198 lines
8.9 KiB
Plaintext
198 lines
8.9 KiB
Plaintext
[/==============================================================================
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Copyright (C) 2001-2015 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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[/////////////////////////////////////////////////////////////////////////////]
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[section:primitive_attributes Attributes of Primitive Components]
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Parsers in __spirit__ are fully attributed. __x3__ parsers always /expose/ an
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attribute specific to their type. This is called /synthesized attribute/ as it
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is returned from a successful match representing the matched input sequence. For
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instance, numeric parsers, such as `int_` or `double_`, return the `int` or
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`double` value converted from the matched input sequence. Other primitive parser
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components have other intuitive attribute types, such as for instance `int_`
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which has `int`, or `ascii::char_` which has `char`. Primitive parsers apply the
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normal C++ convertibility rules: you can use any C++ type to receive the parsed
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value as long as the attribute type of the parser is convertible to the type
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provided. The following example shows how a synthesized parser attribute (the
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`int` value) is extracted by calling the API function `x3::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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x3::parse(strbegin, str.end(), int_, value); // value == 123
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For a full list of available parser primitives and their attribute types please
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see the sections __sec_x3_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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__x3__ implement well defined attribute type propagation rules for all compound
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parsers, such as sequences, alternatives, Kleene star, etc. The main attribute
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propagation rule for a sequences is for instance:
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a: A, b: B --> (a >> b): tuple<A, B>
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which reads as:
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[:Given `a` and `b` are parsers, and `A` is the attribute type of `a`, and `B`
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is the attribute type of `b`, then the attribute type of `a >> b` (`a << b`)
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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 `boost::fusion::tuple<A, B>`
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or `std::pair<A, B>` (for more information 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 parsers consume attributes
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see the sections __sec_x3_compound__.
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[heading The Attribute of Sequence Parsers]
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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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double_ >> double_
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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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x3::phrase_parse(strbegin, input.end(),
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x3::double_ >> x3::double_, // parser grammar
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x3::space, // delimiter grammar
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p); // attribute to fill while parsing
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[tip *For sequences only:* To keep it simple, unlike __Spirit.qi__, __x3__ does
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not support more than one attribute anymore in the `parse` and `phrase_parse` function.
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Just use `std:tuple'. Be sure to include `boost/fusion/adapted/std_tuple.hpp' in this case.
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]
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[heading The Attribute of Alternative Parsers]
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Alternative parsers 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 simplifying 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, 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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'(' >> double_ >> ", " >> double_ >> ')'
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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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x3::parse(strbegin, input.end(),
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'(' >> x3::double_ >> ", " >> x3::double_ >> ')', // parser grammar
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p); // attribute to fill while parsing
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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_` parser.
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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 __x3__'s
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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 *For sequences only:* To keep it simple, unlike __Spirit.qi__, __x3__ does
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not support more than one attribute anymore in the `parse` and `phrase_parse` function.
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Just use `std:tuple'. Be sure to include `boost/fusion/adapted/std_tuple.hpp' in this case.
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]
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Let's take a look at this from a more formal perspective:
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a: A, b: Unused --> (a >> b): A
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which reads as:
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[:Given `a` and `b` are parsers, 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 Nonterminals]
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Nonterminals are the main means of constructing more complex parsers out of
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simpler ones. The nonterminals in the parser world are very similar to functions
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in an imperative programming language. They can be used to encapsulate parser
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expressions for a particular input sequence. After being defined, the
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nonterminals can be used as 'normal' parsers in more complex expressions
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whenever the encapsulated input needs to be recognized. Parser nonterminals in
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__x3__ usually return a value (the synthesized attribute).
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The type of the synthesized attribute as to be explicitly specified while
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defining the particular nonterminal. Example (ignore ID for now):
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x3::rule<ID, int> r;
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[endsect]
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