[antlr-interest] Manually creating tree - infinite loop

[mzukowski at yci.com]

I can't really follow what rule was getting you into trouble, but infinite
loops are usually caused because people forget that trees and nodes are the
same object.  A node contains the down and right pointers.  So if you were
to do something like #(a1, a2, a1) you would create an infinite loop.  Often
people forget to copy a node if they want something like the above.

Monty

-----Original Message-----
From: Edward Povazan [mailto:epovazan at telus.net] 
Sent: Thursday, November 06, 2003 2:17 PM
To: antlr-interest
Subject: [antlr-interest] Manually creating tree - infinite loop

Hello,

I am not too familiar with manipulating trees, and I had an infinite loop. I
'solved' it using ! to exclude the nodes from automatic tree construction,
but I still and not sure of things.

One rule in my grammar is:
rel_op:
add_op (GT^ add_op)*
;

I allow things like:
1 > 2 > 3
and I would like to convert the above to
1 > 2 and 2 > 3
so that it is easier to use in my tree parser for code generation.

The first case creates a tree like
#(> #(> 1 2) 3)
and I want
#(AND #(> 1 2) #(> 2 3))

I broke things down like this so I could build my own tree:
// add_op (GT add_op (GT add_op)*)?

a1:add_op
(
gt1:GT! a2:add_op!
{ ## = #(#gt1, #a1, #a2); }
(
gt2:GT! a3:add_op!
{ ## = #([AND, "and"], ##, #(#gt2, a2, a3)); #a2 = #a3; }
)*
)?
;

If I don't include the ! above, things loop forever in line 340 of
ASTFactory.java:
// Chase tail to last sibling
while (tail.getNextSibling() != null) {
tail = tail.getNextSibling();
}
Is this correct behaviour? I know not including ! adds redundant tree nodes
which are discarded in my own tree building, but just wondering if this
should cause infinite loops.

Also, I end up with a tree like
#(#(AND #(> 1 2 3) #(> 2 3))
and I don't see why. When is 3 being added to ## before I add it as a child
to AND.

And lastly, is there a better way to do this? Are there docs to tree stuff
like this - I found info on ## by searching the lists/internet, but not in
the official docs. Did I miss them?

Thanks for any info,
-Ed


 

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