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 <da...@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@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 >> >> >> >> >> _______________________________________________ >> swift-evolution mailing list >> swift-evolution@swift.org >> https://lists.swift.org/mailman/listinfo/swift-evolution >
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