[swift-evolution] [Draft] open and public protocols

[Zach Waldowski]

On Sun, Feb 19, 2017, at 06:40 PM, Xiaodi Wu via swift-evolution wrote:
> On Sun, Feb 19, 2017 at 5:15 PM, Brent Royal-Gordon via swift-
> evolution <swift-evolution at swift.org> wrote:
>> What is the harm of permitting an outside conformance to
>> `SQLiteValue`?


I'll put words in Brent's mouth here as an attempt at an answer: you
sometimes need to exclusively switch a downcast of an existential.
Consider a serialization type - or SQLiteValue - where need to treat
specific fundamental types ("primitives") in implementation-dependent
ways. In a theoretical serialization design, all non-primitives would
have an explicit protocol of semantics to conform to support
serialization, much like the stdlib's "Custom" protocol pattern, and you
compose from there.


Using an enum to represent this exclusivity remains unconvincing to me,
unless we choose to add implicit conversions to the language. `.int(42)`
is an overwhelming level of duplicate type information in any practical,
non-trivial use case. Yes, (closed) enums model exclusivity. They are
not the only things to do so.


The stdlib alone is reason enough for this feature to exist, even if not
exposed publicly.


Zachary


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