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https://issues.apache.org/jira/browse/LUCENE-3041?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13027080#comment-13027080
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Earwin Burrfoot commented on LUCENE-3041:
-----------------------------------------
I vehemently oppose introducing the "visitor design pattern" (classic
double-dispatch version) into the Query API. It is a badly broken replacement
(ie, cannot be easily extended) for multiple dispatch.
Also, from the looks of it (short IRC discussion), user-written visitors and
rewrite() API have totally different aims.
- rewrite() is very specific (it is a pre-search preparation that produces
runnable query, eg expands multi-term queries into OR sequences or wrapped
filters), but should work over any kinds of user-written Queries with possibly
exotic behaviours (eg, take rewrite from the cache). Consequently, the logic is
tightly coupled to each Query-impl innards.
- user-written visitors on the other hand, may have a multitude of purporses
(wildly varying logic for node handling + navigation - eg, some may want to see
MTQs expanded, and some may not) over relatively fixed number of possible node
types.
So the best possible solution so far is to keep rewrite() asis - it serves its
purporse quite well.
And introduce generic reflection-based multiple-dispatch visitor that can walk
any kind of hierarchies (eg, in my project I rewrite ASTs to ASTs, ASTs to
Queries, and Queries to bags of Terms) so people can transform their query
trees.
The current patch contains a derivative of [my original
version|https://gist.github.com/dfebaf79f5524e6ea8b4]. And here's a
[test/example|https://gist.github.com/e5eb67d762be0bce8d28]
This visitor keeps all logic on itself and thus cannot replace rewrite().
> Support Query Visting / Walking
> -------------------------------
>
> Key: LUCENE-3041
> URL: https://issues.apache.org/jira/browse/LUCENE-3041
> Project: Lucene - Java
> Issue Type: Improvement
> Components: Search
> Reporter: Chris Male
> Priority: Minor
> Attachments: LUCENE-3041.patch, LUCENE-3041.patch, LUCENE-3041.patch,
> LUCENE-3041.patch
>
>
> Out of the discussion in LUCENE-2868, it could be useful to add a generic
> Query Visitor / Walker that could be used for more advanced rewriting,
> optimizations or anything that requires state to be stored as each Query is
> visited.
> We could keep the interface very simple:
> {code}
> public interface QueryVisitor {
> Query visit(Query query);
> }
> {code}
> and then use a reflection based visitor like Earwin suggested, which would
> allow implementators to provide visit methods for just Querys that they are
> interested in.
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