alloy

From predicate calculus style to relation calculus style in Alloy

I have two Alloy facts:

fact A5 {
  all a, b : Filler, s, t : Slot |
    (b in s.slot_of and s in a.fills and a in t.slot_of) implies b in t.slot_of 
}
fact A6 {
  all a, b : Filler, s, t : Slot | ((b in s.slot_of  and s in a.fills) and
      (a in t.slot_of and t in b.fills)) implies a = b
}

As I noticed that relation calculus style seems more efficient than predicate calculus style (less variables and clauses generated), I wanted to rewrite them using relation calculus style.

For A5, I have this:

fact A5 { slot_of.fills.slot_of in slot_of }

which works well and is, if I correctly understand the styles, in relation calculus style.

But for A6, I didn’t achieve to rewrite it. For now, I got this:

fact A6 {
  all a, b : Filler | (a->b in fills.slot_of and b->a in fills.slot_of) implies a = b
}

which is more efficient, but I think it is still in predicate calculus style. So I have some questions: is my current A6 in predicate calculus style? is it possible to rewrite it in relation calculus style? if so, how?

Solution

  • It's still in predicate calculus style because you're using the all quantifier. To be relational, it can't use any quantifiers.

    I think your intended fact is "the only symmetric relations are reflexive ones". The set of all reflexive relations is iden and the set of all symmetric relations in r is r & ~r. So if you want to make A6 fully relational, you'd write fills.slot_of & ~(fills.slot_of) in iden.