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

Matthew Johnson matthew at anandabits.com
Tue Jun 7 20:14:45 CDT 2016

> 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.

The low hanging fruit in the “protocols whose existentials conform to the protocol” space is simple protocols that can already by existentials today (like CustomStringConvertible).  I don’t know enough about Swift’s implementation to comment on how complex it is there, but there aren’t any theoretical problems with making their existentials conform. 

> -- 
> -Dave
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