[swift-evolution] [Pitch] Introduce user-defined dynamically "callable" types

[Joe Groff]

> On Nov 10, 2017, at 8:35 PM, Chris Lattner via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
>> On Nov 10, 2017, at 6:12 PM, Slava Pestov via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> 
>> 
>>> On Nov 10, 2017, at 6:10 PM, Matthew Johnson via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> Setting this aside, I’m very curious to hear whether type providers influence your thinking after you’ve had a chance to look into them.  I have always thought they were very cool.
>> 
>> I’m in favor of solving this problem with something like type providers also. The required compiler changes would be significant but would also clean up the interface between the ClangImporter, Sema and Serialization. If done right it would be a net gain that would benefit all users, instead of just adding YetAnotherCornerCase™ that makes implementation maintainers curse and scream.
> 
> I find it ironic that you’re talking pejoratively about a feature that has very narrow impact, complaining about how much of an impact on the compiler it would have, and then pine for a hugely invasive feature - one that would cause a ton of code churn, and probably wouldn’t actually be enough to eliminate the special cases in place because of ObjC interop.

You underestimate the impact this would have on function call type checking, but since this is an additive, non-ABI-stability feature, I have trouble considering it a candidate for Swift 5, so I don't think there's a time constraint forcing us to consider the "narrow" vs "huge" dimension. What's the best thing for the language and tools in the long term? This is a feature that influences the semantics of potentially any call site in all Swift code, which we'd have to live with forever if we accepted it now. Opening up the compiler architecture to make custom importers easier to write is a great solution to a ton of problems, including yours I think, without adding complexity to the core language. Experience in .NET land seems to show it's a great technique for integrating dynamic systems with static type systems, without poking unnecessary holes in the static language's type system

-Joe

> -Chris
> 
> 
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