Value assignment in Alloy and use of Enum - enums

Question

How to assign variable in Alloy?
Sig ClassA{
variable_1: String,
variable_2: Int
}
Sig ClassB{
variable_1: String,
variable_2: Int
}
pred isLess[a:ClassA,b:ClassB]{
a.variable_2 < b.variable_2
}
assert integrityTest{
all a:ClassA,b:ClassB| isLess[a,b]
}
Now I want to check counterexample of variables when I assign some bigger value in a.variable_2 than b.variable_2. But I am not sure how to assign variable in Alloy. Only thing I came up with is following but it is not working-
pred assignValue[a:ClassA]{
a.variable_2 = Int[4]
}
However, I believe it will only check the equality with 4 and return as false. It has nothing to do with the assignment. So my question is how should I produce counterexample when a.variable_2>b.variable_2
And How can I use Enum of Int in alloy? Like- Enum MetricValue {1,2,3,4,5} to assign a.variable 1.
EDIT
I am still having trouble finding the counterexample, even though I can find one by manually checking when I toggle the value of variable_2 of ca and cb.
sig ClassA{
variable_1: String,
variable_2 = Int
}
sig CA extends ClassA{}{
variable_2 = 1
}
sig ClassB{
variable_1: String,
variable_2 = Int
}
sig CB extends ClassB{}{
variable_2 = 4
}
pred checkAllConstraint [ca: ClassA, cb: ClassB] {
ca.variable_2 > cb.variable_2 }
assert checkAll{
all ca: ClassA, cb: ClassB | checkAllConstraint[ca,cb]
}
check checkAll for 15

Answer 1

You can restrain a field to a single value through facts. In your case, signature facts come handy.
It will look like this:
sig ClassA{
variable_1: String,
variable_2: Int
}{
variable_1="hello world"
variable_2=4
}
To bound a field to one value amongst a set, you can use the "in" keyword instead of "=" as follows:
sig ClassA{
variable_1: String,
variable_2: Int
}{
variable_1 in ("hello" + "world")
variable_2 in (1+2+3+4)
}
Note that in Alloy + is a UNION operator. It doesn't sum nor concatenate as you might expect.
EDIT
It doesn't work for 2 reasons:
you wrote : variable_2 = Int instead of variable_2: Int.
By doing so, no valid instance contains atoms typed by CA or CB, because e.g. ClassA.variable2 is constrained to be the set of all integers and CA.variable2 is constrained to be 1
no String atom is defined. That's one of Alloy's weird part. If you want to use Strings in your model, you need to have string litterals somewhere specified, e.g. in a fact.
Here's your model, corrected:
sig ClassA{
variable_1: String,
variable_2 : Int
}
sig CA extends ClassA{}{
variable_2 = 1
}
sig ClassB{
variable_1: String,
variable_2 : Int
}
sig CB extends ClassB{}{
variable_2 = 4
}
pred checkAllConstraint [ca: ClassA, cb: ClassB] {
ca.variable_2 > cb.variable_2 }
assert checkAll{
all ca: ClassA, cb: ClassB | checkAllConstraint[ca,cb]
}
check checkAll for 15
fact { String in ("test"+"test2"+"test3"+"test4")}
If you still have questions, please create a new one.
-