合金中的 sig 文字

Question

How can I write out a literal for a sig in Alloy? Consider the example below.

sig Foo { a: Int }
fact { #Foo = 1 }

If I execute this, I get

| this/Foo | a |
|----------|---|
| Foo⁰     | 7 |

In the evaluator, I know I can get a reference to the Foo instance with Foo$0 but how can I write a literal that represents the same value?

I've tried {a: 7} , but this is not equal to Foo$0 . This is intentionally a trivial example, but I'm debugging a more complex model and I need to be able to write out literals of sigs with multiple fields.

Answer 1

Ah, this is one of the well hidden secrets! :-) Clearly in your model you cannot refer to atoms since the model is defining all possible values of those atoms. However, quite often you need your hands on some atom to reason about it. That is, you want to be able to name some objects.

The best way to get 'constants' is to create a predicate you call from a run clause. In this predicate, you define names for atoms you want to discuss. You only have to make sure this predicate is true .

pred collision[ car1, car2 : Car, road : Road ] {
   // here you can reason about car1 and car2
}
run collision for 10

Another way is to create a quantification whenever you need to have some named objects:

run  {
   some car1, car2 : Car, road : Road {
      // here you can reason about car1 and car2 and road
   }
} for 10

There was a recent discussion to add these kinds of instances to the language so that Kodkod could take advantage of them. (It would allow faster solving and it is extremely useful for test cases of your model.) However, during a discussion this solution I presented came forward and it does not require any new syntax.

Answer 2

try to put a limitation for 'Integer' in the 'run' command. I mean :

sig Foo {a : Int}
fact{ #Foo = 1}

pred show {}
run show for 1 Foo, 2 Int