覆盖Alloy中的对象

Question

I've been trying to make my project a little bit more dynamic, so I've decided to write an add function, I got inspired from this example they have on the Alloy website :

pred add [b, b': Book, n: Name, t: Target] {
  b'.addr = b.addr + n->t 
}

But what I want to do is override the two objects "b" and "b' " (as in I want to add to the same object) but i can't seem to find a way how to do it.

If anyone has an idea I'll be thankful

Answer 1

You're not missing anything. The reason you cannot find a way to "add to the same object" is that you can't. Alloy doesn't have mutation; you cannot change any object in a model.

Applications that involve mutation are often modeled just as you model add in your example: with two parameters showing the pre-state and the post-state. In such a model, any two Book objects b1 , b2 will show different states of the same mutable Book object in the application domain if and only if there is some n , t such that add[b1, b2, n, t] holds.