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<title>Geyacc: The Generated Parser</title>
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<td><font size="6"><strong>The Generated Parser</strong></font></td>
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<p>For each parser description file given as input, <em>geyacc</em>
will generate an Eiffel class as output. The deferred class <a
href="skeleton.html"><font color="#008080"><em><tt>YY_PARSER</tt></em></font></a>,
which is part of <em>Gobo Eiffel Parse Library</em>, provides an
abstraction for parsers. Every parser class generated by <em>geyacc</em>
will be a descendant of this class. The main feature of <font
color="#008080"><em><tt>YY_PARSER</tt></em></font> is routine <a
href="skeleton.html#parse" name="parse"><font color="#008080"><em><tt>parse</tt></em></font></a><font
color="#008080" size="2" face="Courier New"><em> </em></font>which,
when called, reads tokens, executes actions and ultimately
returns when it encounters the end of input or an unrecoverable
syntax error. <font color="#008080"><em><tt>parse</tt></em></font><font
color="#008080" size="2" face="Courier New"><em> </em></font>sets
<a href="skeleton.html#syntax_error" name="syntax_error"><font
color="#008080"><em><tt>syntax_error</tt></em></font></a><font
color="#008080" size="2" face="Courier New"><em> </em></font>to
false if the parsing was successful. Otherwise, if an
unrecoverable error is detected, <font color="#008080"><em><tt>syntax_error</tt></em></font>
is set to true and the error is reported by calling <a
href="actions.html#report_error"><font color="#008080"><em><tt>report_error</tt></em></font></a>.
By default this routine just prints a message on the screen, but
it can easily be redefined to suit your needs. Also of interest
is feature <a href="actions.html#error_count"><font
color="#008080"><em><tt>error_count</tt></em></font></a> which
keeps track of the number of syntax errors (recovered and fatal)
detected during the last parsing.</p>
<p>The <a name="lexical_analyzer"><em><strong>lexical analyzer</strong></em></a>
routine, <a href="actions.html#read_token"><font color="#008080"><em><tt>read_token</tt></em></font></a>,
recognizes tokens from the input stream and makes them available
to the parser in <a href="actions.html#last_token"><font
color="#008080"><em><tt>last_token</tt></em></font></a>. <em>Geyacc</em>
does not provide a default implementation for <font
color="#008080"><em><tt>read_token</tt></em></font> and <font
color="#008080"><em><tt>last_token</tt></em></font>, so you must
define these two deferred features. In simple programs, <font
color="#008080"><em><tt>read_token</tt></em></font><font
color="#008080" size="2" face="Courier New"><em> </em></font>is
often defined at the end of the <em>geyacc</em> grammar file, in
the <a href="description.html#user_code">user code section</a>.
If <font color="#008080"><em><tt>read_token</tt></em></font><font
color="#008080" size="2" face="Courier New"><em> </em></font>is
defined in a separate class, you need to arrange for the
token-type integer constant definitions to be available there. To
do this, use the <a href="options.html#-t">option <font
color="#800000"><tt>-t</tt></font></a> when you run <em>geyacc</em>,
so that it will write these integer constant definitions into a
separate class. </p>
<p>The value that <font color="#008080"><em><tt>read_token</tt></em></font><font
color="#008080" size="2" face="Courier New"><em> </em></font>returns
in <font color="#008080"><em><tt>last_token</tt></em></font><font
color="#008080" size="2" face="Courier New"><em> </em></font>must
be the numeric code for the <a href="symbols.html">type of token</a>
it has just found, or 0 for end-of-input. When a token is
referred to in the grammar rules by a name, that name in the
parser file becomes an integer constant whose value is the proper
numeric code for that token type. So <font color="#008080"><em><tt>read_token</tt></em></font><font
color="#008080" size="2" face="Courier New"><em> </em></font>can
use the name to indicate that type. When a token is referred to
in the grammar rules by a character literal, the numeric code for
that character is also the code for the token type. So <font
color="#008080"><em><tt>read_token</tt></em></font><font
color="#008080" size="2" face="Courier New"><em> </em></font>can
simply return that character code. The null character must not be
used this way, because its code is zero and that is what
signifies end-of-input. Here is an example showing these things:</p>
<blockquote>
<pre><font color="#008080"><em>read_token</em> <em><strong>is</strong></em>
-- Read a token from input stream.
-- Make result available in <em>last_token</em>.
<em><strong>local</strong></em>
<em>c</em>: <em>CHARACTER</em>
<em><strong>do</strong></em>
...
<em><strong>if</strong></em> <em>c</em> = <em>EOF</em> <em><strong>then</strong></em>
-- Detect end of file.
<em>last_token</em> := <em>0</em>
<em><strong>elseif</strong></em> <em>c</em> = <em>'+' </em><em><strong>or</strong></em><em> c</em> = <em>'-' </em><em><strong>then</strong></em>
-- Assume token type for `+' is '+'.
<em>last_token</em> := <em>c.code</em>
<em><strong>elseif</strong></em> ... <em><strong>then</strong></em>
-- Return the type of the token.
<em>last_token</em> := <em>INT</em>
<em><strong>else</strong></em>
...
<em><strong>end</strong></em>
<em><strong>end</strong></em></font></pre>
</blockquote>
<p>This interface has been designed so that the output from the <a
href="../gelex/index.html"><em>gelex</em></a> utility can be used
without change as the definition of <font color="#008080"><em><tt>read_token</tt></em></font>.</p>
<p><font color="#008080"><em><tt>YY_PARSER</tt></em></font> is
actually a generic class whose generic parameter specifies the <a
href="actions.html#value_types">type</a> of the <a
href="introduction.html#semantic_values">semantic values</a>
associated with each token. When scanning the input stream for a
new token, <font color="#008080"><em><tt>read_token</tt></em></font>
must update the semantic value of the token being read in feature
<a href="actions.html#last_value"><font color="#008080"><em><tt>last_value</tt></em></font></a>.
As for <font color="#008080"><em><tt>read_token</tt></em></font>
and <font color="#008080"><em><tt>last_token</tt></em></font>, <font
color="#008080"><em><tt>last_value</tt></em></font> is a deferred
feature for which you must provide an implementation.</p>
<p>Class <font color="#008080"><em><tt>YY_PARSER</tt></em></font>
is equipped with a procedure <a href="skeleton.html#make"
name="make"><font color="#008080"><em><tt>make</tt></em></font></a>
which initializes the parser. This routine should be used as
creation routine in descendant classes. Also available to
descendants of <font color="#008080"><em><tt>YY_PARSER</tt></em></font><font
color="#008080" size="2" face="Courier New"><em> </em></font>are
a set of features which can be called from the <a
href="actions.html">semantic actions</a>. An implementation for
most of these routines is provided in class <font color="#008080"><em><tt>YY_PARSER_SKELETON</tt></em></font>.</p>
<p><em>Geyacc</em> does not automatically generate the indexing,
class header, formal generics, obsolete, inheritance, creation
and invariant clauses. These have to be specified in <a
href="description.html#eiffel_declarations">Eiffel declarations</a>
in the first section and in the <a
href="description.html#user_code">user code section</a> of the
parser <a href="description.html">description file</a>. The
following example shows a typical parser description file:</p>
<blockquote>
<pre><font color="#0000FF" size="3">%{</font><font size="3">
</font><font color="#008080" size="3"><em><strong>class</strong></em><em> MY_PARSER
</em><em><strong>inherit</strong></em><em>
YY_PARSER_SKELETON </em>[<em>INTEGER</em>]<em>
</em><em><strong>rename</strong></em><em>
make as make_parser_skeleton
</em><em><strong>redefine</strong></em><em>
report_error
</em><em><strong>end</strong></em><em>
MY_SCANNER
</em><em><strong>rename</strong></em><em>
make as make_scanner
</em><em><strong>export</strong></em><em>
</em>{<em>NONE</em>}<em> </em><em><strong>all</strong></em><em>
</em><em><strong>end</strong></em><em>
</em><em><strong>creation</strong></em><em>
make</em></font><font size="3"><em>
</em></font><font color="#0000FF" size="3">%}</font></pre>
<pre><font color="#0000FF" size="3">%token</font><font
size="3"> ...
</font><font color="#0000FF" size="3">%%</font><font size="3">
</font><font color="#FF0000" size="3">...rules...</font><font
size="3">
</font><font color="#0000FF" size="3">%%</font><font size="3">
</font><font color="#008080" size="3"><em><strong>feature</strong></em><em> </em>{<em>NONE</em>} -- Initialization<em>
make </em><em><strong>is</strong></em><em>
</em>-- Create a new parser.<em>
</em><em><strong>do</strong></em><em>
make_scanner
make_parser_skeleton
</em>!!<em> error_messages.make
</em><em><strong>end</strong></em><em>
</em><em><strong>feature</strong></em><em> </em>-- Access<em>
error_messages: LINKED_LIST </em>[<em>STRING</em>]<em>
</em>-- Error messages reported so far<em>
</em><em><strong>
feature</strong></em><em> </em>{<em>NONE</em>} -- Error<em> </em>reporting<em>
report_error </em>(<em>a_message</em>:<em> STRING</em>)<em> </em><em><strong>is</strong></em><em>
-- S</em>tore error message in<em> error_messages.
</em><em><strong>do</strong></em><em>
error_messages.extend </em>(<em>a_message</em>)<em>
</em><em><strong>end</strong></em><em>
</em><em><strong>invariant</strong></em><em>
error_messages_not_void</em>:<em> error_messages </em>/=<em> Void
</em><em><strong>end</strong></em><em> </em>-- class MY_PARSER</font></pre>
</blockquote>
<p>The generated parser class, named <font color="#008080"><em><tt>MY_PARSER</tt></em></font>,
inherits its lexical analyzer features (<font color="#008080"><em><tt>read_token</tt></em></font>,
<font color="#008080"><em><tt>last_token</tt></em></font> and <font
color="#008080"><em><tt>last_value</tt></em></font>) from class <font
color="#008080"><em><tt>MY_SCANNER</tt></em></font> which has
probably been generated using <a href="../gelex/index.html"><em>gelex</em></a>.
The routine <font color="#008080"><em><tt>report_error</tt></em></font>,
inherited from <font color="#008080"><em><tt>YY_PARSER_SKELETON</tt></em></font>,
has been redefined to keep track of the error messages reported
so far. Since the generic parameter of class <font
color="#008080"><em><tt>YY_PARSER_SKELETON</tt></em></font><font
color="#008080" size="2" face="Courier New"><em> </em></font>is <font
color="#008080"><em><tt>INTEGER</tt></em></font>, the semantic
values of the tokens are integers. Finally, the creation routine <font
color="#008080"><em><tt>make</tt></em></font> initializes the
lexical analyzer and the parser and makes sure that the invariant
is preserved.</p>
<hr size="1">
<table border="0" width="100%">
<tr>
<td><address>
<font size="2"><b>Copyright 1998</b></font><font
size="1"><b>, </b></font><font size="2"><strong>Eric
Bezault</strong></font><strong> </strong><font
size="2"><br>
<strong>mailto:</strong></font><a
href="mailto:ericb@gobosoft.com"><font size="2">ericb@gobosoft.com</font></a><font
size="2"><br>
<strong>http:</strong></font><a
href="http://www.gobosoft.com"><font size="2">//www.gobosoft.com</font></a><font
size="2"><br>
<strong>Last Updated:</strong> 21 May 1998</font><br>
<!--webbot bot="PurpleText"
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$Date: 1999/06/12 18:56:56 $
$Revision: 1.10 $"
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</address>
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