[swift-evolution] [Pitch] Generalized supertype constraints

[Rex Fenley]

Huge +1, I've asked for this in the past too.

Have you also found this limitation frustrating?
  - Yes

In what contexts?
  - APIs that have this requirement and end up enforcing them through
runtime type checking and throws. Shows up in some network data mapping
code I have that generalizes over Core Data and Realm (and other
databases). The protocol implementer must specify the subtype for the raw
mapping of JSON and base type for the DB reading/writing layer. Could see
this showing up whenever there's a separation of concerns between what
business logic belongs to the base type and subtypes of a more generalized
system. I could potentially see the same issue showing up in code
generalizing the mapping of data to UI, like UITableView/UITableViewCell.

Does anyone have reservations about introducing this capability?
  - I do not

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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Rex Fenley  |  IOS DEVELOPER

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