[swift-evolution] Union instead of Optional

[frogcjn at 163.com]

Optional should be implement by union concept s instead of using generic.

Enum cannot be easily used to represent many types until you name all of them. 

Both generic and enum solution cannot build a strong relation between optional type and its original type, for example, an Integer type has no reparation with Optional<Int>, until the compiler make a special case for Optional<T> to automatically unwrap or wrap for this.

> 在 2016年5月15日，15:19，David Hart <david at hartbit.com> 写道：
> 
> Enums with associated types in Swift are the equivalent of unions, even if they don’t have the name. I don’t see what your proposal is trying to solve?
> 
>> On 15 May 2016, at 04:33, Cao Jiannan via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> 
>> It’s kind of same idea of TypeScipt 2, at 46:21 in this video.
>> 
>> https://channel9.msdn.com/Events/Build/2016/B881
>> 
>> <屏幕快照 2016-05-15 10.20.36.png>
>> 
>> 
>>> 
>>> 2016-2-16 GMT+8 14:36:28
>>> Hi all,
>>> 
>>> I think the best way to solve the either problem is to separate it from generic. 
>>> Optional and Either shouldn’t work the same way of generic type. 
>>> It’s just a represent of multiple type in one location.
>>> 
>>> Using an old friend, Union in C.
>>> union {
>>> 	case firstType
>>> 	case secondType
>>> }
>>> 
>>> This is the final solution for the sub typing problem of optional.
>>> 
>>> A  == union(A,A)
>>> union(A,B) == union(B,A)
>>> B == union(B,B)
>>> 
>>> B is subtype of union(A,B)
>>> A is subtype of union(A,B)
>>> union(A,B,C) is subtype of union(A,B,C,D,…)
>>> 
>>> suppose 
>>> a is subclass of A
>>> b is subclass of B, then
>>> 	union(a,B) is subtype of union(A,B)
>>> 	union(A,b) is subtype of union(A,B)
>>> 	union(a,b) is subtype of union(a,B)
>>> 	union(a,b) is subtype of union(A,b)
>>> 
>>> union can have as many case as possible. e.g., union(A,B,C,D,…)
>>> 
>>> So the Optional<UITableView> should be union(UITableView, None)
>>> and Optional<MyTableVIew> should be union(MyTableView, None), which is subclass of union(UITableView, None)
>>> 
>>> This is a final rational solution. I think.
>>> 
>>> -Jiannan
>> 
>> 
>> 
>> 
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> 
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