[swift-evolution] [Pitch] Support for pure functions. Part n + 1.

[Dennis Weissmann]

> On Feb 16, 2017, at 10:24 PM, David Sweeris <davesweeris at mac.com> wrote:
> 
> 
>> On Feb 16, 2017, at 13:10, Dennis Weissmann via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> Secondly, are they inherited? So if ClassA has a pure function a(), can I override it in subclass ClassB: ClassA to be impure? (I think no)
> 
> I would agree. IIUC, the "pureness" of A's implementation of a() becomes a moot point if B's implementation doesn't have to have at least as strong of a constraint,  because otherwise all the optimizations that the compiler makes would break as soon as someone passes a B in instead of an A.
> 
>> Can I annotate a function in a protocol to force it to be pure? (not sure about this one)
> 
> I would say yes, for similar reasons: if the attribute doesn't hold for *all* implementations, the compiler can't assume it holds for any. At least not with the way generics work now, IIUC.
> 

That makes a lot of sense and would be consistent with @escaping's behavior.

> - Dave Sweeris.

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