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I'm not sure I agree that Void is a bottom type. If so, void-functions would never be able to return/terminate. Java's void is probably more of a unit type.
They allude to this later, acknowledging that it's sort of a cross between unit and bottom.
No it's not, it is 100% a unit type (except it's not really a type, since you can only use it as return type and nowhere else)
It's not possible to instantiate or assign, which is more like a never type than a unit; and it is not possible to define new types with the same properties, which is also more like bottom than unit. But you're right that it's not actually a true never type since it can't represent function divergence.
I think the truth is just that Java's type system isn't very mathematically disciplined.
It's not possible to instantiate or assign, which is more like a never type than a unit
Actually, this is because
voidis not a type, it is just a keyword, a placeholder used instead of the return type when a function doesn't return anything.If it were a bottom type, that would mean that a method returning
voidmust diverge, which is simply not true.Also, if it were a bottom type, it would be possible to write an "unreachable" method
void unreachable(void bottom) { return bottom; }Even though it couldn't be called, it should be possible to define it, if
voidwas a bottom type. But it is not, becausevoidisn't a bottom type, it's no type at all.The post has been edited; it looks like someone on reddit made essentially the same point. You're right of course that
voidisn't a true type in Java, but the post now also discusses Void, which I suppose just shows how void infects the type system despite not being a type.