Inheritance

Sigs may inherit from other sigs via the extends keyword:

sig <name> extends <parent sig name> {
    <additional fields> ...
}

Sigs may only have at most one parent sig. Moreover, much like how no object can be belong to multiple top-level sigs, no object can belong to more than one immediate child of any sig. That is, any two sigs A and B will never contain an object in common unless one is a descendent of the other.

Different kinds of cat

sig Cat {
    favoriteFood: one Food
}
sig ActorCat extends Cat {
    playName: one Play
}
sig ProgrammerCat extends Cat {}

This means that any ProgrammerCat object is also a Cat object, and so will have a favoriteFood field. But only ActorCats have the playName field. Moreover, any cat may be either an ActorCat, ProgrammerCat, or neither---but not both.

Inheritance and Bounds

Forge must have bounds for every sig, including child sigs. The default of 0-to-4 objects is applied to every top-level sig. Forge can often infer consistent bounds for child sigs, but it cannot always do so and will require you to provide them. This is most often the case in the presence of complex hierarchies involving abstract and one sigs.

More importantly, example and inst syntax require the contents of parent sigs to be defined once the contents of a single child are set. To see why, consider this example:

example workingCats is {myPredicate} for {
    ActorCat = `ActorCat0 + `ActorCat1
    ProgrammerCat = `ProgrammerCat0 + `ProgrammerCat1
}

This produces an error:

run: Please specify an upper bound for ancestors of ActorCat.

This error occurs because Forge knows only that there are 2 specific actors and 2 specific programmers, and can't infer whether any other cats exist. To fix the error, provide bounds for the parent sig:

example workingCats is {myPredicate} for {    
    ActorCat = `ActorCat0 + `ActorCat1
    ProgrammerCat = `ProgrammerCat0 + `ProgrammerCat1
    Cat = `ActorCat0 + `ActorCat1 + `ProgrammerCat0 + `ProgrammerCat1
}

Forge Documentation

Inheritance