[/==============================================================================
    Copyright (C) 2001-2015 Joel de Guzman
    Copyright (C) 2001-2011 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 Introduction]

Boost Spirit X3 is an object-oriented, recursive-descent parser for C++. It
allows you to write grammars using a format similar to
Extended Backus Naur Form (EBNF)[footnote
[@http://www.cl.cam.ac.uk/%7Emgk25/iso-14977.pdf ISO-EBNF]] directly in C++.
These inline grammar specifications can mix freely with other C++ code and,
thanks to the generative power of C++ templates, are immediately executable.
Conventional compiler-compilers or parser-generators have to perform an
additional translation step from the source EBNF code to C or C++ code.

Since the target input grammars are written entirely in C++ we do not need any
separate tools to compile, preprocess or integrate those into the build process.
__spirit__ allows seamless integration of the parsing process with other C++
code. This often allows for simpler and more efficient code.

The created parsers are fully attributed, which allows you to easily build and
handle hierarchical data structures in memory. These data structures resemble
the structure of the input data and can directly be used to generate
arbitrarily-formatted output.

A simple EBNF grammar snippet:

    group       ::= '(' expression ')'
    factor      ::= integer | group
    term        ::= factor (('*' factor) | ('/' factor))*
    expression  ::= term (('+' term) | ('-' term))*

is approximated using facilities of Spirit's /X3/ as seen in this code snippet:

    group       = '(' >> expression >> ')';
    factor      = integer | group;
    term        = factor >> *(('*' >> factor) | ('/' >> factor));
    expression  = term >> *(('+' >> term) | ('-' >> term));

Through the magic of expression templates, this is perfectly valid and
executable C++ code. The production rule `expression` is, in fact, an object
that has a member function `parse` that does the work given a source code
written in the grammar that we have just declared. Yes, it's a calculator. We
shall simplify for now by skipping the type declarations and the definition of
the rule `integer` invoked by `factor`. Now, the production rule `expression` in
our grammar specification, traditionally called the `start` symbol, can
recognize inputs such as:

    12345
    -12345
    +12345
    1 + 2
    1 * 2
    1/2 + 3/4
    1 + 2 + 3 + 4
    1 * 2 * 3 * 4
    (1 + 2) * (3 + 4)
    (-1 + 2) * (3 + -4)
    1 + ((6 * 200) - 20) / 6
    (1 + (2 + (3 + (4 + 5))))

Certainly we have modified the original EBNF syntax. This is done to
conform to C++ syntax rules. Most notably we see the abundance of shift >>
operators. Since there are no juxtaposition operators in C++, it is simply not
possible to write something like:

    a b

as seen in math syntax, for example, to mean multiplication or, in our case,
as seen in EBNF syntax to mean sequencing (b should follow a). __x3__
uses the shift `>>` operator instead for this purpose. We take the `>>`
operator, with arrows pointing to the right, to mean "is followed by". Thus we
write:

    a >> b

The alternative operator `|` and the parentheses `()` remain as is. The
assignment operator `=` is used in place of EBNF's `::=`. Last but not least,
the Kleene star `*`, which in this case is a postfix operator in EBNF becomes a
prefix. Instead of:

    a* //... in EBNF syntax,

we write:

    *a //... in Spirit.

since there are no postfix stars, `*`, in C/C++. Finally, we terminate each
rule with the ubiquitous semi-colon, `;`.

[endsect]