[jira] [Comment Edited] (JENA-1170) QueryTransformLib cannot rename varibales

["Claus Stadler (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/JENA-1170?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15262922#comment-15262922 ] 

Claus Stadler edited comment on JENA-1170 at 4/28/16 8:30 PM:
--------------------------------------------------------------

My use case is an isomorphy cache; or rather the unit test[1] of it:
I take a query and add it to the cache with a fake result table.
Then I rename all of the query's variables and the cache component should be able to find out that there exists a mapping of the variables that makes them equal (or in the case of quad patterns, makes one a subset of the other).

So I don't have to care about scoping issues, because a global rename is just fine for my use case.

The snippet is essentially:
{code:java}
        Collection<Var> vars = PatternVars.vars(baseQuery.getQueryPattern());
        Generator<Var> gen = new VarGeneratorBlacklist(VarGeneratorImpl2.create("v"), vars);

        Map<Var, Node> varMap = vars.stream()
                .collect(Collectors.toMap(
                        v -> v,
                        v -> (Node)gen.next()));

        Query renamedQuery = QueryTransformOps.transform(baseQuery, varMap);
        // ...
        sparqlViewCache.index(baseQfpc, table);
        CacheResult cr = sparqlViewCache.lookup(renamedQfpc);

{code}

The VarGeneratorBlacklist does not generate variables mentioned in 'vars' - not sure if Jena has such a util already - if so, a pointer is appreciated so I could remove my own solution :)
(it could be done with a stream and a filter lambda x -> !vars.contains(x) but still its more convenient just passing vars to an util function)

[1] https://github.com/AKSW/jena-sparql-api/blob/develop/jena-sparql-api-cache/src/test/java/org/aksw/jena_sparql_api/concept_cache/main/TestSparqlViewCacheVariableRenaming.java#L97


was (Author: aklakan):
My use case is an isomorphy cache; or rather the unit test[1] of it:
I take a query and add it to the cache with a fake result table.
Then I rename all of the query's variables and the cache component should be able to find out that there exists a mapping of the variables that makes them equal (or in the case of quad patterns, makes one a subset of the other).

So I don't have to care about scoping issues, because a global rename is just fine for my use case.

The snippet is essentially:
{{code:java}}
        Collection<Var> vars = PatternVars.vars(baseQuery.getQueryPattern());
        Generator<Var> gen = new VarGeneratorBlacklist(VarGeneratorImpl2.create("v"), vars);

        Map<Var, Node> varMap = vars.stream()
                .collect(Collectors.toMap(
                        v -> v,
                        v -> (Node)gen.next()));

        Query renamedQuery = QueryTransformOps.transform(baseQuery, varMap);
        // ...
        sparqlViewCache.index(baseQfpc, table);
        CacheResult cr = sparqlViewCache.lookup(renamedQfpc);

{{code}}

The VarGeneratorBlacklist does not generate variables mentioned in 'vars' - not sure if Jena has such a util already - if so, a pointer is appreciated so I could remove my own solution :)
(it could be done with a stream and a filter lambda x -> !vars.contains(x) but still its more convenient just passing vars to an util function)

[1] https://github.com/AKSW/jena-sparql-api/blob/develop/jena-sparql-api-cache/src/test/java/org/aksw/jena_sparql_api/concept_cache/main/TestSparqlViewCacheVariableRenaming.java#L97

> QueryTransformLib cannot rename varibales
> -----------------------------------------
>
>                 Key: JENA-1170
>                 URL: https://issues.apache.org/jira/browse/JENA-1170
>             Project: Apache Jena
>          Issue Type: Improvement
>          Components: ARQ
>    Affects Versions: Jena 3.0.1
>            Reporter: Claus Stadler
>
> Renaming variables of a query - a rather basic use case when transforming queries - does not work with the snippet below due to a raised exception:
> {code:java}
> Map<Var, Node> vmap = Collections.singletonMap(Var.alloc("a"), (Node)Var.alloc("x"));
> Query foo = QueryFactory.create("Select ?a { ?a ?b ?c }");
> Query bar = QueryTransformOps.transform(foo, vmap);
> {code}
> The reason is, that the way QueryTransformOps.transformVarExprList[1] is implemented, prevents this. Also, a much nicer (unfortunately private) implementation already exists at Transformer.ApplyTransformVisitor.process[2] 
> I suggest to make [2] publicly accessible (e.g. as part of one of the TransformLibs or a VarExprUtils) and implement [1] based on that:
> {code:java}
> class QueryTransformOps {
>     private static void transformVarExprList(VarExprList varExprList, ExprTransform exprTransform)
>     {
>         VarExprList tmp = VarExprListUtils.transform(varExprList, exprTransform);
>         VarExprListUtils.replace(varExprList, tmp);
>     }
> }
> class VarExprListUtils {
>     public static void replace(VarExprList dst, VarExprList src) {
>         if(dst != src) {
>             dst.clear();
>             copy(dst, src);
>         }
>     }
>     public static void copy(VarExprList dst, VarExprList src) {
>         for(Var v : src.getVars()) {
>             Expr e = src.getExpr(v);
>             if(e == null) {
>                 src.add(v);
>             } else {
>                 src.add(v, e);
>             }
>         }
>     }
> }
> {code}
> [1] https://github.com/apache/jena/blob/master/jena-arq/src/main/java/org/apache/jena/sparql/syntax/syntaxtransform/QueryTransformOps.java#L74
> [2] https://github.com/apache/jena/blob/master/jena-arq/src/main/java/org/apache/jena/sparql/algebra/Transformer.java#L243
> As a side note, it would be convenient, if the transform method also accepted a Map<Var, Var> - i.e. changing the method signature to accept a Map<Var, ? extends Node>



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