FnO - Does a Function Triples Map need a Logical Source?

[pmaria]

The current use case for a LogicalSource definition on a FunctionTriplesMap seems to be:

The ability to generate values from a different source and use these values as the result of a Function Term Map.

An example of this is included in one of the proposed FnO test cases: RMLFNOTC009

However, since a FunctionTriplesMap doesn't generate values directly, but generates intermediate function execution triples expressed in FnO, the question of how to handle joins between a TriplesMap and a FunctionTriplesMap with a different LogicalSource arises.

As this is not the same type of join as a join on a RefObjectMap this join would have to be defined. Subsequently, this would require another specific type of join to be implemented by engines.

At the same time we have a very similar mapping challenge for generating literal values by a joining different logical sources: join-on-literal challenge.

I believe it would be advantageous to come up with a solution that covers both generating literals from different LogicalSources using joins, as generating function values from different LogicalSources.
As this solution would not be specific to functions, I think we should look for a solution in the definition of LogicalSources. (pinging @thomas-delva)

[pmaria]

closing this since there is a similar discussion at kg-construct/rml-lv#16. Sorry for the noise :)