Boolean Fields

Often software properties are expressed as booleans, either true or false. But Alloy doesn’t have a native boolean type. There are two ways to emulate this.

For this example we want something akin to

sig Account {
  premium: bool -- invalid, not a real type
}

Subtyping

Using in subtypes is the standard way to model booleans:

sig Account {}
sig PremiumAccount in Account {}

Then the boolean can be tested with a in PremiumAccount. This method has a number of advantages:

• PremiumAccount and Account - PremiumAccount can be used in the signatures of other fields.
• Premium accounts can have additional fields.
• You can create a custom theme for premium accounts.
• The analyzer will generally be faster.

lone fields

Booleans can also be represented using lone:

one sig Premium {}
sig Account {
  , premium: lone Premium
}

Then the boolean can be tested with some a.premium, and the set of all premium accounts is premium.Premium. Using lone is somewhat simpler than using a subtype, but it’s less flexible overall, and the boolean cannot be used in the fields of other signatures.

The Boolean module

If subtyping is insufficient, you can also use the boolean module. This is generlaly not recommended.