Marmotta / KiWi native SPARQL function support

[Sebastian Schaffert <ss...@apache.org>]

Dear all,

I'd like to introduce another improvement: (simplified) native support for
custom SPARQL functions.

What you can do with it: in case you want to implement a specialised custom
SPARQL function based on a function available in the underlying database,
you can now do so using the Java ServiceLoader as "plugin mechanism".

To implement a custom native function, you need to do the following things:
1. implement the
interface org.apache.marmotta.kiwi.sparql.function.NativeFunction
2. add a reference to your implementation class (FQN) to

  META-INF/services/org.apache.marmotta.kiwi.sparql.function.NativeFunction
3. add a reference to your implementation class (FQN) to
    META-INF/services/org.openrdf.query.algebra.evaluation.function.Function


The NativeFunction interface extends the basic Sesame functions with some
methods needed for the translation into SQL. Most importantly, these are
isSupported (return true if there is a mapping for the given dialect) and
getNative (return a string representing the function in SQL, taking the
passed string arguments). An example is given e.g. here:

https://git-wip-us.apache.org/repos/asf?p=marmotta.git;a=blob;f=libraries/kiwi/kiwi-sparql/src/main/java/org/apache/marmotta/kiwi/sparql/function/string/NReplace.java;h=cd1b848618a727f8a8b1206462d8d87c3cee86fb;hb=refs/heads/develop

This also works for aggregation functions. For example, I have a custom
implementation for the standard deviation:

https://git-wip-us.apache.org/repos/asf?p=marmotta.git;a=blob;f=libraries/kiwi/kiwi-sparql/src/main/java/org/apache/marmotta/kiwi/sparql/function/custom/Stddev.java;h=24898de35be95bd73fe44a0265e0bcde542bb5ec;hb=refs/heads/develop

When you are implementing custom SPARQL functions, you should provide also
a Java implementation of the call method so backends that are not supported
can evaluate the function in-memory. At the very least, the call method
should throw an UnsupportedOperationException.

I noticed that there are several people here who would be interested in
such extensions. If you encounter troubles with this, feel free to contact
me here.

Cheers,

Sebastian