Hello,
The method 'clone' of class 'StringPrintContext' in the SPIN API does not copy
the 'initialBindings' member. This causes problems when a query template
contains placeholders in a FILTER expression. A more detailed description
follows:
Let's say I have a query template that contains a FILTER expression:
...
FILTER (?dep >= ?earliestDep }
...
for which the corresponding SPIN description (in Turtle / N3 notation) is
something like:
ex:myTemplate
rdf:type spin:Template ;
spin:body [
# ...
sp:where (
# ...
[ rdf:type sp:Filter ;
sp:expression [ rdf:type sp:ge ;
sp:arg1 _:dep ;
sp:arg2 _:earliestDep ] ]
# ...
)
] .
_:earliestDep sp:varName "earliestDep"^^xsd:string .
Let's say the variable ?earliestDep is a placeholder. Hence, my template
definition contains something like:
ex:myTemplate spin:constraint [
rdf:type spl:Argument ;
spl:predicate ex:earliestDep ;
spl:valueType xsd:double ] .
Now I instantiate this template. A SPIN description for this instantiation
could be:
_:myTemplateInstance
rdf:type ex:myTemplate ;
ex:earliestDep 4.5 .
For this instantiation I expect the resulting SPARQL query to contain this
FILTER expression:
...
FILTER (?dep >= 4.5 }
...
But this does not happen. The placeholder is not replaced by the literal 4.5
as I would expect. I tracked this problem down: The SPIN API initializes a
'StringPrintContext' with the an initial binding that correctly maps the
earliestDep placeholder to the literal. Then, it prints the body of the
template to this 'StringPrintContext'. Several print methods are being called
(recursively) and until -at a certain point- the expression in the FILTER
clause has to be printed and the method 'printExpressionString' in class
'SPINExpressions' is called. Here, the Resource that represents the expression
is casted to a 'FunctionCall', the current print context is cloned, and the
'FunctionCall' is printed to the _cloned_ print context. Since the 'clone'
method of the 'StringPrintContext' does not copy the 'initialBindings', none
of the arguments of the 'FunctionCall' can be subsituted using the initial
bindings.
For this reason I propose to copy the 'initialBindings' when a clone of
'StringPrintContext' is created.
Greetings,
Olaf
----------------------
Olaf Hartig (http://olafhartig.de)
Database and Information Systems Research Group
Department of Computer Science
Humboldt-Universität zu Berlin
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