[swift-evolution] [SE-0011] Re-considering the replacement keyword for "typealias"

Chris Lattner clattner at apple.com
Mon Dec 21 13:33:59 CST 2015

> On Dec 20, 2015, at 8:27 AM, Stephen Celis via swift-evolution <swift-evolution at swift.org> wrote:
>> On Dec 19, 2015, at 11:14 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> On Dec 19, 2015, at 5:09 PM, Brent Royal-Gordon via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> I don't think `required` captures the intended meaning *at all*. You're not required to declare the type of a "required typealias"—it's often, perhaps even usually, inferred.
>> No, but it is required to exist and can't always be inferred.  It puts a constraint on the type that is declared to conform.  This is a requirement in exactly the same sense that other protocol requirements are requirements.  Notably operator requirements may be satisfied "implicitly" by declarations that already exist, but they are still requirements.
> I think reusing "required" here (where "typealias" has already been reused) could make the concept of associated types more opaque to new users.

I agree.  There are a couple of potentially confusing issues here:

- In principle, all of the declarations in the protocol are “requirements” that a type needs to fulfill to conform to the protocol.
- Except for optional requirements in @objc protocols.
- Except for requirements with default implementations (which currently cannot be written inline in the protocol, but should be allowed some day).  Today’s typealiases can have "default implementations” as well.


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