petitparser;
-
-/**
- * A builder that allows the simple definition of expression grammars with
- * prefix, postfix, and left- and right-associative infix operators.
- *
- * The following code creates the empty expression builder:
- *
- *     var builder = new ExpressionBuilder();
- *
- * Then we define the operator-groups in descending precedence. The highest
- * precedence have the literal numbers themselves:
- *
- *     builder.group()
- *       ..primitive(digit().plus()
- *         .seq(char('.').seq(digit().plus()).optional())
- *         .flatten().trim().map((a) => double.parse(a)));
- *
- * Then come the normal arithmetic operators. Note, that the action blocks receive
- * both, the terms and the parsed operator in the order they appear in the parsed
- * input.
- *
- *     // negation is a prefix operator
- *     builder.group()
- *       ..prefix(char('-').trim(), (op, a) => -a);
- *
- *     // power is right-associative
- *     builder.group()
- *       ..right(char('^').trim(), (a, op, b) => math.pow(a, b));
- *
- *     // multiplication and addition is left-associative
- *     builder.group()
- *       ..left(char('*').trim(), (a, op, b) => a * b)
- *       ..left(char('/').trim(), (a, op, b) => a / b);
- *     builder.group()
- *       ..left(char('+').trim(), (a, op, b) => a + b)
- *       ..left(char('-').trim(), (a, op, b) => a - b);
- *
- * Finally we can build the parser:
- *
- *     var parser = builder.build();
- *
- * After executing the above code we get an efficient parser that correctly
- * evaluates expressions like:
- *
- *     parser.parse('-8');      // -8
- *     parser.parse('1+2*3');   // 7
- *     parser.parse('1*2+3');   // 5
- *     parser.parse('8/4/2');   // 2
- *     parser.parse('2^2^3');   // 256
- */
-class ExpressionBuilder {
-  final List<ExpressionGroup> _groups = new List();
-
-  /**
-   * Creates a new group of operators that share the same priority.
-   */
-  ExpressionGroup group() {
-    var group = new ExpressionGroup();
-    _groups.add(group);
-    return group;
-  }
-
-  /**
-   * Builds the expression parser.
-   */
-  Parser build() => _groups.fold(
-      failure('Highest priority group should define a primitive parser.'),
-      (a, b) => b._build(a));
-}
-
-/**
- * Models a group of operators of the same precedence.
- */
-class ExpressionGroup {
-
-  /**
-   * Defines a new primitive or literal [parser].
-   */
-  void primitive(Parser parser) {
-    _primitives.add(parser);
-  }
-
-  Parser _build_primitive(Parser inner) {
-    return _build_choice(_primitives, inner);
-  }
-
-  final List<Parser> _primitives = new List();
-
-  /**
-   * Adds a prefix operator [parser]. Evaluates the optional [action] with the
-   * parsed `operator` and `value`.
-   */
-  void prefix(Parser parser, [action(operator, value)]) {
-    if (action == null) {
-      action = (operator, value) => [operator, value];
-    }
-    _prefix.add(parser.map((operator) => new _ExpressionResult(operator, action)));
-  }
-
-  Parser _build_prefix(Parser inner) {
-    if (_prefix.isEmpty) {
-      return inner;
-    } else {
-      return new SequenceParser([_build_choice(_prefix).star(), inner]).map(
-          (tuple) {
-        return tuple.first.reversed.fold(tuple.last, (value, result) {
-          return result.action(result.operator, value);
-        });
-      });
-    }
-  }
-
-  final List<Parser> _prefix = new List();
-
-  /**
-   * Adds a postfix operator [parser]. Evaluates the optional [action] with the
-   * parsed `value` and `operator`.
-   */
-  void postfix(Parser parser, [action(value, operator)]) {
-    if (action == null) {
-      action = (value, operator) => [value, operator];
-    }
-    _postfix.add(parser.map((operator) => new _ExpressionResult(operator, action)));
-  }
-
-  Parser _build_postfix(Parser inner) {
-    if (_postfix.isEmpty) {
-      return inner;
-    } else {
-      return new SequenceParser([inner, _build_choice(_postfix).star()]).map(
-          (tuple) {
-        return tuple.last.fold(tuple.first, (value, result) {
-          return result.action(value, result.operator);
-        });
-      });
-    }
-  }
-
-  final List<Parser> _postfix = new List();
-
-  /**
-   * Adds a right-associative operator [parser]. Evaluates the optional [action] with
-   * the parsed `left` term, `operator`, and `right` term.
-   */
-  void right(Parser parser, [action(left, operator, right)]) {
-    if (action == null) {
-      action = (left, operator, right) => [left, operator, right];
-    }
-    _right.add(parser.map((operator) => new _ExpressionResult(operator, action)));
-  }
-
-  Parser _build_right(Parser inner) {
-    if (_right.isEmpty) {
-      return inner;
-    } else {
-      return inner.separatedBy(_build_choice(_right)).map((sequence) {
-        var result = sequence.last;
-        for (var i = sequence.length - 2; i > 0; i -= 2) {
-          result =
-              sequence[i].action(sequence[i - 1], sequence[i].operator, result);
-        }
-        return result;
-      });
-    }
-  }
-
-  final List<Parser> _right = new List();
-
-  /**
-   * Adds a left-associative operator [parser]. Evaluates the optional [action] with
-   * the parsed `left` term, `operator`, and `right` term.
-   */
-  void left(Parser parser, [action(left, operator, right)]) {
-    if (action == null) {
-      action = (left, operator, right) => [left, operator, right];
-    }
-    _left.add(parser.map((operator) => new _ExpressionResult(operator, action)));
-  }
-
-  Parser _build_left(Parser inner) {
-    if (_left.isEmpty) {
-      return inner;
-    } else {
-      return inner.separatedBy(_build_choice(_left)).map((sequence) {
-        var result = sequence.first;
-        for (var i = 1; i < sequence.length; i += 2) {
-          result =
-              sequence[i].action(result, sequence[i].operator, sequence[i + 1]);
-        }
-        return result;
-      });
-    }
-  }
-
-  final List<Parser> _left = new List();
-
-  // helper to build an optimal choice parser
-  Parser _build_choice(List<Parser> parsers, [Parser otherwise]) {
-    if (parsers.isEmpty) {
-      return otherwise;
-    } else if (parsers.length == 1) {
-      return parsers.first;
-    } else {
-      return new ChoiceParser(parsers);
-    }
-  }
-
-  // helper to build the group of parsers
-  Parser _build(Parser inner) {
-    return _build_left(_build_right(_build_postfix(_build_prefix(_build_primitive(inner)))));
-  }
-}
-
-// helper class to associate operators and actions
-class _ExpressionResult {
-  final operator;
-  final Function action;
-  _ExpressionResult(this.operator, this.action);
-}