[antlr-interest] AST generation in a recursive rule...the sequel (Tree parser)

[Jeff Vincent]

Great!  Thanks again Monty/Arnar ... I took out the ()+ and it worked
the first time.  I thought I had tried that, but may have tried it
before I had eliminated other errors in my rules.  
 
And, yes, the "eBlock:." is meant not to descend the subtree for the
block portion of the rule.  The reason is that I am building a run-time
scripting language and the act of parsing an AST during runtime in
effect is causing that sub-tree to be executed.  This means that I can't
parse some things unless certain conditions are met.  In a
try-catch-finally clause, an catch block is only exectued (parsed) if an
exception is caught that matches a defined catch clause (obviously).  In
a for-loop, then the block of the for loop will be executed based on the
number of times the condition is met, etc.  Traditionally, I typically
match only the root node of a conditional statement (IF, WHILE, FOR,
TRY, etc.) and manually pull the subcomponents to be evaluated and
executed based on run-time evaluation.  However, there are times when it
is easier if I can get the tree parser to match it directly, as in the
case of the catch statements.  In this case, I end up building up a
Vector of catch clauses that will be searched when an exception is
caught.  If a matching exception is found, then the corresponding block
will be parsed.  It all works pretty slick if I do say so...given that
it is all interpreted at run-time.
 
Anyway, thanks for your input.
 
Jeff

>>> mzukowski at yci.com 10/27/2003 9:05:49 AM >>>

Like Arnar said, I meant the second rule to not have a ()+:

catchBlockList :
  ( catchBlockNode )+
  {   ## = #(#[CATCHBLOCK, "catchBlock"], ##);
       String s = #catchBlockList.toStringList();
       System.out.println(s);
  }
  ;

catchBlockNode :
  CATCH^ LPAREN! identifier IDENTIFIER RPAREN! block 
  { String s = #catchBlockNode.toStringList();
     System.out.println(s);
  }
  ;

Tree parser something like this:

catchBlock :
  #(CATCHBLOCK (catchNodes)+)
  ;

catchNodes :
    #(CATCH identifier IDENTIFIER eBlock:.)
  ;

Note your . for the block means the tree parser will only match the
root and
will not decend into that subtree, I assume that is intended.

Monty

-----Original Message-----
From: Jeff Vincent [mailto:JVincent at Novell.Com] 
Sent: Friday, October 24, 2003 4:02 PM
To: antlr-interest at yahoogroups.com
Subject: [antlr-interest] AST generation in a recursive rule...the
sequel
(Tree parser)

Thanks to all those that have helped me so far (Monty, Arnar)!  I am
still
struggling to wrap my mind around it and I am now stuck trying to get a
rule
to match the tree in the tree parser, so please have patience ('cause
I'm
losing mine ;).  
 
I built the parser catch rules based on Monty's examples (see below)
and it
generates the following AST as shown by ACTUAL output from
toStringList() :
 
      ( CATCHBLOCK ( CATCH ( CATCH ( CATCH ( CATCH Exception1 e1 { )
Exception2 e2 { ) Exception3 e3 { ) Exception4 e4 { ) )
 
Please help me here if I am reading it wrong, but assuming that the
token
nearest an opening parenthesis is a sub-tree root, I think the actual
tree I
am building translates to the following tree-like form (siblings
right,
children down):
 
CATCHBLOCK
   |
   V
CATCH --> Exception4 --> e4 --> block
   |
   V
CATCH --> Exception3 --> e3 --> block
   |
   V
CATCH --> Exception2 --> e2 --> block
   |
   V
CATCH --> null
   |
   V
Exception1 --> e1 --> block
 
I think the output for the tree Monty initially suggested should look
more
like the following toStringList() form :
 
     ( CATCHBLOCK ( CATCH Exception4 e4 { ) ( CATCH  Exception3 e3 { )
(
CATCH  Exception2 e2 { ) ( CATCH Exception1 e1 { ))
 
but I digress.  My problem now is parsing the actual AST from within
the TreeParser.   Here are my TreeParser rules:
 
 catchBlock :
  #(CATCHBLOCK (catchNodes)+)
  ;
 
 catchNodes :
    #(CATCH catchNodes catchNodes)
  | identifier IDENTIFIER eBlock:.
  ;
 
The above TreeParser rules correctly recurse down and match the "CATCH
Exception1 e1 { " sub-tree, but thereafter gets a NullPointerException
because upon attempting to match "Exception2 e2 {" because the sibling
of
the last catch statement is null (I think).  The last catch is also
not
consistent with the others (assuming I am seeing it correctly).  I
would
appreciate any enlightenment.  Below are my Parser rules that builds
this
tree for reference.
 
Thanks for helping me grasp this stuff,

Jeff
 
<----------------Parser Rules----------------->
//Based on examples from Monty taken from previous e-mail thread
catchBlockList :
  ( catchBlockNode )+
  {   ## = #(#[CATCHBLOCK, "catchBlock"], ##);
       String s = #catchBlockList.toStringList();
       System.out.println(s);
  }
  ;
 
 catchBlockNode :
  ( CATCH^ LPAREN! identifier IDENTIFIER RPAREN! block )+
  { String s = #catchBlockNode.toStringList();
     System.out.println(s);
  }
  ;
<----------------Parser Rules: END------------->
 

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