[antlr-interest] Exploit ambiguity in tree rewriter

[nafur]

Hi,

While I'm not completely sure how this would look like, this would
somehow result in implementing the pattern matching myself, right?
As this example is a very easy one, this is no option for me. Actually,
I'm only using ANTLR because of this pattern matching...

Gereon

On 06/13/2012 05:34 AM, Benjamin S Wolf wrote:
> The best way to accomplish this is likely to be an arbitrary action, eg.:
> 
> a : INT -> { new CommonTree(new CommonToken(FLOAT, $INT.text + ".0")) } ;
> 
> This takes an INT and returns a Tree with one node, a FLOAT token with
> text that is the int plus ".0".
> 
> You'll want to make a tree via CommonTree() and add a list of children
> that excludes all tokens that have IDENTITY (or add each child if it
> doesn't have the identity). This means you'll be looping over all the
> nodes in the MULT subtree within the action.
> 
> On Mon, Jun 11, 2012 at 2:01 AM, nafur <nafur42 at gmail.com> wrote:
>> Hi,
>>
>> I asked about this ambiguity quite a while ago. Does nobody has any idea
>> how to do this?
>>
>> Gereon
>>
>> On 05/29/2012 12:48 PM, nafur wrote:
>>> Hi,
>>>
>>> I'm using tree rewriters to manipulate formulas that are represented as
>>> ASTs. One operation I'd like to do is: "remove any identity from a
>>> multiplication".
>>>
>>> The replacement rule I'm currently working with is the following:
>>>
>>> topdown :
>>>       ^( MULT (before += .)* x=. (after += .)* )
>>>       { $x.has("IDENTITY") }?
>>>       -> ^( MULT $before* $after* )
>>> ;
>>>
>>> As you probably see, we have some ambiguity here: before and after can
>>> both take arbitrarily many elements. If I take an AST like
>>> ^( MULT A I x ), the replacement will not match. printing some debug
>>> output reveals, that ANTLR will only check $x.has("IDENTITY"), i.e. will
>>> only try to match the last element in a multiplication.
>>> (That is actually not a surprise, as ANTLR issues a warning...)
>>>
>>> Is there any way to resolve this ambiguity in a way, such that I can
>>> match all occurrences?
>>>
>>> Thanks,
>>> Gereon
>>
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