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

Dave Abrahams dabrahams at apple.com
Wed Jun 8 13:33:12 CDT 2016

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?


> 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]

> 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 :-).


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