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<font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>Quick
Start</b></font>
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<h2>
<b>Why would you want to use Spirit?</b>
</h2>
<p>
Spirit is designed to be a practical parsing tool. At the very least, the
ability to generate a fully-working parser from a formal EBNF
specification inlined in C++ significantly reduces development time.
While it may be practical to use a full-blown, stand-alone parser such as
YACC or ANTLR when we want to develop a computer language such as C or
Pascal, it is certainly overkill to bring in the big guns when we wish to
write extremely small micro-parsers. At that end of the spectrum,
programmers typically approach the job at hand not as a formal parsing
task but through ad hoc hacks using primitive tools such as
<tt>scanf</tt>. True, there are tools such as regular-expression
libraries (such as <a href=
"http://www.boost.org/libs/regex/index.html">boost regex</a>) or scanners
(such as <a href="http://www.boost.org/libs/tokenizer/index.html">boost
tokenizer</a>), but these tools do not scale well when we need to write
more elaborate parsers. Attempting to write even a moderately-complex
parser using these tools leads to code that is hard to understand and
maintain.
</p>
<p>
One prime objective is to make the tool easy to use. When one thinks of a
parser generator, the usual reaction is "it must be big and complex with
a steep learning curve." Not so. Spirit is designed to be fully scalable.
The framework is structured in layers. This permits learning on an
as-needed basis, after only learning the minimal core and basic concepts.
</p>
<p>
For development simplicity and ease in deployment, the entire framework
consists of only header files, with no libraries to link against or
build. Just put the spirit distribution in your include path, compile and
run. Code size? -very tight. In the quick start example that we shall
present in a short while, the code size is dominated by the instantiation
of the <tt>std::vector</tt> and <tt>std::iostream</tt>.
</p>
<h2>
<b>Trivial Example #1</b></h2>
<p>Create a parser that will parse
a floating-point number.
</p>
<pre><code><font color="#000000"> </font></code><span class="identifier">real_p</span>
</pre>
<p>
(You've got to admit, that's trivial!) The above code actually generates
a Spirit <tt>real_parser</tt> (a built-in parser) which parses a floating
point number. Take note that parsers that are meant to be used directly
by the user end with "<tt>_p</tt>" in their names as a Spirit convention.
Spirit has many pre-defined parsers and consistent naming conventions
help you keep from going insane!
</p>
<h2>
<b>Trivial Example #2</b></h2>
<p>
Create a parser that will accept a line consisting of two floating-point
numbers.
</p>
<pre><code><font color="#000000"> </font></code><code><span class=
"identifier">real_p</span> <span class=
"special">>></span> <span class="identifier">real_p</span></code>
</pre>
<p>
Here you see the familiar floating-point numeric parser
<code><tt>real_p</tt></code> used twice, once for each number. What's
that <tt class="operators">>></tt> operator doing in there? Well,
they had to be separated by something, and this was chosen as the
"followed by" sequence operator. The above program creates a parser from
two simpler parsers, glueing them together with the sequence operator.
The result is a parser that is a composition of smaller parsers.
Whitespace between numbers can implicitly be consumed depending on how
the parser is invoked (see below).
</p>
<p>
Note: when we combine parsers, we end up with a "bigger" parser, But it's
still a parser. Parsers can get bigger and bigger, nesting more and more,
but whenever you glue two parsers together, you end up with one bigger
parser. This is an important concept.
</p>
<h2>
<b>Trivial Example #3</b></h2>
<p>
Create a parser that will accept an arbitrary number of floating-point
numbers. (Arbitrary means anything from zero to infinity)
</p>
<pre><code><font color="#000000"> </font></code><code><span class=
"special">*</span><span class="identifier">real_p</span></code>
</pre>
<p>
This is like a regular-expression Kleene Star, though the syntax might
look a bit odd for a C++ programmer not used to seeing the <tt class=
"operators">*</tt> operator overloaded like this. Actually, if you know
regular expressions it may look odd too since the star is <b>before</b>
the expression it modifies. C'est la vie. Blame it on the fact that we
must work with the syntax rules of C++.
</p>
<p>
Any expression that evaluates to a parser may be used with the Kleene
Star. Keep in mind, though, that due to C++ operator precedence rules you
may need to put the expression in parentheses for complex expressions.
The Kleene Star is also known as a Kleene Closure, but we call it the
Star in most places.
</p>
<h3>
<b><a name="list_of_numbers"></a> Example #4 [ A Just Slightly Less Trivial Example</b>
] </h3>
<p>
This example will create a parser that accepts a comma-delimited list of numbers and put the numbers in a vector.
</p>
<h4><strong> Step 1. Create the parser</strong></h4>
<pre><code><font color="#000000"> </font></code><code><span class=
"identifier">real_p</span> <span class=
"special">>></span> <span class="special">*(</span><span class=
"identifier">ch_p</span><span class="special">(</span><span class=
"literal">','</span><span class="special">)</span> <span class=
"special">>></span> <span class=
"identifier">real_p</span><span class="special">)</span></code>
</pre>
<p>
Notice <tt>ch_p(',')</tt>. It is a literal character parser that can
recognize the comma <tt>','</tt>. In this case, the Kleene Star is
modifying a more complex parser, namely, the one generated by the
expression:
</p>
<pre><code><font color="#000000"> </font></code><code><span class=
"special">(</span><span class="identifier">ch_p</span><span class=
"special">(</span><span class="literal">','</span><span class=
"special">)</span> <span class="special">>></span> <span class=
"identifier">real_p</span><span class="special">)</span></code>
</pre>
<p>
Note that this is a case where the parentheses are necessary. The Kleene
star encloses the complete expression above.
</p>
<h4>
<b><strong>Step 2. </strong>Using a Parser (now that it's created)</b></h4>
<p>
Now that we have created a parser, how do we use it? Like the result of
any C++ temporary object, we can either store it in a variable, or call
functions directly on it.
</p>
<p>
We'll gloss over some low-level C++ details and just get to the good
stuff.
</p>
<p>
If <b><tt>r</tt></b> is a rule (don't worry about what rules exactly are
for now. This will be discussed later. Suffice it to say that the rule is
a placeholder variable that can hold a parser), then we store the parser
as a rule like this:
</p>
<pre><code><font color="#000000"> </font></code><code><font color="#000000"><span class=
"identifier">r</span> <span class="special">=</span> <span class=
"identifier">real_p</span> <span class=
"special">>> *(</span><span class=
"identifier">ch_p</span><span class="special">(</span><span class=
"literal">','</span><span class="special">) >></span> <span class=
"identifier">real_p</span><span class="special">);</span></font></code>
</pre>
<p>
Not too exciting, just an assignment like any other C++ expression you've
used for years. The cool thing about storing a parser in a rule is this:
rules are parsers, and now you can refer to it <b>by name</b>. (In this
case the name is <tt><b>r</b></tt>). Notice that this is now a full
assignment expression, thus we terminate it with a semicolon,
"<tt>;</tt>".
</p>
<p>
That's it. We're done with defining the parser. So the next step is now
invoking this parser to do its work. There are a couple of ways to do
this. For now, we shall use the free <tt>parse</tt> function that takes
in a <tt>char const*</tt>. The function accepts three arguments:
</p>
<blockquote>
<p>
<img src="theme/bullet.gif" width="12" height="12"> The null-terminated
<tt>const char*</tt> input<br>
<img src="theme/bullet.gif" width="12" height="12"> The parser
object<br>
<img src="theme/bullet.gif" width="12" height="12"> Another parser
called the <b>skip parser</b>
</p>
</blockquote>
<p>
In our example, we wish to skip spaces and tabs. Another parser named
<tt>space_p</tt> is included in Spirit's repertoire of predefined
parsers. It is a very simple parser that simply recognizes whitespace. We
shall use <tt>space_p</tt> as our skip parser. The skip parser is the one
responsible for skipping characters in between parser elements such as
the <tt>real_p</tt> and the <tt>ch_p</tt>.
</p>
<p>
Ok, so now let's parse!
</p>
<pre><code><font color="#000000"> </font></code><code><font color="#000000"><span class=
"identifier">r</span> <span class="special">=</span> <span class=
"identifier">real_p</span> <span class=
"special">>></span> <span class="special">*(</span><span class=
"identifier">ch_p</span><span class="special">(</span><span class=
"literal">','</span><span class="special">)</span> <span class=
"special">>></span> <span class=
"identifier">real_p</span><span class="special">);
</span> <span class="identifier"> parse</span><span class=
"special">(</span><span class="identifier">str</span><span class=
"special">,</span> <span class="identifier">r</span><span class=
"special">,</span> <span class="identifier">space_p</span><span class=
"special">)</span> <span class=
"comment">// Not a full statement yet, patience...</span></font></code>
</pre>
<p>
The parse function returns an object (called <tt>parse_info</tt>) that
holds, among other things, the result of the parse. In this example, we
need to know:
</p>
<blockquote>
<p>
<img src="theme/bullet.gif" width="12" height="12"> Did the parser
successfully recognize the input <tt>str</tt>?<br>
<img src="theme/bullet.gif" width="12" height="12"> Did the parser
<b>fully</b> parse and consume the input up to its end?
</p>
</blockquote>
<p>
To get a complete picture of what we have so far, let us also wrap this
parser inside a function:
</p>
<pre><code><font color="#000000"> </font></code><code><font color="#000000"><span class=
"keyword">bool
</span> <span class="identifier"> parse_numbers</span><span class=
"special">(</span><span class="keyword">char</span> <span class=
"keyword">const</span><span class="special">*</span> <span class=
"identifier">str</span><span class="special">)
{
</span> <span class="keyword"> return</span> <span class=
"identifier">parse</span><span class="special">(</span><span class=
"identifier">str</span><span class="special">,</span> <span class=
"identifier">real_p</span> <span class=
"special">>></span> <span class="special">*(</span><span class=
"literal">','</span> <span class="special">>></span> <span class=
"identifier">real_p</span><span class="special">),</span> <span class=
"identifier">space_p</span><span class="special">).</span><span class=
"identifier">full</span><span class="special">;
}</span></font></code>
</pre>
<p>
Note in this case we dropped the named rule and inlined the parser
directly in the call to parse. Upon calling parse, the expression
evaluates into a temporary, unnamed parser which is passed into the
parse() function, used, and then destroyed.
</p>
<table border="0" width="80%" align="center">
<tr>
<td class="note_box">
<img src="theme/note.gif" width="16" height="16"><b>char and wchar_t
operands</b><br>
<br>
The careful reader may notice that the parser expression has
<tt class="quotes">','</tt> instead of <tt>ch_p(',')</tt> as the
previous examples did. This is ok due to C++ syntax rules of
conversion. There are <tt>>></tt> operators that are overloaded
to accept a <tt>char</tt> or <tt>wchar_t</tt> argument on its left or
right (but not both). An operator may be overloaded if at least one
of its parameters is a user-defined type. In this case, the
<tt>real_p</tt> is the 2nd argument to <tt>operator<span class=
"operators">>></span></tt>, and so the proper overload of
<tt class="operators">>></tt> is used, converting
<tt class="quotes">','</tt> into a character literal parser.<br>
<br>
The problem with omiting the <tt>ch_p</tt> call should be obvious:
<tt>'a' >> 'b'</tt> is <b>not</b> a spirit parser, it is a
numeric expression, right-shifting the ASCII (or another encoding)
value of <tt class="quotes">'a'</tt> by the ASCII value of
<tt class="quotes">'b'</tt>. However, both <tt>ch_p('a') >>
'b'</tt> and <tt>'a' >> ch_p('b')</tt> are Spirit sequence
parsers for the letter <tt class="quotes">'a'</tt> followed by
<tt class="quotes">'b'</tt>. You'll get used to it, sooner or
later.
</td>
</tr>
</table>
<p>
Take note that the object returned from the parse function has a member
called <tt>full</tt> which returns true if both of our requirements above
are met (i.e. the parser fully parsed the input).
</p>
<h4>
<b> Step 3. Semantic Actions</b></h4>
<p>
Our parser above is really nothing but a recognizer. It answers the
question <i class="quotes">"did the input match our grammar?"</i>, but it
does not remember any data, nor does it perform any side effects.
Remember: we want to put the parsed numbers into a vector. This is done
in an <b>action</b> that is linked to a particular parser. For example,
whenever we parse a real number, we wish to store the parsed number after
a successful match. We now wish to extract information from the parser.
Semantic actions do this. Semantic actions may be attached to any point
in the grammar specification. These actions are C++ functions or functors
that are called whenever a part of the parser successfully recognizes a
portion of the input. Say you have a parser <b>P</b>, and a C++ function
<b>F</b>, you can make the parser call <b>F</b> whenever it matches an
input by attaching <b>F</b>:
</p>
<pre><code><font color="#000000"> </font></code><code><font color="#000000"><span class=
"identifier">P</span><span class="special">[&</span><span class=
"identifier">F</span><span class="special">]</span></font></code>
</pre>
<p>
Or if <b>F</b> is a function object (a functor):
</p>
<pre><code><font color="#000000"> </font></code><code><font color="#000000"><span class=
"identifier">P</span><span class="special">[</span><span class=
"identifier">F</span><span class="special">]</span></font></code>
</pre>
<p>
The function/functor signature depends on the type of the parser to which
it is attached. The parser <tt>real_p</tt> passes a single argument: the
parsed number. Thus, if we were to attach a function <b>F</b> to
<tt>real_p</tt>, we need <b>F</b> to be declared as:
</p>
<pre><code> </code><code><span class=
"keyword">void</span> <span class="identifier">F</span><span class=
"special">(</span><span class="keyword">double</span> <span class=
"identifier">n</span><span class="special">);</span></code></pre>
<p>
For our example however, again, we can take advantage of some predefined
semantic functors and functor generators (<img src="theme/lens.gif"
width="15" height="16"> A functor generator is a function that returns
a functor). For our purpose, Spirit has a functor generator
<tt>push_back_a(c)</tt>. In brief, this semantic action, when called,
<b>appends</b> the parsed value it receives from the parser it is
attached to, to the container <tt>c</tt>.
</p>
<p>
Finally, here is our complete comma-separated list parser:
</p>
<pre><code><font color="#000000"> </font></code><code><font color="#000000"><span class=
"keyword">bool
</span> <span class="identifier">parse_numbers</span><span class=
"special">(</span><span class="keyword">char</span> <span class=
"keyword">const</span><span class="special">*</span> <span class=
"identifier">str</span><span class="special">,</span> <span class=
"identifier">vector</span><span class="special"><</span><span class=
"keyword">double</span><span class=
"special">>&</span> <span class="identifier">v</span><span class=
"special">)
{
</span> <span class="keyword">return</span> <span class=
"identifier">parse</span><span class="special">(</span><span class=
"identifier">str</span><span class="special">,
</span> <span class="comment"> // Begin grammar
</span> <span class="special"> (
</span> <span class="identifier">real_p</span><span class=
"special">[</span><span class="identifier">push_back_a</span><span class=
"special">(</span><span class="identifier">v</span><span class=
"special">)]</span> <span class="special">>></span> <span class=
"special">*(</span><span class="literal">','</span> <span class=
"special">>></span> <span class=
"identifier">real_p</span><span class="special">[</span><span class=
"identifier">push_back_a</span><span class="special">(</span><span class=
"identifier">v</span><span class="special">)])
)
</span> <span class="special"> ,
</span> <span class="comment"> // End grammar
</span> <span class="identifier"> space_p</span><span class=
"special">).</span><span class="identifier">full</span><span class="special">;
}</span></font></code>
</pre>
<p>
This is the same parser as above. This time with appropriate semantic
actions attached to strategic places to extract the parsed numbers and
stuff them in the vector <tt>v</tt>. The parse_numbers function returns
true when successful.
</p>
<p>
<img src="theme/lens.gif" width="15" height="16"> The full source code
can be <a href="../example/fundamental/number_list.cpp">viewed here</a>.
This is part of the Spirit distribution.
</p>
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<p class="copyright">
Copyright © 1998-2003 Joel de Guzman<br>
Copyright © 2002 Chris Uzdavinis<br>
<br>
<font size="2">Use, modification and distribution is subject to 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)</font>
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