[swift-evolution] [Pitch] Remove type inference for associated types

[Austin Zheng]

Hi Dmitri,

Thanks for bringing this up. I surmise this option involves a user optionally marking a specific requirement in a protocol as being "chosen" to bind one or more associated types, with other requirements being forced to defer to that @infers-marked requirement.

I don't think there should be any problematic corner cases - either way, the set of associated types belonging to a type need to be solved in a consistent way, and the main difference would be the type of error emitted if the associated types could not be determined (e.g. "Associated type 'C' is inferred to be 'Int' by the declaration of 'foo(a:)' at line 1234, but was redefined as 'String' at line 1239").

I will add it to the alternatives section. The core team seems to be quite good about picking and choosing whatever they feel are the best parts of whatever alternatives are listed, so I think they will consider it very carefully.

Best,
Austin

> On Jun 25, 2016, at 12:51 AM, Dmitri Gribenko <gribozavr at gmail.com> wrote:
> 
> On Fri, Jun 24, 2016 at 10:50 PM, Austin Zheng via swift-evolution
> <swift-evolution at swift.org> wrote:
>> Hello all,
>> 
>> Per Chris Lattner's list of open Swift 3 design topics
>> (http://article.gmane.org/gmane.comp.lang.swift.evolution/21369), I've put
>> together a proposal for removing type inference for associated types.
> 
> Hi Austin,
> 
> Thank you for starting this discussion!  There's one other alternative
> that I mentioned in one of the previous threads on this subject.  The
> idea is to limit the inference so that the sizes and the complexity of
> the problems that the type checker has to solve become tractable with
> a simple algorithm, not a full constrain solver.
> 
> Currently, as far as I understand, the type checker solves for all
> associated types for a protocol conformance in a single giant step,
> during which every decision can affect every other decision.  My
> suggestion is that we keep associated type inference for the simple
> cases where it is obvious what the user meant.  The author of the
> protocol would be able to identify these simple cases and define how
> exactly the inference should happen.  For example:
> 
> protocol Collection {
>  associatedtype Index
> 
>  @infers(Index)
>  var startIndex: Index
> 
>  // Does not affect associated type inference, types have to match
> with decisions made by other declarations.
>  var endIndex: Index
> 
>  // Does not affect associated type inference.
>  subscript(i: Index) -> Iterator.Element
> 
>  associatedtype Iterator
> 
>  @infers(Iterator)
>  func iterator() -> Iterator
> }
> 
> Under the current system, every declaration in a conforming type that
> matches a requirement that mentions 'Index' can affect the inference.
> That is, 'Index' is inferred from all declarations in the conforming
> type.   But there is no reason to make it that general -- the protocol
> author knows that 'var startIndex' in the conforming type has be of
> the right type, and there is no reason for other declaration to affect
> the decision about what 'Index' is resolved to.  Under the proposed
> rule, there is at most one declaration that the protocol author is
> allowed to designate with @infers, that is allowed to affect the
> inference.  If there is no @infers for a certain associated type, then
> it is never inferred and should always be specified explicitly.
> 
> This is the basic idea, I'm sure there are corner cases I haven't
> thought about (e.g., how does this interact with constrained
> extension, can we still solve everything with a simple algorithm?)
> But the reason why I'm suggesting this alternative is that I'm
> concerned that in simple cases like inferring the 'Index' and
> 'Iterator' typealiases having to specify them manually is just
> boilerplate, that does not add to clarity, and, I believe, can be
> inferred by the type checker without involving a heavy constrain
> solver.
> 
> Dmitri
> 
> -- 
> main(i,j){for(i=2;;i++){for(j=2;j<i;j++){if(!(i%j)){j=0;break;}}if
> (j){printf("%d\n",i);}}} /*Dmitri Gribenko <gribozavr at gmail.com>*/