[/==============================================================================
    Copyright (C) 2001-2008 Joel de Guzman
    Copyright (C) 2001-2008 Hartmut Kaiser

    Distributed under the Boost Software License, Version 1.0. (See accompanying
    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
===============================================================================/]

[section:lexer_quickstart3 Quickstart 3 - Counting Words Using a Parser]

The whole purpose of integrating __lex__ as part of the __spirit__ library was 
to add a library allowing to merge lexical analysis with the parsing 
process as defined by a __spirit__ grammar. __spirit__ parsers read their input 
from an input sequence accessed by iterators. So naturally, we chose iterators 
to be used as the interface beween the lexer and the parser. A second goal of 
the lexer/parser integration was to enable the usage of possibly different 
lexical analyzer libraries. The utilization of iterators seemed to be the 
right choice from this standpoint as well, mainly because these can be used as
an abstraction layer hiding implementation specifics of the used lexer 
library. The [link spirit.lex.flowcontrol picture] below shows the common 
flow control implemented while parsing combined with lexical analysis.

[fig ./images/FlowOfControl.png..The common flow control implemented while parsing combined with lexical analysis..spirit.lex.flowcontrol]

Another problem related to the integration of the lexical analyzer with the 
parser was to find a way how the defined tokens syntactically could be blended
with the grammar definition syntax of __spirit__. For tokens defined as 
instances of the `token_def<>` class the most natural way of integration was 
to allow to directly use these as parser components. Semantically these parser
components succeed matching their input whenever the corresponding token type
has been matched by the lexer. This quick start example will demonstrate this
(and more) by counting words again, simply by adding up the numbers inside
of semantic actions of a parser (for the full example code see here: 
[@../../example/lex/word_count.cpp word_count.cpp]).


[import ../example/lex/word_count.cpp]


[heading Prerequisites]

This example uses two of the __spirit__ library components: __lex__ and __qi__,
consequently we have to `#include` the corresponding header files. Again, we
need to include a couple of header files from the __phoenix2__ library. This 
example shows how to attach functors to parser components, which 
could be done using any type of C++ technique resulting in a callable object.
Using __phoenix2__ for this task simplifies things and avoids adding 
dependencies to other libraries (__phoenix2__ is already in use for 
__spirit__ anyway).

[wcp_includes]

To make all the code below more readable we introduce the following namespaces.

[wcp_namespaces]


[heading Defining Tokens]

If compared to the two previous quick start examples (__sec_lex_quickstart_1__
and __sec_lex_quickstart_2__) the token definition class for this example does 
not reveal any surprises. However, it uses lexer token definition macros to 
simplify the composition of the regular expressions, which will be described in 
more detail in the section __fixme__. Generally, any token definition is usable
without modification either for a standalone lexical analyzer or in conjunction
with a parser.

[wcp_token_definition]


[heading Using Token Definition Instances as Parsers]

While the integration of lexer and parser in the control flow is achieved by 
using special iterators wrapping the lexical analyzer, we still nead a means of
expressing in the grammar what tokens to match and where. The token definition 
class above uses three different ways of defining a token:

* Using an instance of a `token_def<>`, which is handy whenever you need to 
  specify a token attribute (for more information about lexer related 
  attributes please look here: __sec_lex_attributes__).
* Using a single character as the token, in this case the character represents
  itself as a token, where the token id is the ASCII character value.
* Using a regular expression represented as a string, where the token id needs 
  to be specified explicitly to make the token accessible from the grammar 
  level.

All three token definition methods require a different method of grammar 
integration. But as you can see from the following code snippet, each of this 
methods is straightforward and blends the corresponding token instance 
naturally with the surrounding __qi__ grammar syntax.

[table
    [[Token definition]   [Parser integration]]
    [[`token_def<>`]      [The `token_def<>` instance is directly usable as a 
                           parser component. Parsing of this component will 
                           succeed if the regular expression used to define 
                           this has been matched successfully.]]
    [[single character]   [The single character is directly usable in the 
                           grammar, under certain circumstances it needs to be 
                           wrapped by a `char_()` parser component, though.
                           Parsing of this component will succeed if the 
                           single character has been matched.]]
    [[explicit token id]  [To use an explicit token id in a __qi__ grammar you
                           are required to wrap it with the special `token()` 
                           parser component. Parsing of this component will 
                           succeed if the current token has the same token
                           id as specified in the expression `token(<id>)`.]]
]

The grammar definition below uses each of the three types demonstrating their
usage.

[wcp_grammar_definition]

As already described (see: __sec_qi_karma_attributes__), the __qi__ parser 
library builds upon a set of of fully attributed parser components. 
Consequently, all the token definitions do support the this attribute model as
well. The most natural way of implementing this was to use the token values as 
the attributes exposed by the parser component corresponding to the token 
definition (you can read more about this topic here: __sec_lex_tokenvalues__). 
The example above takes advantage of the full integration of the token values 
as the `token_def<>`'s parser attributes: the `word` token definition is 
declared as a `token_def<std::string>`, making every instance of a `word` token
carry the string representation of the matched input sequence as its value.
The semantic action attached to `tok.word` receives this string (represented by 
the `_1` placeholder) and uses it to calculate the number of matched
characters: `ref(c) += size(_1)`.

[important  All placeholders (`_1`, `_2`, etc.) used in /parser/ semantic 
            actions in conjunction with functors created based on __phoenix2__ 
            need to be imported from the `namespace boost::spirit::arg_names` 
            (and *not* `namespace boost::phoenix::arg_names`, which is 
            different from using placeholders in __lex__). 
            Using the wrong placeholders leads to subtle compilation errors 
            which are difficult to backtrack to their cause.
]


[heading Pulling Everything Together]

The main function needs to implement a bit more logic now as we have to 
initialize and start not only the lexical analysis but the parsing process as 
well. The three type definitions (`typedef` statements) simplify the creation 
of the lexical analyzer and the grammar. After reading the contents of the 
given file into memory it calls the function __api_tokenize_and_parse__ to 
initialize the lexical analysis and parsing processes.

[wcp_main]


[endsect]
