[/==============================================================================
    Copyright (C) 2001-2011 Hartmut Kaiser
    Copyright (C) 2001-2011 Joel de Guzman

    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:distinct Qi Distinct Parser Directive]

[heading Description]

The __qi__ `distinct` parser is a directive component allowing to avoid partial 
matches while parsing using a skipper. A simple example is the common task of 
matching a C keyword. 
Consider:

    "description" >> -lit(":") >> *(char_ - eol)

intended to match a line in a configuration file. Let's assume further, that 
this rule is used with a `space` skipper and that we have the following strings 
in the input:

    "description: ident\n"
    "description ident\n"
    "descriptionident\n"

It might seem unexpected, but the parser above matches all three inputs just 
fine, even if the third input should not match at all! In order to avoid the 
unwanted match we are forced to make our rule more complicated:

    lexeme["description" >> !char_("a-zA-Z_0-9")] >> -lit(":") >> *(char_ - eol)

(the rule reads as: match `"description"` as long as it's not /directly/ 
followed by a valid identifier).

The `distinct[]` directive is meant to simplify the rule above:

    distinct(char_("a-zA-Z_0-9"))["description"] >> -lit(":") >> *(char_ - eol)

Using the `distinct[]` component instead of the explicit sequence has the 
advantage of being able to encapsulate the tail (i.e the `char_("a-zA-Z_0-9")`) 
as a separate parser construct. The following code snippet illustrates the idea 
(for the full code of this example please see 
[@../../test/qi/distinct.cpp distinct.cpp]):

[import ../test/qi/distinct.cpp]
[qi_distinct_encapsulation]

These definitions define a new Qi parser recognizing keywords! This allows to 
rewrite our declaration parser expression as:

    keyword["description"] >> -lit(":") >> *(char_ - eol)

which is much more readable and concise if compared to the original parser 
expression. In addition the new `keyword[]` directive has the advantage to be 
usable for wrapping any parser expression, not only strings as in the example 
above.

[heading Header]

    // forwards to <boost/spirit/repository/home/qi/directive/distinct.hpp>
    #include <boost/spirit/repository/include/qi_distinct.hpp>

[heading Synopsis]

    distinct(tail)[subject]

[heading Parameters]

[table
    [[Parameter]            [Description]]
    [[`tail`]               [The parser construct specifying what whould not
                             follow the subject in order to match the overall 
                             expression.]]
    [[`subject`]            [The parser construct to use to match the current
                             input. The distinct directive makes sure that no
                             unexpected partial matches occur.]]
]

All two parameters can be arbitrary complex parsers themselves.

[heading Attribute]

The `distinct` component exposes the attribute type of its subject as its own 
attribute type. If the `subject` does not expose any attribute (the type is
`unused_type`), then the `distinct` does not expose any attribute either.

    a: A, b: B --> distinct(b)[a]: A

[heading Example]

The following example shows simple use cases of the `distinct` parser. 
[@../../example/qi/distinct.cpp distinct.cpp])

[import ../example/qi/distinct.cpp]

[heading Prerequisites]

In addition to the main header file needed to include the core components 
implemented in __qi__ we add the header file needed for the new `distinct` 
generator.

[qi_distinct_includes]

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

[qi_distinct_namespace]

[heading Using The Distinct Directive to Match keywords]

We show several examples of how the `distinct[]` directive can be used to force
correct behavior while matching keywords. The first two code snippets show 
the correct matching of the `description` keyword (in this hypothetical example 
we allow keywords to be directly followed by an optional `"--"`):

[qi_distinct_description_ident]
[qi_distinct_description__ident]

The last example shows that the `distinct[]` parser component correctly refuses
to match "description-ident":

[qi_distinct_description_ident_error]

[endsect]