[swift-evolution] [swift-evolution-announce] [Review] SE-0089: Replace protocol<P1, P2> syntax with Any<P1, P2>

Thorsten Seitz tseitz42 at icloud.com
Wed Jun 8 14:34:47 CDT 2016

> Am 08.06.2016 um 20:33 schrieb Dave Abrahams via swift-evolution <swift-evolution at swift.org>:
> on Tue Jun 07 2016, Matthew Johnson <matthew-AT-anandabits.com> wrote:
>>> On Jun 7, 2016, at 9:15 PM, Dave Abrahams <dabrahams at apple.com> wrote:
>>> on Tue Jun 07 2016, Matthew Johnson <matthew-AT-anandabits.com <http://matthew-at-anandabits.com/>> wrote:
>>>>> On Jun 7, 2016, at 4:13 PM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>>>>> on Tue Jun 07 2016, Matthew Johnson <swift-evolution at swift.org> wrote:
>>>>>>> , but haven't realized
>>>>>>> that if you step around the type relationships encoded in Self
>>>>>>> requirements and associated types you end up with types that appear to
>>>>>>> interoperate but in fact trap at runtime unless used in exactly the
>>>>>>> right way.
>>>>>> Trap at runtime?  How so?  Generalized existentials should still be
>>>>>> type-safe.  
>>>>> There are two choices when you erase static type relationships:
>>>>> 1. Acheive type-safety by trapping at runtime
>>>>> FloatingPoint(3.0 as Float) + FloatingPoint(3.0 as Double) // trap
>>>>> 2. Don't expose protocol requirements that involve these relationships,
>>>>> which would prevent the code above from compiling and prevent
>>>>> FloatingPoint from conforming to itself.
>>>>>> Or are you talking about the hypothetical types / behaviors people
>>>>>> think they want when they don’t fully understand what is happening...
>>>>> I don't know what you mean here.  I think generalized existentials will
>>>>> be nice to have, but I think most people will want them to do something
>>>>> they can't possibly do.
>>>> Exactly.  What I meant is that people think they want that expression
>>>> to compile because they don’t understand that the only thing it can do
>>>> is trap.  I said “hypothetical” because producing a compile time error
>>>> rather than a runtime trap is the only sane thing to do.  Your comment
>>>> surprised me because I can’t imagine we would move forward in Swift
>>>> with the approach of trapping.
>>> I would very much like to be able to create instances of “Collection
>>> where Element == Int” so we can throw away the wrappers in the stdlib.
>>> That will require some type mismatches to be caught at runtime via
>>> trapping.
>> For invalid index because the existential accepts a type erased index?
> Exactly.
>> How do you decide where to draw the line here?  It feels like a very
>> slippery slope for a language where safety is a stated priority to
>> start adopting a strategy of runtime trapping for something as
>> fundamental as how you expose members on an existential.
> If you don't do this, the alternative is that “Collection where Element
> == Int” does not conform to Collection.  That's weird and not very
> useful.  You could expose all the methods that were on protocol
> extensions of Collection on this existential, unless they used
> associated types other than the element type.  But you couldn't pass the
> existential to a generic function like
>   func scrambled<C: Collection>(_ c: C) -> [C.Element]

I don’t understand. Why couldn’t an existential be passed to that function?


>> IMO you should *have* to introduce unsafe behavior like that manually.
>  Collection where Element == Int & Index == *
> ?
>> Collection indices are already something that isn’t fully statically
>> safe so I understand why you might want to allow this.  
> By the same measure, so are Ints :-)
> The fact that a type's methods have preconditions does *not* make it
> “statically unsafe.”
>> But I don’t think having the language's existentials do this
>> automatically is the right approach.  Maybe there is another approach
>> that could be used in targeted use cases where the less safe behavior
>> makes sense and is carefully designed.
> Whether it makes sense or not really depends on the use-cases.  There's
> little point in generalizing existentials if the result isn't very useful.
> The way to find out is to take a look at the examples we currently have
> of protocols with associated types or Self requirements and consider
> what you'd be able to do with their existentials if type relationships
> couldn't be erased.  
> We have known use-cases, currently emulated in the standard library, for
> existentials with erased type relationships.  *If* these represent the
> predominant use cases for something like generalized existentials, it
> seems to me that the language feature should support that.  Note: I have
> not seen anyone build an emulation of the other kind of generalized
> existential.  My theory: there's a good reason for that :-).
> -- 
> Dave
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