Hi all. Yep, I put ^ and ! suffix operators into ANTLR in about '93.
Makes building ASTs really easy for things like expressions. Note
that turning on AST construction with no operators builds a linked
list (flat tree) of entire input; very convenient.
Here's an example. This builds proper ASTs for additive expressions:
add : atom ('+'^ atom)* ;
For non-expressions, i like my rewrite syntax:
ifstat : 'if' '(' expr ')' s1=stat ('else' s2=stat)? -> ^('if' expr s1
s2?) ;
Grammar to tree grammar rewrite.
Terence
On Jun 14, 2009, at 12:00 PM, David-Sarah Hopwood wrote:
> Peter Goodman wrote:
>> I wanted to mention one of the way in which I have been building an
>> AST from
>> my parse tree while the tree itself is being constructed (a more
>> appropriate
>> term might be: reduced parse tree). I use one rule within the
>> parser and also
>> two operators within the grammar. One of these operators is already
>> well-
>> understood and has been mentioned on this list several times.
>>
>> My two operators are:
>>
>> i) A non-excludable operator (-), which requires that a non/
>> terminal appear
>> in the reduced parse tree. If no such operator is used, then
>> terminals are
>> *not* added to the tree as leafs, and non-terminals, where only one
>> child
>> exists, are replaced with their child (my automatic reduction rule).
>
> ANTLR (v3) has a '!' operator which excludes a child from appearing
> in the
> reduced tree (i.e. the opposite of your '-'). It has a similar rule to
> yours for automatically reducing nodes with only one child. I think
> that
> I prefer the approach of including children by default; it means
> that the
> default at least includes all of the necessary information.
>
>> ii) A raise children operator (^), which puts the derivations of a
>> particular
>> non-terminal in a production in place of the non-terminal. This
>> allows me to
>> collect lists of nodes.
>
> This is also similar to ANTLR.
>
> --
> David-Sarah Hopwood ⚥ http://davidsarah.livejournal.com
>
>
>
>
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