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<div class="titlepage"><div><div><h3 class="title">
<a name="spirit.abstracts.syntax_diagram"></a><a class="link" href="syntax_diagram.html" title="Syntax Diagram">Syntax Diagram</a>
</h3></div></div></div>
<p>
In the next section, we will deal with Parsing Expression Grammars (PEG)
<sup>[<a name="id835167" href="#ftn.id835167" class="footnote">3</a>]</sup>, a variant of Extended Backus-Naur Form (EBNF) <sup>[<a name="id835179" href="#ftn.id835179" class="footnote">4</a>]</sup> with a different interpretation. It is easier to understand PEG
using Syntax Diagrams. Syntax diagrams represent a grammar graphically. It
was used extensibly by Niklaus Wirth <sup>[<a name="id835191" href="#ftn.id835191" class="footnote">5</a>]</sup> in the "Pascal User Manual". Syntax Diagrams are easily
understandable by programmers due to their similarity to flow charts. The
isomorphism of the diagrams and functions make them ideal for representing
Recursive Descent parsers which are essentially mutually recursive functions.
</p>
<p>
Historically, Parsing Expression Grammars have been used for describing grammars
for parsers only (hence the name). In <a href="http://boost-spirit.com" target="_top">Spirit</a>
we use a very similar notation for output generation as well. Almost all
the concepts described here are equally applicable both to <span class="emphasis"><em>Spirit.Qi</em></span>
parsers and to <span class="emphasis"><em>Spirit.Karma</em></span> generators.
</p>
<a name="spirit.abstracts.syntax_diagram.elements"></a><h5>
<a name="id835219"></a>
<a class="link" href="syntax_diagram.html#spirit.abstracts.syntax_diagram.elements">Elements</a>
</h5>
<p>
All diagrams have one entry and one exit point. Arrows connect all possible
paths through the grammar from the entry point to the exit point.
</p>
<div class="blockquote"><blockquote class="blockquote"><p>
<span class="inlinemediaobject"><img src="../.././images/start_stop.png" alt="start_stop"></span>
</p></blockquote></div>
<p>
Terminals are represented by round boxes. Terminals are atomic and usually
represent plain characters, strings or tokens.
</p>
<div class="blockquote"><blockquote class="blockquote"><p>
<span class="inlinemediaobject"><img src="../.././images/terminal.png" alt="terminal"></span>
</p></blockquote></div>
<p>
Non-terminals are represented by boxes. Diagrams are modularized using named
non-terminals. A complex diagram can be broken down into a set of non-terminals.
Non-terminals also allow recursion (i.e. a non-terminal can call itself).
</p>
<div class="blockquote"><blockquote class="blockquote"><p>
<span class="inlinemediaobject"><img src="../.././images/non-terminal.png" alt="non-terminal"></span>
</p></blockquote></div>
<a name="spirit.abstracts.syntax_diagram.constructs"></a><h5>
<a name="id835308"></a>
<a class="link" href="syntax_diagram.html#spirit.abstracts.syntax_diagram.constructs">Constructs</a>
</h5>
<p>
The most basic composition is the Sequence. B follows A:
</p>
<div class="blockquote"><blockquote class="blockquote"><p>
<span class="inlinemediaobject"><img src="../.././images/sequence.png" alt="sequence"></span>
</p></blockquote></div>
<p>
The ordered choice henceforth we will call <span class="emphasis"><em>alternatives</em></span>.
In PEG, ordered choice and alternatives are not quite the same. PEG allows
ambiguity of choice where one or more branches can succeed. In PEG, in case
of ambiguity, the first one always wins.
</p>
<div class="blockquote"><blockquote class="blockquote"><p>
<span class="inlinemediaobject"><img src="../.././images/alternative.png" alt="alternative"></span>
</p></blockquote></div>
<p>
The optional (zero-or-one):
</p>
<div class="blockquote"><blockquote class="blockquote"><p>
<span class="inlinemediaobject"><img src="../.././images/optional.png" alt="optional"></span>
</p></blockquote></div>
<p>
Now, the loops. We have the zero-or-more and one-or-more:
</p>
<div class="blockquote"><blockquote class="blockquote"><p>
<span class="inlinemediaobject"><img src="../.././images/kleene.png" alt="kleene"></span>
</p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
<span class="inlinemediaobject"><img src="../.././images/plus.png" alt="plus"></span>
</p></blockquote></div>
<p>
Take note that, as in PEG, these loops behave greedily. If there is another
'A' just before the end-point, it will always fail because the preceding
loop has already exhausted all 'A's and there is nothing more left. This
is a crucial difference between PEG and general Context Free Grammars (CFGs).
This behavior is quite obvious with syntax diagrams as they resemble flow-charts.
</p>
<a name="spirit.abstracts.syntax_diagram.predicates"></a><h5>
<a name="id835451"></a>
<a class="link" href="syntax_diagram.html#spirit.abstracts.syntax_diagram.predicates">Predicates</a>
</h5>
<p>
Now, the following are Syntax Diagram versions of PEG predicates. These are
not traditionally found in Syntax Diagrams. These are special extensions
we invented to closely follow PEGs.
</p>
<p>
First, we introduce a new element, the Predicate:
</p>
<div class="blockquote"><blockquote class="blockquote"><p>
<span class="inlinemediaobject"><img src="../.././images/predicate.png" alt="predicate"></span>
</p></blockquote></div>
<p>
This is similar to the conditionals in flow charts where the 'No' branch
is absent and always signals a failed parse.
</p>
<p>
We have two versions of the predicate, the <span class="emphasis"><em>And-Predicate</em></span>
and the <span class="emphasis"><em>Not-Predicate</em></span>:
</p>
<div class="blockquote"><blockquote class="blockquote"><p>
<span class="inlinemediaobject"><img src="../.././images/and_predicate.png" alt="and_predicate"></span>
</p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
<span class="inlinemediaobject"><img src="../.././images/not_predicate.png" alt="not_predicate"></span>
</p></blockquote></div>
<p>
The <span class="emphasis"><em>And-Predicate</em></span> tries the predicate, P, and succeeds
if P succeeds, or otherwise fail. The opposite is true with the <span class="emphasis"><em>Not-Predicate</em></span>.
It tries the predicate, P, and fails if P succeeds, or otherwise succeeds.
Both versions do a look-ahead but do not consume any input regardless if
P succeeds or not.
</p>
<div class="footnotes">
<br><hr width="100" align="left">
<div class="footnote"><p><sup>[<a name="ftn.id835167" href="#id835167" class="para">3</a>] </sup>
Bryan Ford: Parsing Expression Grammars: A Recognition-Based Syntactic
Foundation, <a href="http://pdos.csail.mit.edu/~baford/packrat/popl04/" target="_top">http://pdos.csail.mit.edu/~baford/packrat/popl04/</a>
</p></div>
<div class="footnote"><p><sup>[<a name="ftn.id835179" href="#id835179" class="para">4</a>] </sup>
Richard E. Pattis: EBNF: A Notation to Describe Syntax, <a href="http://www.cs.cmu.edu/~pattis/misc/ebnf.pdf" target="_top">http://www.cs.cmu.edu/~pattis/misc/ebnf.pdf</a>
</p></div>
<div class="footnote"><p><sup>[<a name="ftn.id835191" href="#id835191" class="para">5</a>] </sup>
Niklaus Wirth: The Programming Language Pascal. (July 1973)
</p></div>
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