> On May 26, 2016, at 8:51 AM, Jan E. Schotsman via swift-evolution
> <[email protected]> wrote:
>
>
> On May 26, 2016, at 3:44 PM, Austin Zheng wrote:
>
>> The inimitable Joe Groff provided me with an outline as to how the design
>> could be improved. I've taken the liberty of rewriting parts of the
>> proposal to account for his advice.
>
> Looks like great progress!
>
> A few remarks:
>
> 1.I don't see the need for AllOf. A nested existential also acts as a
> constraint bundle, so you would have to write
>
> Any<ProtocolA, AllOf< ProtocolB, ProtocolC>>
>
> for consistency.
I’ve been thinking about this as well, especially in the context of Brent’s
idea to distinguish existentials from generic constraints. My conclusion is
that disallowing use of `Any` (or typealiases of `Any`) in generic constraints
is an arbitrary restriction and introducing a new name `AllOf` for something
that is structurally the same is unnecessary and potentially confusing.
I would like to see us eventually introduce a first class mechanism for
abstracting constraints. That mechanism would become the preferred mechanism
for factoring constraints, used in both generics and existentials. When we
have that mechanism it might be reasonable to push people to use it rather than
“existential constraints”. Maybe this can even happen along side enhanced
existentials in Swift 4.
>
> In a "is subclass of" constraint the superclass also acts as a constraint
> bundle, in a sense.
>
> 2.If openas never fails why do we need to use optional binding with it?
I assume you’re talking about this:
let e1: Equatable = ...
let e2: Equatable = ...
if let storedInE1 = e1 openas T {
// is e2 also a T?
if let storedInE2 = e2 as? T {
// okay: storedInT1 and storedInE2 are both of type T, which we know is
Equatable
if storedInE1 == storedInE2 { ... }
}
}
That is a good question. I imagine the purpose is to require a new scope to be
introduced, inside which `T` is bound to the dynamic type of `e1` and
`storedInE1` is bound to a value of that type. I am not sure why we need a new
scope for these bindings rather than allowing them to be in effect in the
surrounding scope (this may have been discussed already). If the new scope is
necessary it might be worth elaborating on why. Otherwise, it would be simper
if we could just do this:
let e1: Equatable = ...
let e2: Equatable = ...
let storedInE1 = e1 openas T
// is e2 also a T?
if let storedInE2 = e2 as? T {
// okay: storedInT1 and storedInE2 are both of type T, which we know is
Equatable
if storedInE1 == storedInE2 { ... }
}
>
> 3. You have a typo in the "where clause" section: where Collection.Element
> == Streamable
>
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