Hiding a field for a subtype in Alloy

Question

Suppose I have the following signature declarations, in Alloy 4.2: ``` sig Target {} abstract sig A { parent: lone A, r: some Target } sig B extends A {} sig C extends A {} ``` When running, the resulting instances would have arrows from every `B` to some `Target`, and from `C` to some `Target`. How would I be able to hide only the arrows from `B`? I tried the following, at first: ``` abstract sig A { parent: lone A } sig B extends A { r: some Target } sig C extends A { r: some Target } ``` This would give me control over `r` in `B`, but it introduces a lot of ambiguity when writing properties. I would like to keep these as simple as possible. For example: ``` all a: A | a.r = parent.a.r ``` The above says that `a`'s `Targets` is the set of `a`'s children's `Targets`. With the latter declarations, I would have to rewrite this to ``` all b: B | b.r = parent.b.((B <: r) + (C <: r)) all c: C | c.r = parent.c.((B <: r) + (C <: r)) ``` which is undesirable. Is there any workaround to be ab…

Accepted Answer

You can define a function that exactly corresponds to (C <: r):

```
fun C_r : A -> Target {
    (C <: r)
}
```

In the Alloy visualizer, you wil have access to relation $C_r. Then, you can turn off "Show as arcs" for relation r, but still have $C_r visible. This should hide the set of edges that belong to (B <: r).

Hiding a field for a subtype in Alloy

Solution

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