Yes, I agree, we need variadic generics before we can have tuples conform
At the end of the day, you want to be able to treat “(U, V, W)” as sugar
for Tuple<U,V,W> just like we handle array sugar. When that is possible,
Tuple is just a type like any other in the system (but we need variadics to
Once you have that, then you could write conformances in general, as well
as conditional conformances that depend on (e.g.) all the element types
We also need that to allow functions conform to protocols, because
functions aren’t "T1->T2” objects, the actual parameter list is an
inseparable part of the function type, and the parameter list needs
On Nov 20, 2017, at 6:10 PM, Slava Pestov <firstname.lastname@example.org> wrote:
Ignoring synthesized conformances for a second, think about how you would
manually implement a conformance of a tuple type to a protocol. You would
need some way to statically “iterate” over all the component types of the
tuple — in fact this is the same as having variadic generics.
If we had variadic generics, we could implement tuples conforming to
protocols, either by refactoring the compiler to allow conforming types to
be non-nominal, or by reworking things so that a tuple is a nominal type
with a single variadic generic parameter.
On Nov 20, 2017, at 9:06 PM, Tony Allevato via swift-evolution < > email@example.com> wrote:
This is something I've wanted to look at for a while. A few weeks ago I
pushed out https://github.com/apple/swift/pull/12598 to extend the
existing synthesis to handle structs/enums when a field/payload has a tuple
of things that are Equatable/Hashable, and in that PR it was (rightly)
observed, as Chris just did, that making tuples conform to protocols would
be a more general solution that solves the same problem you want to solve
I'd love to dig into this more, but last time I experimented with it I got
stuck on places where the protocol conformance machinery expects
NominalTypeDecls, and I wasn't sure where the right place to hoist that
logic up to was (since tuples don't have a corresponding Decl from what I
can tell). Any pointers?
On Mon, Nov 20, 2017 at 5:51 PM Chris Lattner via swift-evolution < > firstname.lastname@example.org> wrote:
On Nov 20, 2017, at 5:48 PM, Kelvin Ma <email@example.com> wrote:
the end goal here is to use tuples as a compatible currency type, to that
end it makes sense for these three protocols to be handled as “compiler
magic” and to disallow users from manually defining tuple conformances
themselves. i’m not a fan of compiler magic, but Equatable, Hashable, and
Comparable are special because they’re the basis for a lot of standard
library functionality so i think the benefits of making this a special
supported case outweigh the additional language opacity.
I understand your goal, but that compiler magic can’t exist until there
is something to hook it into. Tuples can’t conform to protocols right now,
so there is nothing that can be synthesized.
On Mon, Nov 20, 2017 at 8:42 PM, Chris Lattner <firstname.lastname@example.org> >> wrote:
On Nov 20, 2017, at 5:39 PM, Kelvin Ma via swift-evolution < >>> email@example.com> wrote:
went through swift evolution, it was agreed that the next logical step
is synthesizing these conformances for tuple types, though it was left out
of the original proposal to avoid mission creep. I think now is the time to
start thinking about this. i’m also tacking on Comparable to the other
two protocols because there is precedent in the language from SE-15
that tuple comparison is something that makes sense to write.
EHC conformance is even more important for tuples than it is for structs
because tuples effectively have no workaround whereas in structs, you could
just manually implement the conformance.
In my opinion, you’re approaching this from the wrong direction. The
fundamental problem here is that tuples can’t conform to a protocol. If
they could, synthesizing these conformances would be straight-forward.
If you’re interested in pushing this forward, the discussion is “how do
non-nominal types like tuples and functions conform to protocols”?
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