[/==============================================================================
    Copyright (C) 2001-2011 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 Preface]

[:['["Examples of designs that meet most of the criteria for
"goodness" (easy to understand, flexible, efficient) are a
recursive-descent parser, which is traditional procedural
code. Another example is the STL, which is a generic library of
containers and algorithms depending crucially on both traditional
procedural code and on parametric polymorphism.]] [*--Bjarne
Stroustrup]]

[heading History]

[heading /80s/]

In the mid-80s, Joel wrote his first calculator in Pascal. Such an
unforgettable coding experience, he was amazed at how a mutually
recursive set of functions can model a grammar specification. In time,
the skills he acquired from that academic experience became very
practical as he was tasked to do some parsing. For instance, whenever he
needed to perform any form of binary or text I/O, he tried to approach
each task somewhat formally by writing a grammar using Pascal-like
syntax diagrams and then a corresponding recursive-descent parser. This
process worked very well.

[heading /90s/]

The arrival of the Internet and the World Wide Web magnified the need
for parsing a thousand-fold. At one point Joel had to write an HTML
parser for a Web browser project. Using the W3C formal specifications,
he easily wrote a recursive-descent HTML parser. With the influence of
the Internet, RFC specifications were abundent. SGML, HTML, XML, email
addresses and even those seemingly trivial URLs were all formally
specified using small EBNF-style grammar specifications. Joel had more
parsing to do, and he wished for a tool similar to larger parser
generators such as YACC and ANTLR, where a parser is built automatically
from a grammar specification. 

This ideal tool would be able to parse anything from email addresses and
command lines, to XML and scripting languages. Scalability was a primary
goal. The tool would be able to do this without incurring a heavy
development load, which was not possible with the above mentioned parser
generators. The result was Spirit.

Spirit was a personal project that was conceived when Joel was involved
in R&D in Japan. Inspired by the GoF's composite and interpreter
patterns, he realized that he can model a recursive-descent parser with
hierarchical-object composition of primitives (terminals) and composites
(productions). The original version was implemented with run-time
polymorphic classes. A parser was generated at run time by feeding in
production rule strings such as: 

    "prod ::= {'A' | 'B'} 'C';"

A compile function compiled the parser, dynamically creating a hierarchy
of objects and linking semantic actions on the fly. A very early text
can be found here: __early_spirit__.

[heading /2001 to 2006/]

Version 1.0 to 1.8 was a complete rewrite of the original Spirit parser
using expression templates and static polymorphism, inspired by the
works of Todd Veldhuizen (__exprtemplates__, C++ Report, June
1995). Initially, the static-Spirit version was meant only to replace
the core of the original dynamic-Spirit. Dynamic-Spirit needed a parser
to implement itself anyway. The original employed a hand-coded
recursive-descent parser to parse the input grammar specification
strings. It was at this time when Hartmut Kaiser joined the Spirit
development. 

After its initial "open-source" debut in May 2001, static-Spirit became
a success. At around November 2001, the Spirit website had an activity
percentile of 98%, making it the number one parser tool at Source Forge
at the time. Not bad for a niche project like a parser library. The
"static" portion of Spirit was forgotten and static-Spirit simply became
Spirit. The library soon evolved to acquire more dynamic features.

Spirit was formally accepted into __boost__ in October 2002. Boost is a
peer-reviewed, open collaborative development effort around a collection
of free Open Source C++ libraries covering a wide range of domains. The
Boost Libraries have become widely known as an industry standard for
design and implementation quality, robustness, and reusability.

[heading /2007/]

Over the years, especially after Spirit was accepted into Boost, Spirit
has served its purpose quite admirably. [*/Classic-Spirit/] (versions
prior to 2.0) focused on transduction parsing, where the input string is
merely translated to an output string. Many parsers fall into the
transduction type. When the time came to add attributes to the parser
library, it was done in a rather ad-hoc manner, with the goal being 100%
backward compatible with Classic Spirit. As a result, some parsers have
attributes, some don't. 

Spirit V2 is another major rewrite. Spirit V2 grammars are fully
attributed (see __attr_grammar__) which means that all parser components
have attributes. To do this efficiently and elegantly, we had to use a
couple of infrastructure libraries. Some did not exist, some were quite
new when Spirit debuted, and some needed work. __mpl__ is an important
infrastructure library, yet is not sufficient to implement Spirit V2.
Another library had to be written: __fusion__. Fusion sits between MPL
and STL --between compile time and runtime -- mapping types to values.
Fusion is a direct descendant of both MPL and __boost_tuples__. Fusion
is now a full-fledged __boost__ library. __phoenix__ also had to be
beefed up to support Spirit V2. The result is __phoenix__. Last
but not least, Spirit V2 uses an __exprtemplates__ library called
__boost_proto__.

Even though it has evolved and matured to become a multi-module library,
Spirit is still used for micro-parsing tasks as well as scripting
languages. Like C++, you only pay for features that you need. The power
of Spirit comes from its modularity and extensibility. Instead of giving
you a sledgehammer, it gives you the right ingredients to easily create
a sledgehammer.

[heading New Ideas: Spirit V2]

Just before the development of Spirit V2 began, Hartmut came across the
__string_template__ library that is a part of the ANTLR parser
framework. [footnote Quote from http://www.stringtemplate.org/: It is a
Java template engine (with ports for C# and Python) for generating
source code, web pages, emails, or any other formatted text output.]
The concepts presented in that library lead Hartmut to
the next step in the evolution of Spirit. Parsing and generation are
tightly connected to a formal notation, or a grammar. The grammar
describes both input and output, and therefore, a parser library should
have a grammar driven output. This duality is expressed in Spirit by the
parser library __qi__ and the generator library __karma__ using the same
component infrastructure. 

The idea of creating a lexer library well integrated with the Spirit
parsers is not new. This has been discussed almost since Classic-Spirit
(pre V2) initially debuted. Several attempts to integrate existing lexer
libraries and frameworks with Spirit have been made and served as a
proof of concept and usability (for example see __wave__: The Boost
C/C++ Preprocessor Library, and __slex__: a fully dynamic C++ lexer
implemented with Spirit). Based on these experiences we added __lex__: a
fully integrated lexer library to the mix, allowing the user to take
advantage of the power of regular expressions for token matching,
removing pressure from the parser components, simplifying parser
grammars. Again, Spirit's modular structure allowed us to reuse the same
underlying component library as for the parser and generator libraries.

[heading How to use this manual]

Each major section (there are 3: __sec_qi__, __sec_karma__, and
__sec_lex__) is roughly divided into 3 parts:

# Tutorials: A step by step guide with heavily annotated code. These
  are meant to get the user acquainted with the library as quickly as
  possible. The objective is to build the confidence of the user in
  using the library through abundant examples and detailed
  instructions. Examples speak volumes and we have volumes of
  examples!

# Abstracts: A high level summary of key topics. The objective is to
  give the user a high level view of the library, the key concepts,
  background and theories.

# Reference: Detailed formal technical reference. We start with a quick
  reference -- an easy to use table that maps into the reference proper.
  The reference proper starts with C++ concepts followed by
  models of the concepts.

Some icons are used to mark certain topics indicative of their relevance.
These icons precede some text to indicate:

[table Icons

    [[Icon]             [Name]          [Meaning]]

    [[__note__]         [Note]          [Generally useful information (an aside that
                                        doesn't fit in the flow of the text)]]

    [[__tip__]          [Tip]           [Suggestion on how to do something
                                        (especially something that is not obvious)]]

    [[__important__]    [Important]     [Important note on something to take
                                        particular notice of]]

    [[__caution__]      [Caution]       [Take special care with this - it may
                                        not be what you expect and may cause bad
                                        results]]

    [[__danger__]       [Danger]        [This is likely to cause serious
                                        trouble if ignored]]
]

This documentation is automatically generated by Boost QuickBook
documentation tool. QuickBook can be found in the __boost_tools__.

[heading Support]

Please direct all questions to Spirit's mailing list. You can subscribe
to the __spirit_list__. The mailing list has a searchable archive. A
search link to this archive is provided in __spirit__'s home page. You
may also read and post messages to the mailing list through
__spirit_general__ (thanks to __gmane__). The news group mirrors the
mailing list. Here is a link to the archives: __mlist_archive__.

[endsect] [/ Preface]
