> On Jan 19, 2017, at 9:07 PM, David Sweeris via swift-evolution
> <[email protected]> wrote:
> It causes other issues, too. For instance, if we have
> protocol Initable { init() }
> And
> struct Foo { init(_ x: Int = 0) {} }
> We're left in an odd situation where `Foo` can't meaningfully conform to
> `Initable` because while "init(_: Int = 0)" is not the same as "init()", if
> you add a "init()" to `Foo`
> you'll get an ambiguous somethingerather error because there's no mechanism
> for the compiler to know whether you want the actual "0 argument" function or
> the "1 argument with 1 default value" function.
>
> Aside from re-architecting the default argument system (which I'm not even
> sure is possible, let alone a good idea), I think I see couple ways forward
> for the protocol conformance issue. Both have downsides, though.
>
> 1) Require any potentially conflicting protocol functions to be in an
> extension so the compiler knows what's going on, have "Foo()" call the one
> defined in the type, and use "(Foo as Initable)()" for the protocol version
> defined in an extension. This could get real confusing real fast if people
> don't realize there's two functions with, as far as they can tell, the same
> signature.
>
> 2) Add default argument support to protocols. The syntax that makes sense to
> me would be something like
> protocol Bar {
> func baz(_: Int = _)
> }
> On the downside, I suspect this would necessarily add a phantom "Self or
> associated type requirement" so that the compiler could have a way to get at
> each implementation's default value. It's not ideal... You'd get an error
> kinda out of the blue if you tried to use the function non-generically, but
> at least you couldn't have a function change out from under you.
I think in this specific example, the best solution is to allow init(_ x: Int =
0) to witness the init() requirement, and have the compiler emit the necessary
glue in-between so that a call to the init() requirement calls init(_) with the
appropriate default value. This will address the ‘ambiguous reference’ issue,
and should not require too much work to implement. I am also inclined to
believe this is (mostly) a source-compatible change. It also fits in with the
current default argument model, but Doug Gregor can correct me if I’m wrong.
If anyone is interested, the code for matching protocol requirements to
witnesses is in lib/Sema/TypeCheckProtocol.cpp, matchWitness() and surrounding
functions, and the code for emitting a protocol witness thunk (where you would
actually apply the default arguments) is in lib/SILGen/SILGenPoly.cpp,
emitProtocolWitness(). It would be a good not-quite-starter-project ;-)
Slava
>
> - Dave Sweeris
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