[o:XML] Query on YAPP Usage

[sudhir.parisaboyina at wipro.com]

Hi,
Thanks a zillion for letting all of us know on how to use YAPP. Now I am
able to generate the parser xsl file & call it from my own style sheet
using the same as illustrated by you. But I have an observation over
here. I was trying to see what this resulting YAPP parser will do if I
pass an invalid  data (as my project requirement is to validate the data
with BNF rule). Hence I passed the in param in the call template as '123
+ abc', and I see this output as:

<term name="AdditiveExpression">
<term name="number">123</term>
<term name="AdditiveExpression-rest">
<term name="plus">+</term>
<term name="number">abc</term>
<term name="AdditiveExpression-rest"/>

Where as my BNF rule is as below

minus ::= '-' ;
plus ::= '+' ;
number ::= '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9' ;
Expression ::= AdditiveExpression end ;
AdditiveExpression ::= number | AdditiveExpression minus number |
AdditiveExpression plus number ; 


After going through the mailing archive, I have sensed that YAPP does
not support "error detection & reporting". I want to re confirm if this
is true and also willing to know whether YAPP, in future is going to
have such enhancements done. If no, can you pls give us some guidelines
on how can we make YAPP as an error detecting + reporting tool?




Thanks & Regards,
Pnvd Sudhir 
 

-----Original Message-----
From: Martin Klang [mailto:martin at o-xml.org] 
Sent: Wednesday, August 22, 2007 7:05 PM
To: Sudhir Parisaboyina (WT01 - TES-Transport Infrastructure)
Cc: o-xml at lists.pingdynasty.com; Karuna Kosaraju (WT01 - TES-Transport
Infrastructure); Siddesh P R (WT01 - TES-Transport Infrastructure)
Subject: Re: [o:XML] Query on YAPP Usage

Hi there,

Thanks for using YAPP, apologies that documentation and examples  
aren't quite up to scratch. If you would like to contribute e.g. with  
code examples or docs, please feel free.

Your grammar contains a construct that is left recursive, which makes  
it unparseable by recursive descent parsers - see for example http:// 
en.wikipedia.org/wiki/Left_recursion for more details.

YAPP is a recursive descent parser, hence the generated grammar  
doesn't produce a valid parser.
The solution is quite simple, YAPP comes with a stylesheet that will  
eliminate left recursion.

I managed to get your example working with the following process:

1) create YAPP BNF parser and lexer.
xalan -XSL generator.xsl -IN bnf-grammar.xml > bnf-parser.xsl
xalan -XSL tokenizer.xsl -IN bnf-grammar.xml > bnf-lexer.xsl

2) generate XML grammar from calculator BNF grammar:
xalan -XSL bnf-parser.xsl -IN calculator-grammar.bnf > calculator- 
grammar.xml

3) fix left recursion problem:
xalan -XSL eliminator.xsl -IN calculator-grammar.xml > fixed- 
calculator-grammar.xml

3) generate parser and lexer:
xalan -XSL generator.xsl -IN fixed-calculator-grammar.xml >  
calculator-parser.xsl
xalan -XSL tokenizer.xsl -IN fixed-calculator-grammar.xml >  
calculator-lexer.xsl

All files apart from calculator-grammar.bnf are supplied with YAPP, I  
created it from your example as follows:
<grammar>
   <terminal name="end">
     <end/>
   </terminal>
   <ignore char=" "/>
   <bnf>
minus ::= '-' ;
plus ::= '+' ;
number ::= [.0123456789] ;
Expression ::= AdditiveExpression end ;
AdditiveExpression ::= number | AdditiveExpression minus number |  
AdditiveExpression plus number ;
   </bnf>
</grammar>

I then called the resulting parser with this stylesheet:
<xsl:stylesheet version="1.0"
                 xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                 xmlns:p="http://www.pingdynasty.com/namespaces/parser">
   <xsl:import href="calculator-lexer.xsl"/>
   <xsl:import href="calculator-parser.xsl"/>
   <xsl:template match="/">
     <output>
       <xsl:call-template name="p:Expression">
         <xsl:with-param name="in" select="'123 + 456 - 789'"/>
       </xsl:call-template>
     </output>
   </xsl:template>
</xsl:stylesheet>

... and I got this result:
<output>
   <term name="Expression">
     <term name="AdditiveExpression">
       <term name="number">123</term>
       <term name="AdditiveExpression-rest">
         <term name="plus">+</term>
         <term name="number">456</term>
         <term name="AdditiveExpression-rest">
           <term name="minus">-</term>
           <term name="number">789</term>
           <term name="AdditiveExpression-rest"/>
         </term>
       </term>
     </term>
     <term name="end"/>
   </term>
   <remainder/>
</output>


The construct called AdditiveExpression-rest has been created by  
YAPP, it can be ignored, or filtered out with another XSL.

I hope this helps, let me know how it goes!

/m




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