sig是抽象的，但合金分析仪是它的一个实例

Question

I have the following very simple model in Alloy:

abstract sig Object {}
pred show(){}
run show for 5

when I execute this model, Why alloy analyzer still makes an instance of Object while it is defined to be abstract!

Answer 1

An abstract signature can contain atoms so long as it has no subsignatures. This is designed to allow you to build and animate a model incrementally, so you can mark a signature as abstract with the expectation that subsignatures will be introduced later.