使用util / ordering在Alloy中订购

Question

I am trying to learn about how ordering works in Alloy. I have a time signature which I have used to instantiate the ordering module. I want the predicate addPage to add a page to the book at time t' where t' = t.next. (Basically add a page to the Book on the next time) However it is not working as expected and instead Time2 has lesser number of pages than Time1. Can someone explain to me why this is happening? Thanks.

open util/ordering[Page] as P0
open util/ordering[Time] as T0


sig Page {}


sig Time {}


sig Book
{   
  pages: Page -> Time
}


pred addPage(b:Book, p:Page, t: Time)
{
     t != T0/last implies
   {
       let t' = t.next |
           b.pages.t' = b.pages.t + p
   }
}


run addPage {} for 3

Answer 1

The problem are the extra curly braces in the run statement. I think Alloy executes an empty predicate in this case.

Try:

run addPage for 3

instead. You will see a visualization where the selected instances for b, t and p are marked.

Answer 2

You're trying to change state which can only be simulated in constraint logic. Please notice that the expression in addPage is basically ineffective /run your model without it/ and that there's only one Book atom in the solution.

Here's a model you can start with and gradually refine.

open util/ordering[Time]

sig Page {}
sig Time {}

sig Book {   
  pages : Page lone -> Time // each Time atom is mapped to at most one Page atom
}

pred addPage(b0, b1 : Book, pg : Page, t0, t1 : Time) {
    one pg                          and // one page at a time (it's likely redundant)

    not pg in b0.pages.Time         and // it's a 'new' page
    b0.pages + pg->t1 = b1.pages    and // 'new state' of b0
    t1 = t0.next                        // pg is 'added' with the next time stamp
}

run addPage for 3 but 2 Book

I used the optional 'and' operators, placed t1 = t0.next at the end of the constraint, positioned b1.pages /representing the 'new state'/ on the right and used quotes in the comments to emphasize that there's no real state change and sequence of operation in the sense imperative programming works.