I see, that makes totally sense. I thought about `inout` only in terms of
mutation not reassignment from within the scope itself.
Am 10. Dezember 2017 um 17:56:56, Jonathan Keller (jkeller...@icloud.com)
schrieb:
No, inout should *not* be covariant, since foo can assign any subtype of P to
its parameter, not just F:
protocol P {}
struct F: P {}
struct G: P {}
func foo(_ p: inout P) {
p = G()
}
var f = F()
foo(&f) // assigns a value of type G to a variable of type F
>From Jonathan
On Dec 10, 2017, at 12:51 AM, Adrian Zubarev via swift-evolution
<swift-evolution@swift.org> wrote:
Hello Matthew, I have more more question about the generalized supertype
constraints. Does the generalization also covers inout? I found a really
annoying case with inout.
protocol P {}
func foo(_ p: inout P) {
print(p)
}
struct F : P {}
var f = F()
foo(&f) // won't compile until we explicitly write `var f: P = F()`
Shouldn’t we allow inout to have subtypes as well, like inout F : inout P?
This is actually one of the pain points with the generic version of the print
function. We cannot pass an arbitrary TextOutputStream to it without
sacrificing the type. I doubt the generic print function is justified, because
TextOuputStream does not have associated types nor a Self constraint.
Furthermore it forces you to create a custom type-erased wrapper that can hold
an arbitrary TextOutputSteram.
If this is part of a totally different topic, I’ll move it in it’s own thread.
Am 2. Dezember 2017 um 19:03:24, Matthew Johnson via swift-evolution
(swift-evolution@swift.org) schrieb:
This thread received very light, but positive feedback. I would really like to
see this feature added and am willing to draft and official proposal but am not
able to implement it. If anyone is interested in collaborating just let me
know.
On Nov 24, 2017, at 5:03 PM, Matthew Johnson via swift-evolution
<swift-evolution@swift.org> wrote:
One of the most frequent frustrations I encounter when writing generic code in
Swift is the requirement that supertype constraints be concrete. When I
mentioned this on Twitter
(https://twitter.com/anandabits/status/929958479598534656) Doug Gregor
mentioned that this feature is smaller and mostly straightforward to design and
implement (https://twitter.com/dgregor79/status/929975472779288576).
I currently have a PR open to add the high-level description of this feature
found below to the generics manifesto
(https://github.com/apple/swift/pull/13012):
Currently, supertype constraints may only be specified using a concrete class
or protocol type. This prevents us from abstracting over the supertype.
```swift
protocol P {
associatedtype Base
associatedtype Derived: Base
}
```
In the above example `Base` may be any type. `Derived` may be the same as
`Base` or may be _any_ subtype of `Base`. All subtype relationships supported
by Swift should be supported in this context including, but not limited to,
classes and subclasses, existentials and conforming concrete types or refining
existentials, `T?` and `T`, `((Base) -> Void)` and `((Derived) -> Void)`, etc.
Generalized supertype constraints would be accepted in all syntactic locations
where generic constraints are accepted.
I would like to see generalized supertype constraints make it into Swift 5 if
possible. I am not an implementer so I will not be able to bring a proposal
forward alone but am interested in collaborating with anyone interested in
working on implementation.
I am also interested in hearing general feedback on this feature from the
community at large. Have you also found this limitation frustrating? In what
contexts? Does anyone have reservations about introducing this capability? If
so, what are they?
Matthew
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