Ticket #2610: introduction.qbk

[/==============================================================================
    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 Introduction]

Boost Spirit is an object-oriented, recursive-descent parser and output generation
library for C++. It allows you to write grammars and format descriptions using a
format similar to EBNF (Extended Backus Naur Form, see [4]) 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.
In retrospect, conventional compiler-compilers or parser-generators have to
perform an additional translation step from the source EBNF code to C or C++
code.

The syntax and semantics of the libraries' API directly form domain-specific
embedded languages (DSEL). In fact, Spirit exposes 3 different DSELs to the
user:

* one for creating parser grammars,
* one for the specification of the required tokens to be used for parsing,
* and one for the description of the required output formats.

Since the target input grammars and output formats 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
and output generation process with other C++ code. Often this allows for
simpler and more efficient code.

Both the created parsers and generators 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.

The [link spirit.spiritstructure figure] below depicts the overall structure
of the Boost Spirit library. The library consists of 4 major parts:

* __classic__: This is the almost-unchanged code base taken from the
  former Boost Spirit V1.8 distribution. It has been moved into the namespace
  boost::spirit::classic. A special compatibility layer has been added to
  ensure complete compatibility with existing code using Spirit V1.8.
* __qi__: This is the parser library allowing you to build recursive
  descent parsers. The exposed domain-specific language can be used to describe
  the grammars to implement, and the rules for storing the parsed information.
* __lex__: This is the library usable to create tokenizers (lexers). The
  domain-specific language exposed by __lex__
* __karma__: This is the generator library allowing you to create code for
  recursive descent, data type-driven output formatting. The exposed
  domain-specific language is almost equivalent to the parser description language
  used in __qi__, except that it is used to describe the required output
  format to generate from a given data structure.

[fig ./images/spiritstructure.png..The overall structure of the Boost Spirit library..spirit.spiritstructure]

The separate sublibraries __qi__, __karma__ and __lex__ are well integrated
with any of the other parts. Because of their similar structure and identical
underlying technology these are usable either separately or together at the
same time. For instance is it possible to directly feed the hierarchical data
structures generated by __qi__ into output generators created using __karma__;
or to use the token sequence generated by __lex__ as the input for a parser
generated by __qi__.


The [link spirit.spiritkarmaflow figure] below shows the typical data flow of
some input being converted to some internal representation. After some
(optional) transformation these data are converted back into some different,
external representation. The picture highlights Spirit's place in this data
transformation flow.

[fig ./images/spiritkarmaflow.png..The place of __qi__ and __karma__ in a data transformation flow of a typical application..spirit.spiritkarmaflow]

[heading A Quick Overview of Parsing with __qi__]

__qi__ is Spirit's sublibrary dealing with generating parsers based on a given
target grammar (essentially a format description of the input data to read).

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 /Qi/ sublibrary 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 done some modifications to 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 'empty' 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). Spirit
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 used to be 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, `;`.


[heading A Quick Overview of Output Generation with __karma__]

Spirit not only allows you to describe the structure of the input. Starting with
Version 2.0 it enables the specification of the output format for your data
in a similar way, and based on a single syntax and compatible semantics.

Let's assume we need to generate a textual representation from a simple data
structure such as a `std::vector<int>`. Conventional code probably would look like:

    std::vector<int> v (initialize_and_fill());
    std::vector<int>::iterator end = v.end();
    for (std::vector<int>::iterator it = v.begin(); it != end; ++it)
        std::cout << *it << std::endl;

which is not very flexible and quite difficult to maintain when it comes to
changing the required output format. Spirit's sublibrary /Karma/ allows you to
specify output formats for arbitrary data structures in a very flexible way.
The following snippet is the /Karma/ format description used to create the
same output as the traditional code above:

    *(int_ << eol)

Here are some more examples of format descriptions for different output
representations of the same `std::vector<int>`:

[table Different output formats for `std::vector<int>`
    [ [Format]                         [Example]          [Description] ]
    [ [`'[' << *(int_ << ',') << ']'`] [`[1,8,10,]`]      [Comma separated list of integers] ]
    [ [`*('(' << int_ << ')' << ',')`] [`(1),(8),(10),]`] [Comma separated list of integers in parenthesis] ]
    [ [`*hex`]                         [`18a`]            [A list of hexadecimal numbers] ]
    [ [`*(double_ << ',')`]            [`1.0,8.0,10.0,`]  [A list of floating point numbers] ]
]

The syntax is similar to /Qi/ with the exception that we use the `<<`
operator for output concatenation. This should be easy to understand as it
follows the conventions used in the Standard's I/O streams.

Another important feature of /karma/ allows you to fully decouple the data
type from the output format. You can use the same output format with different
data types as long as these conform conceptually. The next table gives some
related examples.

[table Different data types usable with the output format `(*int_ << eol)`
    [ [Data type] ]
    [ [`int i[4]`]                [C style arrays] ]
    [ [`std::vector<int>`]        [Standard vector] ]
    [ [`std::list<int>`]          [Standard list] ]
    [ [`boost::array<long, 20>`]  [Boost array] ]
]

[endsect]