Adding getRelation and tuple iterator to SWIG Interface

[cwarden]

I've got a simple go application calling a souffle program using SWIG.

I'd like to update the SWIG interface so I can get a relation's tuples. I updated the SWIG interface to add getRelation, but am not sure how to go about adding bindings for the iterator. Instead of returning souffle::Relation* directly, should I wrap it in a new SWIGSouffleRelation class similar to SWIGSouffleProgram, and then implement a method to iterate using the underlying iterator? Is there a simpler or more idiomatic solution?

Ultimately, I'm thinking the interface would allow for something like

	p := SwigInterface.NewInstance("example")
	p.LoadAll(".")
	p.Run()
	r := p.GetRelation("path")
	for {
		tuple, ok := r.Next()
		if !ok {
			break
		}
		fmt.Printf("%+v", tuple)
	}

[b-scholz]

I can vaguely remember we had an initial prototype accessing the tuples of a relation. Indeed, you will need a wrapper because we could not directly use Souffle's native C++ interface in SWIG. Great that you are looking into this issue!

[b-scholz]

I found an earlier version from Andres. Unfortunately, it seems that the relational interface got lost. Please can you have a look:
Interfaces with examples.zip

[cwarden]

Thanks. It's looking promising: cwarden/souffle-go-example@bbeb0ea

[b-scholz]

I can remember now that students who worked on the swig interface had some issues w.r.t. relations. I cannot remember the details.

If you have time, it would be great if you could re-introduce the relations again. You find the swig sources of soufflé in src/include/souffle/swig.

[cwarden]

@b-scholz how does #2014 look? Suggestions for improvement welcome.

Here's the example using the updated interface: https://github.com/cwarden/souffle-go-example/blob/688c8f33d5ae73cf875e52b994032d46166cac5b/example.go