311 lines
15 KiB
Plaintext
311 lines
15 KiB
Plaintext
[/==============================================================================
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Copyright (C) 2001-2011 Hartmut Kaiser
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Copyright (C) 2001-2011 Joel de Guzman
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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===============================================================================/]
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[section:attributes Attributes]
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[/////////////////////////////////////////////////////////////////////////////]
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[section:primitive_attributes Attributes of Primitive Components]
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Parsers and generators in __spirit__ are fully attributed. __qi__ parsers always
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/expose/ an attribute specific to their type. This is called /synthesized
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attribute/ as it is returned from a successful match representing the matched
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input sequence. For instance, numeric parsers, such as `int_` or `double_`,
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return the `int` or `double` value converted from the matched input sequence.
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Other primitive parser components have other intuitive attribute types, such as
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for instance `int_` which has `int`, or `ascii::char_` which has `char`. For
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primitive parsers apply the normal C++ convertibility rules: you can use any
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C++ type to receive the parsed value as long as the attribute type of the
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parser is convertible to the type provided. The following example shows how a
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synthesized parser attribute (the `int` value) is extracted by calling the
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API function `qi::parse`:
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int value = 0;
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std::string str("123");
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std::string::iterator strbegin = str.begin();
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qi::parse(strbegin, str.end(), int_, value); // value == 123
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The attribute type of a generator defines what data types this generator is
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able to consume in order to produce its output. __karma__ generators always
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/expect/ an attribute specific to their type. This is called /consumed
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attribute/ and is expected to be passed to the generator. The consumed
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attribute is most of the time the value the generator is designed to emit
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output for. For primitive generators the normal C++ convertibility rules apply.
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Any data type convertible to the attribute type of a primitive generator can be
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used to provide the data to generate. We present a similar example as above,
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this time the consumed attribute of the `int_` generator (the `int` value)
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is passed to the API function `karma::generate`:
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int value = 123;
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std::string str;
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std::back_insert_iterator<std::string> out(str);
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karma::generate(out, int_, value); // str == "123"
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Other primitive generator components have other intuitive attribute types, very
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similar to the corresponding parser components. For instance, the
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`ascii::char_` generator has `char` as consumed attribute. For a full list of
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available parser and generator primitives and their attribute types please see
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the sections __sec_qi_primitive__ and __sec_karma_primitive__.
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[endsect]
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[/////////////////////////////////////////////////////////////////////////////]
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[section:compound_attributes Attributes of Compound Components]
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__qi__ and __karma__ implement well defined attribute type propagation rules
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for all compound parsers and generators, such as sequences, alternatives,
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Kleene star, etc. The main attribute propagation rule for a sequences is for
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instance:
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[table
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[[Library] [Sequence attribute propagation rule]]
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[[Qi] [`a: A, b: B --> (a >> b): tuple<A, B>`]]
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[[Karma] [`a: A, b: B --> (a << b): tuple<A, B>`]]
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]
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which reads as:
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[:Given `a` and `b` are parsers (generators), and `A` is the attribute type of
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`a`, and `B` is the attribute type of `b`, then the attribute type of
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`a >> b` (`a << b`) will be `tuple<A, B>`.]
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[note The notation `tuple<A, B>` is used as a placeholder expression for any
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fusion sequence holding the types A and B, such as
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`boost::fusion::tuple<A, B>` or `std::pair<A, B>` (for more information
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see __fusion__).]
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As you can see, in order for a type to be compatible with the attribute type
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of a compound expression it has to
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* either be convertible to the attribute type,
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* or it has to expose certain functionalities, i.e. it needs to conform to a
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concept compatible with the component.
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Each compound component implements its own set of attribute propagation rules.
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For a full list of how the different compound generators consume attributes
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see the sections __sec_qi_compound__ and __sec_karma_compound__.
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[heading The Attribute of Sequence Parsers and Generators]
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Sequences require an attribute type to expose the concept of a fusion sequence,
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where all elements of that fusion sequence have to be compatible with the
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corresponding element of the component sequence. For example, the expression:
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[table
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[[Library] [Sequence expression]]
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[[Qi] [`double_ >> double_`]]
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[[Karma] [`double_ << double_`]]
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]
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is compatible with any fusion sequence holding two types, where both types have
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to be compatible with `double`. The first element of the fusion sequence has to
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be compatible with the attribute of the first `double_`, and the second element
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of the fusion sequence has to be compatible with the attribute of the second
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`double_`. If we assume to have an instance of a `std::pair<double, double>`,
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we can directly use the expressions above to do both, parse input to fill the
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attribute:
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// the following parses "1.0 2.0" into a pair of double
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std::string input("1.0 2.0");
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std::string::iterator strbegin = input.begin();
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std::pair<double, double> p;
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qi::phrase_parse(strbegin, input.end(),
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qi::double_ >> qi::double_, // parser grammar
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qi::space, // delimiter grammar
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p); // attribute to fill while parsing
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and generate output for it:
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// the following generates: "1.0 2.0" from the pair filled above
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std::string str;
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std::back_insert_iterator<std::string> out(str);
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karma::generate_delimited(out,
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karma::double_ << karma::double_, // generator grammar (format description)
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karma::space, // delimiter grammar
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p); // data to use as the attribute
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(where the `karma::space` generator is used as the delimiter, allowing to
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automatically skip/insert delimiting spaces in between all primitives).
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[tip *For sequences only:* __qi__ and __karma__ expose a set of API functions
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usable mainly with sequences. Very much like the functions of the `scanf`
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and `printf` families these functions allow to pass the attributes for
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each of the elements of the sequence separately. Using the corresponding
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overload of /Qi's/ parse or /Karma's/ `generate()` the expression above
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could be rewritten as:
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``
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double d1 = 0.0, d2 = 0.0;
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qi::phrase_parse(begin, end, qi::double_ >> qi::double_, qi::space, d1, d2);
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karma::generate_delimited(out, karma::double_ << karma::double_, karma::space, d1, d2);
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``
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where the first attribute is used for the first `double_`, and
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the second attribute is used for the second `double_`.
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]
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[heading The Attribute of Alternative Parsers and Generators]
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Alternative parsers and generators are all about - well - alternatives. In
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order to store possibly different result (attribute) types from the different
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alternatives we use the data type __boost_variant__. The main attribute
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propagation rule of these components is:
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a: A, b: B --> (a | b): variant<A, B>
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Alternatives have a second very important attribute propagation rule:
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a: A, b: A --> (a | b): A
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often allowing to simplify things significantly. If all sub expressions of
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an alternative expose the same attribute type, the overall alternative
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will expose exactly the same attribute type as well.
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[endsect]
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[/////////////////////////////////////////////////////////////////////////////]
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[section:more_compound_attributes More About Attributes of Compound Components]
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While parsing input or generating output it is often desirable to combine some
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constant elements with variable parts. For instance, let us look at the example
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of parsing or formatting a complex number, which is written as `(real, imag)`,
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where `real` and `imag ` are the variables representing the real and imaginary
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parts of our complex number. This can be achieved by writing:
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[table
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[[Library] [Sequence expression]]
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[[Qi] [`'(' >> double_ >> ", " >> double_ >> ')'`]]
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[[Karma] [`'(' << double_ << ", " << double_ << ')'`]]
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]
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Fortunately, literals (such as `'('` and `", "`) do /not/ expose any attribute
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(well actually, they do expose the special type `unused_type`, but in this
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context `unused_type` is interpreted as if the component does not expose any
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attribute at all). It is very important to understand that the literals don't
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consume any of the elements of a fusion sequence passed to this component
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sequence. As said, they just don't expose any attribute and don't produce
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(consume) any data. The following example shows this:
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// the following parses "(1.0, 2.0)" into a pair of double
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std::string input("(1.0, 2.0)");
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std::string::iterator strbegin = input.begin();
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std::pair<double, double> p;
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qi::parse(strbegin, input.end(),
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'(' >> qi::double_ >> ", " >> qi::double_ >> ')', // parser grammar
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p); // attribute to fill while parsing
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and here is the equivalent __karma__ code snippet:
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// the following generates: (1.0, 2.0)
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std::string str;
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std::back_insert_iterator<std::string> out(str);
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generate(out,
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'(' << karma::double_ << ", " << karma::double_ << ')', // generator grammar (format description)
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p); // data to use as the attribute
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where the first element of the pair passed in as the data to generate is still
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associated with the first `double_`, and the second element is associated with
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the second `double_` generator.
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This behavior should be familiar as it conforms to the way other input and
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output formatting libraries such as `scanf`, `printf` or `boost::format` are
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handling their variable parts. In this context you can think about __qi__'s
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and __karma__'s primitive components (such as the `double_` above) as of being
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type safe placeholders for the attribute values.
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[tip Similarly to the tip provided above, this example could be rewritten
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using /Spirit's/ multi-attribute API function:
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``
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double d1 = 0.0, d2 = 0.0;
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qi::parse(begin, end, '(' >> qi::double_ >> ", " >> qi::double_ >> ')', d1, d2);
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karma::generate(out, '(' << karma::double_ << ", " << karma::double_ << ')', d1, d2);
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``
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which provides a clear and comfortable syntax, more similar to the
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placeholder based syntax as exposed by `printf` or `boost::format`.
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]
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Let's take a look at this from a more formal perspective. The sequence attribute
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propagation rules define a special behavior if generators exposing `unused_type`
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as their attribute are involved (see __sec_karma_compound__):
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[table
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[[Library] [Sequence attribute propagation rule]]
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[[Qi] [`a: A, b: Unused --> (a >> b): A`]]
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[[Karma] [`a: A, b: Unused --> (a << b): A`]]
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]
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which reads as:
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[:Given `a` and `b` are parsers (generators), and `A` is the attribute type of
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`a`, and `unused_type` is the attribute type of `b`, then the attribute type
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of `a >> b` (`a << b`) will be `A` as well. This rule applies regardless of
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the position the element exposing the `unused_type` is at.]
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This rule is the key to the understanding of the attribute handling in
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sequences as soon as literals are involved. It is as if elements with
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`unused_type` attributes 'disappeared' during attribute propagation. Notably,
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this is not only true for sequences but for any compound components. For
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instance, for alternative components the corresponding rule is:
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a: A, b: Unused --> (a | b): A
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again, allowing to simplify the overall attribute type of an expression.
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[endsect]
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[/////////////////////////////////////////////////////////////////////////////]
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[section:nonterminal_attributes Attributes of Rules and Grammars]
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Nonterminals are well known from parsers where they are used as the main means
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of constructing more complex parsers out of simpler ones. The nonterminals in
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the parser world are very similar to functions in an imperative programming
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language. They can be used to encapsulate parser expressions for a particular
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input sequence. After being defined, the nonterminals can be used as 'normal'
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parsers in more complex expressions whenever the encapsulated input needs to be
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recognized. Parser nonterminals in __qi__ may accept /parameters/ (inherited
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attributes) and usually return a value (the synthesized attribute).
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Both, the types of the inherited and the synthesized attributes have to be
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explicitly specified while defining the particular `grammar` or the `rule`
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(the Spirit __repo__ additionally has `subrules` which conform to a similar
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interface). As an example, the following code declares a __qi__ `rule`
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exposing an `int` as its synthesized attribute, while expecting a single
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`double` as its inherited attribute (see the section about the __qi__ __rule__
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for more information):
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qi::rule<Iterator, int(double)> r;
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In the world of generators, nonterminals are just as useful as in the parser
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world. Generator nonterminals encapsulate a format description for a particular
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data type, and, whenever we need to emit output for this data type, the
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corresponding nonterminal is invoked in a similar way as the predefined
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__karma__ generator primitives. The __karma__ [karma_nonterminal nonterminals]
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are very similar to the __qi__ nonterminals. Generator nonterminals may accept
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/parameters/ as well, and we call those inherited attributes too. The main
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difference is that they do not expose a synthesized attribute (as parsers do),
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but they require a special /consumed attribute/. Usually the consumed attribute
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is the value the generator creates its output from. Even if the consumed
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attribute is not 'returned' from the generator we chose to use the same
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function style declaration syntax as used in __qi__. The example below declares
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a __karma__ `rule` consuming a `double` while not expecting any additional
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inherited attributes.
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karma::rule<OutputIterator, double()> r;
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The inherited attributes of nonterminal parsers and generators are normally
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passed to the component during its invocation. These are the /parameters/ the
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parser or generator may accept and they can be used to parameterize the
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component depending on the context they are invoked from.
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[/
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* attribute propagation
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* explicit and operator%=
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]
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[endsect]
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[endsect] [/ Attributes]
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