Joel de Guzman <[EMAIL PROTECTED]> wrote in message news:[EMAIL PROTECTED] > Mat Marcus <[EMAIL PROTECTED]> wrote: > > --On Monday, September 01, 2003 3:37 PM -0300 Fernando Cacciola > > <[EMAIL PROTECTED]> wrote: > > > >> Joel de Guzman <[EMAIL PROTECTED]> wrote in message > >>> One can think of an optional<T> as conceptually a specialized but > >>> nevertheless, *IS-A* T, with the added specialization that it can > >>> be in a dead-uninitialized state. Maybe we'll call it a zombie > >>> object, undead object, you name it ;-) > > > > Hmmm. I'm not so sure about this. When I hear the phrase optional<T> > > IS-A T with an added specialization I am reminded of the phrase a > > Square IS-A Rectangle with an added specialization which usually gets > > folks into trouble. [Theoretical aside: I still see optional<T> as a > > sum/union, e.g. T + nil/ T | nil. That is I don't think we really want > > A + B < B (the sum/union of A and B) to be a subtype of A.] > > This is the model that I was trying to *sell* from the very very start when > optional first came out for review.
And it is a good model indeed, which I liked from the very moment you mentioned it. > I never really understood why people > didn't see it that way. This is exactly the reason why I suggested looking > at other languages: to be able to get a solid grasp of the concepts behind > such a *thing* so as to be able to answer with utmost certainty the question: > : what is optional? > > Some people say it is a container. Not! > Why? What's wrong with saying that an object of optional<T> *contains* an object of type T or is empty? The fact that Haskell doesn't model it this way, or that a discriminated union, T+nil, is a good model, doesn't mean that the container model is wrong. I don't see any significant advantage with the union model over the container model, and in fact, the union model implies the explicit usage of 'nil', something I don't find particularly convenient. > Some people say it is like a pointer > that can be NULL. Not! Here I agree. > And this uncertainty leads us to confusion, and, > ultimately: missguided syntax and semantics. > And you know here I disagree.. but let's leave this out. > [snipped] > I never really understood why I wasn't able to sell the idea that an > optional<T> is *REALLY REALLY REALLY* nothing else but a > union of T and nil. > You did sell the idea that it can be a union, but I held to the idea that it can just as well be considered as *REALLY REALLY REALLY* nothing else but a container that has a T or is empty. I agree there is nothing wrong with the union model, but I don't see why is it better than the other. > [snipped] > > I would certainly hope to hear more from you. In fact, > I wish to hear more from the type-theory people such as Mat and Vesa > ;-) > I adhere to this wish! Fernando Cacciola _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost
