It is no a mistake. since fn1: (A|B)->Void is subtype of fn0: A->Void
Detail explain:
var fn0: A->Void = {print($0)}
var fn1: (A|B)->Void = {print(v0)}
let a = A()
let b = B()
So:
fn0( a ) // this is OK
fn1( a ) // this is also OK
fn1 is subtype of fn0, because fn1 can do anything fn0 do.
Thus fn0 = fn1 is OK.
But:
fn1( b ) // this is OK
fn0( b ) // this is not OK
So fn0 is not subtype of fn1
At 2016-08-11 10:41:02, "Step C" <schristop...@bignerdranch.com> wrote:
Shouldn't it be "fn1 = fn0"? Same for the fn2 statements.
`var fn0: A->Void= {print(v0)}
var fn1: (A|B)->Void= {print(v0)}
fn0 = fn1 // OK, because Original Type and Union Type has a sub-typing
relationshipvar
fn2: (A|B|C)->Void= {print($0)}
fn0 = fn2 // OK
fn1 = fn2 // OK`
On Aug 10, 2016, at 9:28 PM, Cao Jiannan via swift-evolution
<swift-evolution@swift.org> wrote:
Hi all,
I want to make a discussion about union type for swift 4.
See
https://github.com/frogcjn/swift-evolution/blob/master/proposals/xxxx-union-type.md
Add union type grammar, represents the type which is one of other types.
var stringOrURL: String| URL ="https://www.apple.com";
Now, if we using the new union type feature, we can declare type conveniently,
No other type declaration, and compiler will automatically calculate the common
interface.
funcinput(value: A | B | C) {
print(value.commonProperty) // type checker will calculate the common
interface, developer just use it out of boxswitch value {
caselet value as A:
// value is type Aprint(value.propertyInA)
caselet value as B:
// value is type Bprint(value.propertyInB)
caselet value as C:
// value is type Cprint(value.propertyInC)
}
// there is no default case other than A, B or C. we already declared that.
}
Note: A, B, C can be either class or protocol, or any other types. This leaves
developer more freedom.
Impact on existing code
This is a new feature, developer who need declare common type will alter to
this new grammar.
Enum based version optional or IUO will be replaced by Union-based ones. Any
optional type will automatically replaced by union type
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