Relational Join and Operators in Alloy

Question

I've been going over Alloy in general and found some concepts that I need some clearance on.

First of all, the . (Dot Join). I got how it works for trivial examples but in a case like this:

sig B {}
sig A {
rel: B -> C
}
sig C {
rel1: B -> A
}
rel = {(a1,b1,c1), (a2,b2,c2)}
rel1 = {(c1,b1,a1),(c2,b2,a2)}
rel.rel1 = {(a1,b1,b1,a1),(a2,b2,b2,a2)}
rel1.rel = {(c1,b1,b1,c1),(c2,b2,b2,c2)

I don't get the result of rel.rel1 or rel1.rel. Can somebody explain how it works please?

I am also having problems with <: and >: operators.

Thanks in advance.

Answer 1

This is explained in detail with lots of examples in my book (Software Abstractions, MIT Press, 2012).

Also, you might find these slides from an old talk helpful:

From slide 80 on, there's an extended example of various dot joins.