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

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

Claus Stadler created JENA-1170:
-----------------------------------

             Summary: QueryTransformLib cannot rename varibales
                 Key: JENA-1170
                 URL: https://issues.apache.org/jira/browse/JENA-1170
             Project: Apache Jena
          Issue Type: Bug
          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:

        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);

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:

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);
            }
        }
    }
}


[1] https://github.com/apache/jena/blob/master/jena-arq/src/main/java/org/apache/jena/sparql/algebra/Transformer.java#L243


[2] https://github.com/apache/jena/blob/master/jena-arq/src/main/java/org/apache/jena/sparql/syntax/syntaxtransform/QueryTransformOps.java#L74

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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