[swift-evolution] [Pitch] Generalized supertype constraints

[Jonathan Keller]

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 at 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 at 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 at 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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>>> swift-evolution at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>> 
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