This is the example implemented with generic and enum.
enum UnionOf3 <T,U,V>{
case type0(T)
case type1(U)
case type2(V)
}
func input(value: UnionOf3<A,B,C>) {
switch value {
case let .type0(a):
print(a)
case let .type1(b):
print(b)
case let .type2(c):
print(c)
}
}
let a = A()
input(UnionOf3.type0(a))
For each different number N of types to get union, you should announce UnionOfN
for each.
it is more complicated then the union solution.
func input(value: (A | B | C)) {
switch value {
case a as A:
print(a)
case b as B:
print(b)
case let .type2(c):
print(c)
}
}
let a= A()
input(a)
And the most bad thing is,
compiler doesn’t know the type relation between generic union with original
type,
for example, the type relation between A and A | B and A | B | C.
compare:
enum UnionOf3 <T,U,V>{
case type0(T)
case type1(U)
case type2(V)
}
let a = A()
let union: UnionOf3<A,B,C> = UnionOf3.type0(a) // You should wrap by yourself.
a == union // probably the compiler will refuse to compare.
sub-typing:
var fn0: A->Void = {print($0)}
var fn1: UnionOf2<A,B>->Void = {print($0)}
fn0 = fn1 // This should be allowed. But will be rejected since using enum.
var fn2: UnionOf3<A,B,C>->Void = {print($0)}
fn0 = fn2 // This should be allowed. But will be rejected since using enum.
fn1 = fn2 // This should be allowed. But will be rejected since using enum.
---------------------------------------------------------------------------
If using union, this problem can be solved easily.
let a = A()
let union: (A|B|C) = a // Automatically wrap.
a == union // Yes
sub-typing:
var fn0: A->Void = {print(v0)}
var fn1: (A|B)->Void = {print(v0)}
fn0 = fn1 // OK
var fn2: (A|B|C)->Void = {print($0)}
fn0 = fn2 // OK
fn1 = fn2 // OK
> 下面是被转发的邮件：
>
> 发件人: Cao Jiannan via swift-evolution <[email protected]>
> 主题: [swift-evolution] Fwd: Union instead of Optional
> 日期: 2016年5月15日 GMT+8 18:34:44
> 收件人: Adrian Zubarev via swift-evolution <[email protected]>
> 回复－收件人: Cao Jiannan <[email protected]>
>
> for example, there is a method input union of 3 types: A, B, C,
>
> This is the three class.
>
> class A {}
>
> class B {}
>
> class C {}
>
> This is how it implemented under Swift 2:
>
> enum UnionABC {
> case classA(A)
> case classB(B)
> case classC(C)
> }
>
> func input(value: UnionABC) {
>
> }
>
> let a = A()
> let b = B()
> let c = C()
> input(UnionABC.classA(a))
>
>
> It needs announce all the cases and name each cases and cannot use class
> names as their case names.
>
> what about using union? It is more easy and rational.
>
>
> func input(value: (A | B | C)) {
>
> }
>
> let a= A()
> input(a)
>
> Or you can implement it with protocol and extension, but compiler will not
> know how many cases it should have.
>
>
> protocol UnionABC {
>
> }
>
> extension A: UnionABC {}
> extension B: UnionABC {}
> extension C: UnionABC {}
>
>
> func input(value: UnionABC) {
> if value is A {
>
> } else if value is B {
>
> } else if value is C {
>
> } else {
> // There are other cases? Compiler doesn't know
> }
> }
>
> let a = A()
> input(a)
>
>
>
>> 下面是被转发的邮件：
>>
>> 发件人: [email protected] <mailto:[email protected]>
>> 主题: 回复： [swift-evolution] Union instead of Optional
>> 日期: 2016年5月15日 GMT+8 18:00:55
>> 收件人: Austin Zheng <[email protected] <mailto:[email protected]>>
>>
>>
>> Enum and Union are two things.
>>
>> If you use Enum to implement Union, you should announce it with case name.
>>
>> Another issue using enum instead of union is that, union can combine types
>> as many as possible, you just write ( A | B | C ... | Z), but for enum, you
>> should carefully announce it for each case.
>>
>> 在 2016年5月15日，15:22，Austin Zheng <[email protected]
>> <mailto:[email protected]>> 写道：
>>
>>> In addition, not everything in Swift can be modeled in terms of inheritance
>>> relationships.
>>>
>>> I'm a little curious as to why union types keep on coming up, when enums
>>> can do everything they can and much more (methods, constraints on generic
>>> types, conformance to protocols).
>>>
>>> Austin
>>>>> [email protected] <mailto:[email protected]>
>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>> <https://lists.swift.org/mailman/listinfo/swift-evolution>
>>>>
>>>> _______________________________________________
>>>> swift-evolution mailing list
>>>> [email protected] <mailto:[email protected]>
>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>> <https://lists.swift.org/mailman/listinfo/swift-evolution>
>
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