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<body><div style="font-family:Arial;">On Sun, Feb 19, 2017, at 06:40 PM, Xiaodi Wu via swift-evolution wrote:<br></div>
<blockquote type="cite"><div dir="ltr"><div><div defang_data-gmailquote="yes"><div style="font-family:Arial;">On Sun, Feb 19, 2017 at 5:15 PM, Brent Royal-Gordon via swift-evolution <span dir="ltr"><<a href="mailto:swift-evolution@swift.org">swift-evolution@swift.org</a>></span> wrote:<br></div>
<blockquote defang_data-gmailquote="yes" style="margin-top:0px;margin-right:0px;margin-bottom:0px;margin-left:0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204, 204, 204);padding-left:1ex;"><div style="font-family:Arial;">What is the harm of permitting an outside conformance to `SQLiteValue`?<br></div>
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<div style="font-family:Arial;">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.<br></div>
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<div style="font-family:Arial;">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.<br></div>
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<div style="font-family:Arial;">The stdlib alone is reason enough for this feature to exist, even if not exposed publicly.<br></div>
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<div style="font-family:Arial;">Zachary</div>
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