Re: [swift-evolution] [RFC] Associated type inference

[Matthew Johnson via swift-evolution]

Sent from my iPad

> On Dec 7, 2017, at 5:27 PM, Douglas Gregor via swift-evolution 
>  wrote:
> 
> 
> 
>> On Dec 2, 2017, at 9:23 PM, Dave Abrahams  wrote:
>> 
>> 
>>> On Nov 30, 2017, at 2:28 PM, Douglas Gregor via swift-evolution 
>>>  wrote:
>> 
>>> What’s a Good Solution Look Like?
>>> Our current system for associated type inference and associated type 
>>> defaults is buggy and complicated.
>> 
>> Well, that’s the problem, then.  Don’t worry, I won’t suggest that you 
>> simply fix the implementation, because even if there weren’t bugs and the 
>> system were predictable I’d still think we could improve the situation for 
>> users by making associated type default declarations more explicit.
>> 
>>> The compiler gets it right often enough that people depend on it, but I 
>>> don’t think anyone can reasonably be expected to puzzle out what’s going to 
>>> happen, and this area is rife with bugs. If we were to design a new 
>>> solution from scratch, what properties should it have?
>>> 
>>> It should allow the author of a protocol to provide reasonable defaults, so 
>>> the user doesn’t have to write them
>>> It shouldn’t require users to write typealiases for “obvious” cases, even 
>>> when they aren’t due to defaults
>>> It shouldn’t infer an inconsistent set of typealiases
>>> It should be something that a competent Swift programmer could reason about 
>>> when it will succeed, when and why it will fail, and what the resulting 
>>> inferred typealiases would be
>>> It should admit a reasonable implementation in the compiler that is 
>>> performant and robust
>> • It should cover all of the existing use cases.
>> • It should not break code at this point.
>> • We should have a migration strategy for existing code that avoids traps 
>> like silent semantic changes.
>> 
>> My bullet is important to me; I don’t think existing use cases are 
>> (inherently) so complex that we can sacrifice almost any of them and still 
>> end up with a sufficiently useful system.  At the very least, existing use 
>> cases provide the only guidance we really have as to what the feature should 
>> do.
> 
> I honestly don’t feel like a have a good handle on all of the use cases for 
> associated type inference, and it’s not something we can simply search for on 
> GitHub. But I think it covers most of them—and Matthew and Greg’s positive 
> feedback helps my confidence here. The biggest potential issue, I think, is 
> that we’ll no longer infer associated types from default implementations, 
> which protocol vendors might be relying on.

Hi Doug.  FWIW, I always explicitly state defaults in protocol declarations so 
inference in default implementations is something that I don’t use.  I think 
it’s very reasonable to require explicit declaration of the default.

There are a few areas where I can imagine a real impact.  The main one that I 
don’t think has been discussed yet is when conformance is declared in an 
extension but inference would need to consider a member declared in the 
original declaration.  This is likely to be pretty common given the requirement 
to declare stored properties in the original declaration and the common pattern 
of declaring conformance in an extension.  If implementation is feasible you 
might want to also consider the original declaration for a conformance that is 
stated in the same module (or even just the same file).

> 
>> 
>> I think we need to acknowledge that my second bullet is unattainable, at 
>> least if we want to improve type checking performance. Not breaking any code 
>> means that given any existing code, the compiler would have to explore the 
>> same solution space it currently does, and come up with the same answers.  
>> Improving performance would require new  declarations to use totally 
>> optional explicit syntax to prevent some explorations, and that’s an 
>> untenable user experience.
> 
> Yes, I agree.
> 
>> Which brings me to my third bullet: unless we are willing to break the code 
>> of protocol users (as opposed to vendors) we need to ensure that vendors can 
>> confidently convert code to use the new system without changing semantics.
> 
> Yeah, (2) below is basically that feature.
> 
>>  
>>> 
>>> A Rough Proposal
>>> I’ve been thinking about this for a bit, and I think there are three ways 
>>> in which we should be able to infer an associated type witness:
>>> 
>>> Associated type defaults, which are specified with the associated type 
>>> itself, e.g.,
>>> 
>>>   associatedtype Indices = DefaultIndices
>>> 
>>> These are easy to reason about for both the programmer and the compiler.
>>> Typealiases in (possibly constrained) protocol extensions, e.g.,
>>> 
>>>   extension RandomAccessCollection where Index : Strideable, Index.Stride 
>>> == IndexDistance {
>>> typealias RandomAccessCollection.Indices = CountableRange
>>>   }
>>> 
>>> I’m intentionally using some odd ‘.’ syntax 

Re: [swift-evolution] [RFC] Associated type inference

[Douglas Gregor via swift-evolution]

> On Dec 2, 2017, at 9:23 PM, Dave Abrahams  wrote:
> 
> 
> On Nov 30, 2017, at 2:28 PM, Douglas Gregor via swift-evolution 
> > wrote:
>> What’s a Good Solution Look Like?
>> Our current system for associated type inference and associated type 
>> defaults is buggy and complicated.
> 
> Well, that’s the problem, then.  Don’t worry, I won’t suggest that you simply 
> fix the implementation, because even if there weren’t bugs and the system 
> were predictable I’d still think we could improve the situation for users by 
> making associated type default declarations more explicit.
> 
>> The compiler gets it right often enough that people depend on it, but I 
>> don’t think anyone can reasonably be expected to puzzle out what’s going to 
>> happen, and this area is rife with bugs. If we were to design a new solution 
>> from scratch, what properties should it have?
>> 
>> It should allow the author of a protocol to provide reasonable defaults, so 
>> the user doesn’t have to write them
>> It shouldn’t require users to write typealiases for “obvious” cases, even 
>> when they aren’t due to defaults
>> It shouldn’t infer an inconsistent set of typealiases
>> It should be something that a competent Swift programmer could reason about 
>> when it will succeed, when and why it will fail, and what the resulting 
>> inferred typealiases would be
>> It should admit a reasonable implementation in the compiler that is 
>> performant and robust
> • It should cover all of the existing use cases.
> • It should not break code at this point.
> • We should have a migration strategy for existing code that avoids traps 
> like silent semantic changes.
> 
> My bullet is important to me; I don’t think existing use cases are 
> (inherently) so complex that we can sacrifice almost any of them and still 
> end up with a sufficiently useful system.  At the very least, existing use 
> cases provide the only guidance we really have as to what the feature should 
> do.

I honestly don’t feel like a have a good handle on all of the use cases for 
associated type inference, and it’s not something we can simply search for on 
GitHub. But I think it covers most of them—and Matthew and Greg’s positive 
feedback helps my confidence here. The biggest potential issue, I think, is 
that we’ll no longer infer associated types from default implementations, which 
protocol vendors might be relying on.

> 
> I think we need to acknowledge that my second bullet is unattainable, at 
> least if we want to improve type checking performance. Not breaking any code 
> means that given any existing code, the compiler would have to explore the 
> same solution space it currently does, and come up with the same answers.  
> Improving performance would require new  declarations to use totally optional 
> explicit syntax to prevent some explorations, and that’s an untenable user 
> experience.

Yes, I agree.

> Which brings me to my third bullet: unless we are willing to break the code 
> of protocol users (as opposed to vendors) we need to ensure that vendors can 
> confidently convert code to use the new system without changing semantics.

Yeah, (2) below is basically that feature.

>  
>> 
>> A Rough Proposal
>> I’ve been thinking about this for a bit, and I think there are three ways in 
>> which we should be able to infer an associated type witness:
>> 
>> Associated type defaults, which are specified with the associated type 
>> itself, e.g.,
>> 
>>   associatedtype Indices = DefaultIndices
>> 
>> These are easy to reason about for both the programmer and the compiler.
>> Typealiases in (possibly constrained) protocol extensions, e.g.,
>> 
>>   extension RandomAccessCollection where Index : Strideable, Index.Stride == 
>> IndexDistance {
>> typealias RandomAccessCollection.Indices = CountableRange
>>   }
>> 
>> I’m intentionally using some odd ‘.’ syntax here to indicate that this 
>> typealias is intended only to be found when trying to satisfy an associated 
>> type requirement, and is not a general typealias that could be found by 
>> normal name lookup. Let’s set the syntax bike shed aside for the moment. The 
>> primary advantage of this approach (vs. inferring Indices from “var Indices: 
>> CountableRange” in a constrained protocol extension) is that there’s 
>> a real typealias declaration that compiler and programmer alike can look at 
>> and reason about based just on the name “Indices”. 
>> 
>> Note that this mechanism technically obviates the need for (1), in the same 
>> sense that default implementations in protocols 
>> 
>>  are merely syntactic sugar.
>> Declarations within the nominal type declaration or extension that declares 
>> conformance to the protocol in question. This is generally the same approach 
>> as described in “associated type 

Re: [swift-evolution] [RFC] Associated type inference

[Douglas Gregor via swift-evolution]

> On Dec 3, 2017, at 2:39 PM, Jens Persson  wrote:
> 
> Would this help sorting out the behavior of typealiases in constrained 
> extensions?

Sadly, no.

> 
> If not, please ignore the following and accept my apologies for posting OT.
> 
> Typealiases in constrained extensions are - and have been, for a long time - 
> very broken.
> The following program (which is clearly crazy in several ways) compiles and 
> runs using the latest version of the compiler:
> 
> struct S {
>   var v: This
> }
> extension S where T == Int {
>   typealias This = Is
> }
> extension S where T == Bool {
>   typealias Is = Fine
> }
> extension S where T == String {
>   typealias Fine = T
> }
> let x = S(v: "uh")
> print(x.v) // uh
> 
> ( SR-5440 )
> The current behavior is so allowing and strange that I'm having trouble 
> seeing what the correct behavior would be if things worked as intended.

I’d said that “var v: This”, “typealias This = Is”, and “typealias Is = Fine” 
are ill-formed and the compiler should reject them. You should only be able to 
use types from another extension if your extra constraints imply the 
constraints of that extension. I *think* it’s actually a simple model, but it 
didn’t get implemented.

> For example should the following program still compile, and if so, should the 
> last line also compile (if uncommented)?
> 
> protocol P {
> associatedtype A = Int
> associatedtype B = Bool
> typealias C = Float
> }
> extension P where B == A {
> typealias C = String
> }

I think this should be ill-formed, because we shouldn’t allow two typealiases 
with the same name to “overload” within the same type.

> struct S : P {
> var v: (A, B, C)
> }
> extension S where A == Int, B == Bool {
> typealias C = [String]
> }
> let s1 = S(v: (1, true, [""]))
> // let s2 = S(v: ("a", "b", "c")) // Not (currently) ok.
> 
> Again, sorry for the noise if this is unrelated to the discussion.

- Doug

> /Jens
> 
> 
> On Sun, Dec 3, 2017 at 6:23 AM, Dave Abrahams via swift-evolution 
> > wrote:
> 
> On Nov 30, 2017, at 2:28 PM, Douglas Gregor via swift-evolution 
> > wrote:
> 
>> Hello Swift community,
>> 
>> Associated type inference, which is the process by which the Swift compiler 
>> attempts to infer typealiases to satisfy associated-type requirements based 
>> on other requirements, has caused both implementation problems and user 
>> confusion for a long time. Some of you might remember a previous (failed) 
>> attempt to remove this feature from the Swift language, in SE-0108 “Remove 
>> associated type inference”. 
>> 
>>  
>> 
>> I’m not sure we can remove this feature outright (i.e., the concerns that 
>> sank that proposal are still absolutely valid), because it is so very 
>> convenient and a lot of user code likely depends on it in some form or 
>> other. So, here I’d like to describe the uses of the feature, its current 
>> (very problematic) implementation approach, and a half-baked proposal to 
>> narrow the scope of associated type inference to something that I think is 
>> more tractable. I need help with the design of this feature, because I feel 
>> like it’s going to be a delicate balance between implementability and 
>> expressiveness.
> 
> Aloha, Doug!
> 
>> 
>> A Motivating Example
>> As a motivating example, let’s consider a “minimal” random access collection:
>> 
>> struct MyCollection {
>> var contents: [T]
>> }
>> 
>> extension MyCollection: RandomAccessCollection {
>> var startIndex: Int { return contents.startIndex }
>> var endIndex: Int { return contents.endIndex }
>> subscript(index: Int) -> T { return contents[index] }
>> }
>> 
>> This is actually pretty awesome: by providing just two properties and a 
>> subscript, we get the full breadth of the random access collection API! This 
>> is relying heavily on associated type inference (for associated type 
>> requirements) and default implementations specified on protocol extensions. 
>> Without associated type inference, we would have had to write:
>> 
>> 
>> extension MyCollection: RandomAccessCollection {
>> typealias Element = T
>> typealias Index = Int
>> typealias Indices = CountableRange
>> typealias Iterator = IndexingIterator
>> typealias SubSequence = RandomAccessSlice
>> 
>> var startIndex: Int { return contents.startIndex }
>> var endIndex: Int { return contents.endIndex }
>> subscript(index: Int) -> T { return contents[index] }
>> }
>> 
>> where the bolded typealiases are currently inferred. It was worse back when 
>> we reviewed SE-0108, because IIRC there were a few underscored associated 
>> types (e.g., _Element) that have since been removed. Still, that’s a bit of 
>> 

Re: [swift-evolution] [RFC] Associated type inference

[Douglas Gregor via swift-evolution]

> On Dec 1, 2017, at 11:42 AM, Nevin Brackett-Rozinsky 
>  wrote:
> 
> On Thu, Nov 30, 2017 at 7:28 PM, Douglas Gregor via swift-evolution 
> > wrote:
> A Rough Proposal
> I’ve been thinking about this for a bit, and I think there are three ways in 
> which we should be able to infer an associated type witness:
> 
> Associated type defaults, which are specified with the associated type 
> itself, e.g.,
> 
>   associatedtype Indices = DefaultIndices
> 
> These are easy to reason about for both the programmer and the compiler.
> Typealiases in (possibly constrained) protocol extensions, e.g.,
> 
>   extension RandomAccessCollection where Index : Strideable, Index.Stride == 
> IndexDistance {
> typealias RandomAccessCollection.Indices = CountableRange
>   }
> 
> I’m intentionally using some odd ‘.’ syntax here to indicate that this 
> typealias is intended only to be found when trying to satisfy an associated 
> type requirement, and is not a general typealias that could be found by 
> normal name lookup. Let’s set the syntax bike shed aside for the moment. The 
> primary advantage of this approach (vs. inferring Indices from “var Indices: 
> CountableRange” in a constrained protocol extension) is that there’s a 
> real typealias declaration that compiler and programmer alike can look at and 
> reason about based just on the name “Indices”. 
> 
> Note that this mechanism technically obviates the need for (1), in the same 
> sense that default implementations in protocols 
> 
>  are merely syntactic sugar.
> Declarations within the nominal type declaration or extension that declares 
> conformance to the protocol in question. This is generally the same approach 
> as described in “associated type inference” above, where we match 
> requirements of the protocol against declarations that could satisfy those 
> requirements and infer associated types from there. However, I want to turn 
> it around: instead of starting with the requirements of the protocol any 
> looking basically anywhere in the type or any protocol to which it conforms 
> (for implementations in protocol extensions), start with the declarations 
> that the user explicitly wrote at the point of the conformance and look for 
> requirements they might satisfy. For example, consider our initial example:
> 
>   extension MyCollection: RandomAccessCollection {
> var startIndex: Int { return contents.startIndex }
> var endIndex: Int { return contents.endIndex }
> subscript(index: Int) -> T { return contents[index] }
>   }
> 
> Since startIndex, endIndex, and subscript(_:) are declared in the same 
> extension that declares conformance to RandomAccessIterator, we should look 
> for requirements with the same name as these properties and subscript within 
> RandomAccessCollection (or any protocol it inherits) and infer Index := Int 
> and Element := T by matching the type signatures. This is still the most 
> magical inference rule, because there is no declaration named “Index” or 
> “Element” to look at. However, it is much narrower in scope than the current 
> implementation, because it’s only going to reason from the (probably small) 
> set of declarations that the user wrote alongside the conformance, so it’s 
> more likely to be intentional. Note that this is again nudging programmers 
> toward the style of programming where one puts one protocol conformance per 
> extension, which is admittedly my personal preference.
> 
> Thoughts?
> I think this approach is more predictable and more implementable than the 
> current model. I’m curious whether the above makes sense to someone other 
> than me, and whether it covers existing use cases well enough. Thoughts?
> 
>   - Doug
> 
> 
> How does this work with retroactive conformance, especially where all the 
> protocol requirements already exist on a type and an empty extension declares 
> conformance? For example, suppose Module A declares a protocol with 
> associated types, and Module B has a struct which naturally possesses all the 
> required members to conform (maybe B handles Double concretely, while A can 
> work with any FloatingPoint, or some such). As a client importing both 
> modules and providing an empty extension to conform B’s struct to A’s 
> protocol, will the associated types be inferred?

No, the associated types will not be inferred in this case. That will be a 
change in behavior (and a source compatibility regression).

> Also, have you considered the possibility of allowing protocol authors to 
> specify which types should be inferred from which requirements? For example 
> Collection might demarcate “startIndex” as the source-of-truth for inferring 
> “Index”, and “subscript (Index)->Element” as the source-of-truth for 
> inferring “Element”.

This did 

Re: [swift-evolution] [RFC] Associated type inference

[Jens Persson via swift-evolution]

Would this help sorting out the behavior of typealiases in constrained
extensions?

If not, please ignore the following and accept my apologies for posting OT.

Typealiases in constrained extensions are - and have been, for a long time
- very broken.
The following program (which is clearly crazy in several ways) compiles and
runs using the latest version of the compiler:

struct S {
  var v: This
}
extension S where T == Int {
  typealias This = Is
}
extension S where T == Bool {
  typealias Is = Fine
}
extension S where T == String {
  typealias Fine = T
}
let x = S(v: "uh")
print(x.v) // uh

( SR-5440 )
The current behavior is so allowing and strange that I'm having trouble
seeing what the correct behavior would be if things worked as intended.
For example should the following program still compile, and if so, should
the last line also compile (if uncommented)?

protocol P {
associatedtype A = Int
associatedtype B = Bool
typealias C = Float
}
extension P where B == A {
typealias C = String
}
struct S : P {
var v: (A, B, C)
}
extension S where A == Int, B == Bool {
typealias C = [String]
}
let s1 = S(v: (1, true, [""]))
// let s2 = S(v: ("a", "b", "c")) // Not (currently) ok.

Again, sorry for the noise if this is unrelated to the discussion.
/Jens


On Sun, Dec 3, 2017 at 6:23 AM, Dave Abrahams via swift-evolution <
swift-evolution@swift.org> wrote:

>
> On Nov 30, 2017, at 2:28 PM, Douglas Gregor via swift-evolution <
> swift-evolution@swift.org> wrote:
>
> Hello Swift community,
>
> Associated type inference, which is the process by which the Swift
> compiler attempts to infer typealiases to satisfy associated-type
> requirements based on other requirements, has caused both implementation
> problems and user confusion for a long time. Some of you might remember a
> previous (failed) attempt to remove this feature from the Swift language,
> in SE-0108 “Remove associated type inference”.
> 
>
>
> I’m not sure we can remove this feature outright (i.e., the concerns that
> sank that proposal are still absolutely valid), because it is so very
> convenient and a lot of user code likely depends on it in some form or
> other. So, here I’d like to describe the uses of the feature, its current
> (very problematic) implementation approach, and a half-baked proposal to
> narrow the scope of associated type inference to something that I think is
> more tractable. I need help with the design of this feature, because I feel
> like it’s going to be a delicate balance between implementability and
> expressiveness.
>
>
> Aloha, Doug!
>
>
> *A Motivating Example*
> As a motivating example, let’s consider a “minimal” random access
> collection:
>
> struct MyCollection {
> var contents: [T]
> }
>
> extension MyCollection: RandomAccessCollection {
> var startIndex: Int { return contents.startIndex }
> var endIndex: Int { return contents.endIndex }
> subscript(index: Int) -> T { return contents[index] }
> }
>
>
> This is actually pretty awesome: by providing just two properties and a
> subscript, we get the full breadth of the random access collection API!
> This is relying heavily on associated type inference (for associated type
> requirements) and default implementations specified on protocol extensions.
> Without associated type inference, we would have had to write:
>
>
> extension MyCollection: RandomAccessCollection {
> *typealias Element = T*
> *typealias Index = Int*
> *typealias Indices = CountableRange*
> *typealias Iterator = IndexingIterator*
> *typealias SubSequence = RandomAccessSlice*
>
> var startIndex: Int { return contents.startIndex }
> var endIndex: Int { return contents.endIndex }
> subscript(index: Int) -> T { return contents[index] }
> }
>
> where the bolded typealiases are currently inferred. It was worse back
> when we reviewed SE-0108, because IIRC there were a few underscored
> associated types (e.g., _Element) that have since been removed. Still,
> that’s a bit of additional work to define a “minimal” collection, and
> requires quite a bit more understanding: how do I know to choose
> IndexingIterator, and CountableRange, and RandomAccessSlice?
>
> The issue would get worse with, e.g., SE-0174 “Change filter to return an
> associated type”
> ,
> which adds an associated type Filtered that almost nobody will ever
> customize, and isn’t really fundamental to the way collections work. Adding
> Filtered to the standard library would be a source-breaking change, because
> users would have to write a typealias giving it its default.
>
> *Associated Type Defaults*
> One of the ways in which we avoid having to specify typealiases is to use
> associated type defaults. For example, the standard library 

Re: [swift-evolution] [RFC] Associated type inference

[Dave Abrahams via swift-evolution]

> On Nov 30, 2017, at 2:28 PM, Douglas Gregor via swift-evolution 
>  wrote:
> 
> Hello Swift community,
> 
> Associated type inference, which is the process by which the Swift compiler 
> attempts to infer typealiases to satisfy associated-type requirements based 
> on other requirements, has caused both implementation problems and user 
> confusion for a long time. Some of you might remember a previous (failed) 
> attempt to remove this feature from the Swift language, in SE-0108 “Remove 
> associated type inference”. 
> 
> I’m not sure we can remove this feature outright (i.e., the concerns that 
> sank that proposal are still absolutely valid), because it is so very 
> convenient and a lot of user code likely depends on it in some form or other. 
> So, here I’d like to describe the uses of the feature, its current (very 
> problematic) implementation approach, and a half-baked proposal to narrow the 
> scope of associated type inference to something that I think is more 
> tractable. I need help with the design of this feature, because I feel like 
> it’s going to be a delicate balance between implementability and 
> expressiveness.

Aloha, Doug!

> 
> A Motivating Example
> As a motivating example, let’s consider a “minimal” random access collection:
> 
> struct MyCollection {
> var contents: [T]
> }
> 
> extension MyCollection: RandomAccessCollection {
> var startIndex: Int { return contents.startIndex }
> var endIndex: Int { return contents.endIndex }
> subscript(index: Int) -> T { return contents[index] }
> }
> 
> This is actually pretty awesome: by providing just two properties and a 
> subscript, we get the full breadth of the random access collection API! This 
> is relying heavily on associated type inference (for associated type 
> requirements) and default implementations specified on protocol extensions. 
> Without associated type inference, we would have had to write:
> 
> 
> extension MyCollection: RandomAccessCollection {
> typealias Element = T
> typealias Index = Int
> typealias Indices = CountableRange
> typealias Iterator = IndexingIterator
> typealias SubSequence = RandomAccessSlice
> 
> var startIndex: Int { return contents.startIndex }
> var endIndex: Int { return contents.endIndex }
> subscript(index: Int) -> T { return contents[index] }
> }
> 
> where the bolded typealiases are currently inferred. It was worse back when 
> we reviewed SE-0108, because IIRC there were a few underscored associated 
> types (e.g., _Element) that have since been removed. Still, that’s a bit of 
> additional work to define a “minimal” collection, and requires quite a bit 
> more understanding: how do I know to choose IndexingIterator, and 
> CountableRange, and RandomAccessSlice?
> 
> The issue would get worse with, e.g., SE-0174 “Change filter to return an 
> associated type”, which adds an associated type Filtered that almost nobody 
> will ever customize, and isn’t really fundamental to the way collections 
> work. Adding Filtered to the standard library would be a source-breaking 
> change, because users would have to write a typealias giving it its default.
> 
> Associated Type Defaults
> One of the ways in which we avoid having to specify typealiases is to use 
> associated type defaults. For example, the standard library contains 
> associated type defaults for Indices, Iterator, and SubSequence. Let’s focus 
> on Indices:
> 
> protocol Collection : Sequence {
>   associatedtype Indices = DefaultIndices
>   // ...
> }
> 
> protocol BidirectionalCollection : Collection {
>   associatedtype Indices = DefaultBidirectionalIndices
>   // ...
> }
> 
> protocol RandomAccessCollection : BidirectionalCollection {
>   associatedtype Indices = DefaultRandomAccessIndices
>   // ...
> }
> 
> The basic idea here is that different protocols in the hierarchy provide 
> different defaults, and you presumably want the default from the most 
> specific protocol. If I define a type and make it conform to 
> BidirectionalCollection, I’d expect to get DefaultBidirectionalIndices 
> for Indices. If a define a type and make it conform to RandomAccessIterator, 
> I’d expect to get DefaultRandomAccessIndices.
> 
> (Aside: DefaultRandomAccessIndices and DefaultBidirectionalIndices got 
> collapsed into DefaultIndices now that we have conditional conformances for 
> the standard library, but the issues I’m describing remain).
> 
> Associated type defaults seem like a reasonable feature that fits well enough 
> into the design. However, it’s not the only thing in place with our 
> MyCollection example, for which Indices was inferred to CountableRange. How’s 
> that happen?
> 
> Associated Type Inference
> Associated type inference attempts to look at the requirements of a protocol, 
> and then looks into the conforming type for declarations that might satisfy 
> those requirements, and infers associated types 

Re: [swift-evolution] [RFC] Associated type inference

[Howard Lovatt via swift-evolution]

We also know that the current situation isn’t acceptable to a great proposition 
of the community, that is why we are still discussing the issue!

A notable example of reversal of an evolution decision is String’s conformance 
to Collection. Which I think on the 2nd attempt was a much better decision. 

For requiring typedefs for associated types, a fix it and error would be quite 
successful, e.g. Xcode already suggests the typedefs (which I currently accept 
before letting Xcode insert blanks for the missing methods etc.).

-- Howard. 

> On 3 Dec 2017, at 8:15 am, Xiaodi Wu  wrote:
> 
>> On Sat, Dec 2, 2017 at 2:30 PM, Howard Lovatt via swift-evolution 
>>  wrote:
>> Definitely in favour of doing something, I always define the associated 
>> types since I have had so much trouble with the inference.
>> 
>> Personally I would prefer just 1 and 2 and forget 3. I know this would break 
>> a lot of code, but I think we should do that because it is the lesser of the 
>> evils.
> 
> As Doug wrote, an approach that's essentially that was reviewed and rejected 
> in SE-0108. We already know that it's not acceptable to a great proportion of 
> the community.
> 
> 
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Re: [swift-evolution] [RFC] Associated type inference

[Matthew Johnson via swift-evolution]

Sent from my iPad

> On Dec 2, 2017, at 4:40 PM, Douglas Gregor  wrote:
> 
> 
> 
> Sent from my iPhone
> 
>> On Dec 2, 2017, at 1:37 PM, Matthew Johnson  wrote:
>> 
>> 
>>> On Nov 30, 2017, at 6:28 PM, Douglas Gregor via swift-evolution 
>>>  wrote:
>>> 
>>> Hello Swift community,
>>> 
>>> Associated type inference, which is the process by which the Swift compiler 
>>> attempts to infer typealiases to satisfy associated-type requirements based 
>>> on other requirements, has caused both implementation problems and user 
>>> confusion for a long time. Some of you might remember a previous (failed) 
>>> attempt to remove this feature from the Swift language, in SE-0108 “Remove 
>>> associated type inference”. 
>>> 
>>> I’m not sure we can remove this feature outright (i.e., the concerns that 
>>> sank that proposal are still absolutely valid), because it is so very 
>>> convenient and a lot of user code likely depends on it in some form or 
>>> other. So, here I’d like to describe the uses of the feature, its current 
>>> (very problematic) implementation approach, and a half-baked proposal to 
>>> narrow the scope of associated type inference to something that I think is 
>>> more tractable. I need help with the design of this feature, because I feel 
>>> like it’s going to be a delicate balance between implementability and 
>>> expressiveness.
>>> 
>>> A Motivating Example
>>> As a motivating example, let’s consider a “minimal” random access 
>>> collection:
>>> 
>>> struct MyCollection {
>>> var contents: [T]
>>> }
>>> 
>>> extension MyCollection: RandomAccessCollection {
>>> var startIndex: Int { return contents.startIndex }
>>> var endIndex: Int { return contents.endIndex }
>>> subscript(index: Int) -> T { return contents[index] }
>>> }
>>> 
>>> This is actually pretty awesome: by providing just two properties and a 
>>> subscript, we get the full breadth of the random access collection API! 
>>> This is relying heavily on associated type inference (for associated type 
>>> requirements) and default implementations specified on protocol extensions. 
>>> Without associated type inference, we would have had to write:
>>> 
>>> 
>>> extension MyCollection: RandomAccessCollection {
>>> typealias Element = T
>>> typealias Index = Int
>>> typealias Indices = CountableRange
>>> typealias Iterator = IndexingIterator
>>> typealias SubSequence = RandomAccessSlice
>>> 
>>> var startIndex: Int { return contents.startIndex }
>>> var endIndex: Int { return contents.endIndex }
>>> subscript(index: Int) -> T { return contents[index] }
>>> }
>>> 
>>> where the bolded typealiases are currently inferred. It was worse back when 
>>> we reviewed SE-0108, because IIRC there were a few underscored associated 
>>> types (e.g., _Element) that have since been removed. Still, that’s a bit of 
>>> additional work to define a “minimal” collection, and requires quite a bit 
>>> more understanding: how do I know to choose IndexingIterator, and 
>>> CountableRange, and RandomAccessSlice?
>>> 
>>> The issue would get worse with, e.g., SE-0174 “Change filter to return an 
>>> associated type”, which adds an associated type Filtered that almost nobody 
>>> will ever customize, and isn’t really fundamental to the way collections 
>>> work. Adding Filtered to the standard library would be a source-breaking 
>>> change, because users would have to write a typealias giving it its default.
>>> 
>>> Associated Type Defaults
>>> One of the ways in which we avoid having to specify typealiases is to use 
>>> associated type defaults. For example, the standard library contains 
>>> associated type defaults for Indices, Iterator, and SubSequence. Let’s 
>>> focus on Indices:
>>> 
>>> protocol Collection : Sequence {
>>>   associatedtype Indices = DefaultIndices
>>>   // ...
>>> }
>>> 
>>> protocol BidirectionalCollection : Collection {
>>>   associatedtype Indices = DefaultBidirectionalIndices
>>>   // ...
>>> }
>>> 
>>> protocol RandomAccessCollection : BidirectionalCollection {
>>>   associatedtype Indices = DefaultRandomAccessIndices
>>>   // ...
>>> }
>>> 
>>> The basic idea here is that different protocols in the hierarchy provide 
>>> different defaults, and you presumably want the default from the most 
>>> specific protocol. If I define a type and make it conform to 
>>> BidirectionalCollection, I’d expect to get 
>>> DefaultBidirectionalIndices for Indices. If a define a type and make 
>>> it conform to RandomAccessIterator, I’d expect to get 
>>> DefaultRandomAccessIndices.
>>> 
>>> (Aside: DefaultRandomAccessIndices and DefaultBidirectionalIndices got 
>>> collapsed into DefaultIndices now that we have conditional conformances for 
>>> the standard library, but the issues I’m describing remain).
>>> 
>>> Associated type defaults seem like a reasonable feature that fits well 
>>> 

Re: [swift-evolution] [RFC] Associated type inference

[Douglas Gregor via swift-evolution]

Sent from my iPhone

> On Dec 2, 2017, at 1:37 PM, Matthew Johnson  wrote:
> 
> 
>> On Nov 30, 2017, at 6:28 PM, Douglas Gregor via swift-evolution 
>>  wrote:
>> 
>> Hello Swift community,
>> 
>> Associated type inference, which is the process by which the Swift compiler 
>> attempts to infer typealiases to satisfy associated-type requirements based 
>> on other requirements, has caused both implementation problems and user 
>> confusion for a long time. Some of you might remember a previous (failed) 
>> attempt to remove this feature from the Swift language, in SE-0108 “Remove 
>> associated type inference”. 
>> 
>> I’m not sure we can remove this feature outright (i.e., the concerns that 
>> sank that proposal are still absolutely valid), because it is so very 
>> convenient and a lot of user code likely depends on it in some form or 
>> other. So, here I’d like to describe the uses of the feature, its current 
>> (very problematic) implementation approach, and a half-baked proposal to 
>> narrow the scope of associated type inference to something that I think is 
>> more tractable. I need help with the design of this feature, because I feel 
>> like it’s going to be a delicate balance between implementability and 
>> expressiveness.
>> 
>> A Motivating Example
>> As a motivating example, let’s consider a “minimal” random access collection:
>> 
>> struct MyCollection {
>> var contents: [T]
>> }
>> 
>> extension MyCollection: RandomAccessCollection {
>> var startIndex: Int { return contents.startIndex }
>> var endIndex: Int { return contents.endIndex }
>> subscript(index: Int) -> T { return contents[index] }
>> }
>> 
>> This is actually pretty awesome: by providing just two properties and a 
>> subscript, we get the full breadth of the random access collection API! This 
>> is relying heavily on associated type inference (for associated type 
>> requirements) and default implementations specified on protocol extensions. 
>> Without associated type inference, we would have had to write:
>> 
>> 
>> extension MyCollection: RandomAccessCollection {
>> typealias Element = T
>> typealias Index = Int
>> typealias Indices = CountableRange
>> typealias Iterator = IndexingIterator
>> typealias SubSequence = RandomAccessSlice
>> 
>> var startIndex: Int { return contents.startIndex }
>> var endIndex: Int { return contents.endIndex }
>> subscript(index: Int) -> T { return contents[index] }
>> }
>> 
>> where the bolded typealiases are currently inferred. It was worse back when 
>> we reviewed SE-0108, because IIRC there were a few underscored associated 
>> types (e.g., _Element) that have since been removed. Still, that’s a bit of 
>> additional work to define a “minimal” collection, and requires quite a bit 
>> more understanding: how do I know to choose IndexingIterator, and 
>> CountableRange, and RandomAccessSlice?
>> 
>> The issue would get worse with, e.g., SE-0174 “Change filter to return an 
>> associated type”, which adds an associated type Filtered that almost nobody 
>> will ever customize, and isn’t really fundamental to the way collections 
>> work. Adding Filtered to the standard library would be a source-breaking 
>> change, because users would have to write a typealias giving it its default.
>> 
>> Associated Type Defaults
>> One of the ways in which we avoid having to specify typealiases is to use 
>> associated type defaults. For example, the standard library contains 
>> associated type defaults for Indices, Iterator, and SubSequence. Let’s focus 
>> on Indices:
>> 
>> protocol Collection : Sequence {
>>   associatedtype Indices = DefaultIndices
>>   // ...
>> }
>> 
>> protocol BidirectionalCollection : Collection {
>>   associatedtype Indices = DefaultBidirectionalIndices
>>   // ...
>> }
>> 
>> protocol RandomAccessCollection : BidirectionalCollection {
>>   associatedtype Indices = DefaultRandomAccessIndices
>>   // ...
>> }
>> 
>> The basic idea here is that different protocols in the hierarchy provide 
>> different defaults, and you presumably want the default from the most 
>> specific protocol. If I define a type and make it conform to 
>> BidirectionalCollection, I’d expect to get DefaultBidirectionalIndices 
>> for Indices. If a define a type and make it conform to RandomAccessIterator, 
>> I’d expect to get DefaultRandomAccessIndices.
>> 
>> (Aside: DefaultRandomAccessIndices and DefaultBidirectionalIndices got 
>> collapsed into DefaultIndices now that we have conditional conformances for 
>> the standard library, but the issues I’m describing remain).
>> 
>> Associated type defaults seem like a reasonable feature that fits well 
>> enough into the design. However, it’s not the only thing in place with our 
>> MyCollection example, for which Indices was inferred to CountableRange. 
>> How’s that happen?
>> 
>> Associated Type Inference
>> Associated 

Re: [swift-evolution] [RFC] Associated type inference

[Matthew Johnson via swift-evolution]

> On Nov 30, 2017, at 6:28 PM, Douglas Gregor via swift-evolution 
>  wrote:
> 
> Hello Swift community,
> 
> Associated type inference, which is the process by which the Swift compiler 
> attempts to infer typealiases to satisfy associated-type requirements based 
> on other requirements, has caused both implementation problems and user 
> confusion for a long time. Some of you might remember a previous (failed) 
> attempt to remove this feature from the Swift language, in SE-0108 “Remove 
> associated type inference”. 
> 
>  
> 
> I’m not sure we can remove this feature outright (i.e., the concerns that 
> sank that proposal are still absolutely valid), because it is so very 
> convenient and a lot of user code likely depends on it in some form or other. 
> So, here I’d like to describe the uses of the feature, its current (very 
> problematic) implementation approach, and a half-baked proposal to narrow the 
> scope of associated type inference to something that I think is more 
> tractable. I need help with the design of this feature, because I feel like 
> it’s going to be a delicate balance between implementability and 
> expressiveness.
> 
> A Motivating Example
> As a motivating example, let’s consider a “minimal” random access collection:
> 
> struct MyCollection {
> var contents: [T]
> }
> 
> extension MyCollection: RandomAccessCollection {
> var startIndex: Int { return contents.startIndex }
> var endIndex: Int { return contents.endIndex }
> subscript(index: Int) -> T { return contents[index] }
> }
> 
> This is actually pretty awesome: by providing just two properties and a 
> subscript, we get the full breadth of the random access collection API! This 
> is relying heavily on associated type inference (for associated type 
> requirements) and default implementations specified on protocol extensions. 
> Without associated type inference, we would have had to write:
> 
> 
> extension MyCollection: RandomAccessCollection {
> typealias Element = T
> typealias Index = Int
> typealias Indices = CountableRange
> typealias Iterator = IndexingIterator
> typealias SubSequence = RandomAccessSlice
> 
> var startIndex: Int { return contents.startIndex }
> var endIndex: Int { return contents.endIndex }
> subscript(index: Int) -> T { return contents[index] }
> }
> 
> where the bolded typealiases are currently inferred. It was worse back when 
> we reviewed SE-0108, because IIRC there were a few underscored associated 
> types (e.g., _Element) that have since been removed. Still, that’s a bit of 
> additional work to define a “minimal” collection, and requires quite a bit 
> more understanding: how do I know to choose IndexingIterator, and 
> CountableRange, and RandomAccessSlice?
> 
> The issue would get worse with, e.g., SE-0174 “Change filter to return an 
> associated type” 
> ,
>  which adds an associated type Filtered that almost nobody will ever 
> customize, and isn’t really fundamental to the way collections work. Adding 
> Filtered to the standard library would be a source-breaking change, because 
> users would have to write a typealias giving it its default.
> 
> Associated Type Defaults
> One of the ways in which we avoid having to specify typealiases is to use 
> associated type defaults. For example, the standard library contains 
> associated type defaults for Indices, Iterator, and SubSequence. Let’s focus 
> on Indices:
> 
> protocol Collection : Sequence {
>   associatedtype Indices = DefaultIndices
>   // ...
> }
> 
> protocol BidirectionalCollection : Collection {
>   associatedtype Indices = DefaultBidirectionalIndices
>   // ...
> }
> 
> protocol RandomAccessCollection : BidirectionalCollection {
>   associatedtype Indices = DefaultRandomAccessIndices
>   // ...
> }
> 
> The basic idea here is that different protocols in the hierarchy provide 
> different defaults, and you presumably want the default from the most 
> specific protocol. If I define a type and make it conform to 
> BidirectionalCollection, I’d expect to get DefaultBidirectionalIndices 
> for Indices. If a define a type and make it conform to RandomAccessIterator, 
> I’d expect to get DefaultRandomAccessIndices.
> 
> (Aside: DefaultRandomAccessIndices and DefaultBidirectionalIndices got 
> collapsed into DefaultIndices now that we have conditional conformances for 
> the standard library , but the 
> issues I’m describing remain).
> 
> Associated type defaults seem like a reasonable feature that fits well enough 
> into the design. However, it’s not the only thing in place with our 
> MyCollection example, for which Indices was inferred to CountableRange. How’s 
> that happen?
> 

Re: [swift-evolution] [RFC] Associated type inference

[Xiaodi Wu via swift-evolution]

On Sat, Dec 2, 2017 at 2:30 PM, Howard Lovatt via swift-evolution <
swift-evolution@swift.org> wrote:

> Definitely in favour of doing something, I always define the associated
> types since I have had so much trouble with the inference.
>
> Personally I would prefer just 1 and 2 and forget 3. I know this would
> break a lot of code, but I think we should do that because it is the lesser
> of the evils.
>

As Doug wrote, an approach that's essentially that was reviewed and
rejected in SE-0108. We already know that it's not acceptable to a great
proportion of the community.
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Re: [swift-evolution] [RFC] Associated type inference

[Howard Lovatt via swift-evolution]

Definitely in favour of doing something, I always define the associated types 
since I have had so much trouble with the inference. 

Personally I would prefer just 1 and 2 and forget 3. I know this would break a 
lot of code, but I think we should do that because it is the lesser of the 
evils. 

-- Howard. 

> On 1 Dec 2017, at 11:20 pm, Johannes Weiß via swift-evolution 
>  wrote:
> 
> Hi Douglas,
> 
> First of all, thanks very much for looking into this seemingly dark corner of 
> the compiler. This caused us a lot of trouble already.
> 
> Comments inline.
> 
>> On 1 Dec 2017, at 12:28 am, Douglas Gregor via swift-evolution 
>>  wrote:
>> 
>> Hello Swift community,
>> 
>> Associated type inference, which is the process by which the Swift compiler 
>> attempts to infer typealiases to satisfy associated-type requirements based 
>> on other requirements, has caused both implementation problems and user 
>> confusion for a long time. Some of you might remember a previous (failed) 
>> attempt to remove this feature from the Swift language, in SE-0108 “Remove 
>> associated type inference”. 
>> 
>> I’m not sure we can remove this feature outright (i.e., the concerns that 
>> sank that proposal are still absolutely valid), because it is so very 
>> convenient and a lot of user code likely depends on it in some form or 
>> other. So, here I’d like to describe the uses of the feature, its current 
>> (very problematic) implementation approach, and a half-baked proposal to 
>> narrow the scope of associated type inference to something that I think is 
>> more tractable. I need help with the design of this feature, because I feel 
>> like it’s going to be a delicate balance between implementability and 
>> expressiveness.
>> 
>> A Motivating Example
>> As a motivating example, let’s consider a “minimal” random access collection:
>> 
>> struct MyCollection {
>>  var contents: [T]
>> }
>> 
>> extension MyCollection: RandomAccessCollection {
>>  var startIndex: Int { return contents.startIndex }
>>  var endIndex: Int { return contents.endIndex }
>>  subscript(index: Int) -> T { return contents[index] }
>> }
>> 
>> This is actually pretty awesome: by providing just two properties and a 
>> subscript, we get the full breadth of the random access collection API! This 
>> is relying heavily on associated type inference (for associated type 
>> requirements) and default implementations specified on protocol extensions. 
>> Without associated type inference, we would have had to write:
>> 
>> 
>> extension MyCollection: RandomAccessCollection {
>>  typealias Element = T
>>  typealias Index = Int
>>  typealias Indices = CountableRange
>>  typealias Iterator = IndexingIterator
>>  typealias SubSequence = RandomAccessSlice
>> 
>>  var startIndex: Int { return contents.startIndex }
>>  var endIndex: Int { return contents.endIndex }
>>  subscript(index: Int) -> T { return contents[index] }
>> }
>> 
>> where the bolded typealiases are currently inferred. It was worse back when 
>> we reviewed SE-0108, because IIRC there were a few underscored associated 
>> types (e.g., _Element) that have since been removed. Still, that’s a bit of 
>> additional work to define a “minimal” collection, and requires quite a bit 
>> more understanding: how do I know to choose IndexingIterator, and 
>> CountableRange, and RandomAccessSlice?
>> 
>> The issue would get worse with, e.g., SE-0174 “Change filter to return an 
>> associated type”, which adds an associated type Filtered that almost nobody 
>> will ever customize, and isn’t really fundamental to the way collections 
>> work. Adding Filtered to the standard library would be a source-breaking 
>> change, because users would have to write a typealias giving it its default.
>> 
>> Associated Type Defaults
>> One of the ways in which we avoid having to specify typealiases is to use 
>> associated type defaults. For example, the standard library contains 
>> associated type defaults for Indices, Iterator, and SubSequence. Let’s focus 
>> on Indices:
>> 
>> protocol Collection : Sequence {
>> associatedtype Indices = DefaultIndices
>> // ...
>> }
>> 
>> protocol BidirectionalCollection : Collection {
>> associatedtype Indices = DefaultBidirectionalIndices
>> // ...
>> }
>> 
>> protocol RandomAccessCollection : BidirectionalCollection {
>> associatedtype Indices = DefaultRandomAccessIndices
>> // ...
>> }
>> 
>> The basic idea here is that different protocols in the hierarchy provide 
>> different defaults, and you presumably want the default from the most 
>> specific protocol. If I define a type and make it conform to 
>> BidirectionalCollection, I’d expect to get DefaultBidirectionalIndices 
>> for Indices. If a define a type and make it conform to RandomAccessIterator, 
>> I’d expect to get DefaultRandomAccessIndices.
>> 
>> (Aside: DefaultRandomAccessIndices and DefaultBidirectionalIndices got 
>> collapsed 

Re: [swift-evolution] [RFC] Associated type inference

[Nevin Brackett-Rozinsky via swift-evolution]

On Thu, Nov 30, 2017 at 7:28 PM, Douglas Gregor via swift-evolution <
swift-evolution@swift.org> wrote:

> *A Rough Proposal*
> I’ve been thinking about this for a bit, and I think there are three ways
> in which we should be able to infer an associated type witness:
>
>
>1. Associated type defaults, which are specified with the associated
>type itself, e.g.,
>
>  associatedtype Indices = DefaultIndices
>
>These are easy to reason about for both the programmer and the
>compiler.
>2. Typealiases in (possibly constrained) protocol extensions, e.g.,
>
>  extension RandomAccessCollection where Index : Strideable,
>Index.Stride == IndexDistance {
>typealias RandomAccessCollection.Indices = CountableRange
>  }
>
>I’m intentionally using some odd ‘.’ syntax here to indicate that this
>typealias is intended only to be found when trying to satisfy an associated
>type requirement, and is not a general typealias that could be found by
>normal name lookup. Let’s set the syntax bike shed aside for the moment.
>The primary advantage of this approach (vs. inferring Indices from “var
>Indices: CountableRange” in a constrained protocol extension) is
>that there’s a real typealias declaration that compiler and programmer
>alike can look at and reason about based just on the name “Indices”.
>
>Note that this mechanism technically obviates the need for (1), in the
>same sense that default implementations in protocols
>
> 
>  are
>merely syntactic sugar.
>3. Declarations within the nominal type declaration or extension that
>declares conformance to the protocol in question. This is generally the
>same approach as described in “associated type inference” above, where we
>match requirements of the protocol against declarations that could satisfy
>those requirements and infer associated types from there. However, I want
>to turn it around: instead of starting with the requirements of the
>protocol any looking basically anywhere in the type or any protocol to
>which it conforms (for implementations in protocol extensions), start with
>the declarations that the user explicitly wrote at the point of the
>conformance and look for requirements they might satisfy. For example,
>consider our initial example:
>
>  extension MyCollection: RandomAccessCollection {
>var startIndex: Int { return contents.startIndex }
>var endIndex: Int { return contents.endIndex }
>subscript(index: Int) -> T { return contents[index] }
>  }
>
>Since startIndex, endIndex, and subscript(_:) are declared in the same
>extension that declares conformance to RandomAccessIterator, we should look
>for requirements with the same name as these properties and subscript
>within RandomAccessCollection (or any protocol it inherits) and infer Index
>:= Int and Element := T by matching the type signatures. This is still the
>most magical inference rule, because there is no declaration named “Index”
>or “Element” to look at. However, it is much narrower in scope than the
>current implementation, because it’s only going to reason from the
>(probably small) set of declarations that the user wrote alongside the
>conformance, so it’s more likely to be intentional. Note that this is again
>nudging programmers toward the style of programming where one puts one
>protocol conformance per extension, which is admittedly my personal
>preference.
>
>
> *Thoughts?*
> I think this approach is more predictable and more implementable than the
> current model. I’m curious whether the above makes sense to someone other
> than me, and whether it covers existing use cases well enough. Thoughts?
>
> - Doug
>


How does this work with retroactive conformance, especially where all the
protocol requirements already exist on a type and an empty extension
declares conformance? For example, suppose Module A declares a protocol
with associated types, and Module B has a struct which naturally possesses
all the required members to conform (maybe B handles Double concretely,
while A can work with any FloatingPoint, or some such). As a client
importing both modules and providing an empty extension to conform B’s
struct to A’s protocol, will the associated types be inferred?

Also, have you considered the possibility of allowing protocol authors to
specify which types should be inferred from which requirements? For example
Collection might demarcate “startIndex” as the source-of-truth for
inferring “Index”, and “subscript (Index)->Element” as the source-of-truth
for inferring “Element”.

Nevin
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Re: [swift-evolution] [RFC] Associated type inference

[Douglas Gregor via swift-evolution]

> On Dec 1, 2017, at 10:07 AM, Greg Titus  wrote:
> 
> 
> 
>> On Dec 1, 2017, at 9:11 AM, Ben Langmuir via swift-evolution 
>> > wrote:
>> Hey Doug,
>> 
>> I'm very much in favour of reducing the scope of associated type inference.  
>> Can you outline why you believe that (3) is necessary?  If I am following 
>> correctly, if we had (1) and (2) the only thing you'd need to add to the 
>> "minimal collection" implementation would be a typealias for `Element`, 
>> which seems reasonable to me.
>> 
>> Ben
> 
> If nothing else, dropping (3) would be source breaking for 90%+ of current 
> associated type uses. Whereas even the very minimal inference in (3) probably 
> brings that figure down to 1% or so (outside of the stdlib, which would need 
> to adopt a bunch of (2)). Obviously these percentages are just my guesses and 
> not based on any real survey, but certainly would be the case for all Swift 
> code I’ve seen.

Many of the associated-type inference bugs I’ve seen were from people expecting 
something like (3), but the current implementation either fails to infer 
anything (the common case!) or we get inference from some seemingly-unrelated 
place. I included (3) specifically because I think taking away (3) will break 
source compatibility significantly (as Greg suggests)… and despite the fact 
that the complexity of implementation for (3) is fairly high.

- Doug


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Re: [swift-evolution] [RFC] Associated type inference

[Douglas Gregor via swift-evolution]

> On Dec 1, 2017, at 9:11 AM, Ben Langmuir  wrote:
> 
>> 
>> On Nov 30, 2017, at 4:28 PM, Douglas Gregor via swift-evolution 
>> > wrote:
>> 
>> Hello Swift community,
>> 
>> Associated type inference, which is the process by which the Swift compiler 
>> attempts to infer typealiases to satisfy associated-type requirements based 
>> on other requirements, has caused both implementation problems and user 
>> confusion for a long time. Some of you might remember a previous (failed) 
>> attempt to remove this feature from the Swift language, in SE-0108 “Remove 
>> associated type inference”. 
>> 
>>  
>> 
>> I’m not sure we can remove this feature outright (i.e., the concerns that 
>> sank that proposal are still absolutely valid), because it is so very 
>> convenient and a lot of user code likely depends on it in some form or 
>> other. So, here I’d like to describe the uses of the feature, its current 
>> (very problematic) implementation approach, and a half-baked proposal to 
>> narrow the scope of associated type inference to something that I think is 
>> more tractable. I need help with the design of this feature, because I feel 
>> like it’s going to be a delicate balance between implementability and 
>> expressiveness.
>> 
>> A Motivating Example
>> As a motivating example, let’s consider a “minimal” random access collection:
>> 
>> struct MyCollection {
>> var contents: [T]
>> }
>> 
>> extension MyCollection: RandomAccessCollection {
>> var startIndex: Int { return contents.startIndex }
>> var endIndex: Int { return contents.endIndex }
>> subscript(index: Int) -> T { return contents[index] }
>> }
>> 
>> This is actually pretty awesome: by providing just two properties and a 
>> subscript, we get the full breadth of the random access collection API! This 
>> is relying heavily on associated type inference (for associated type 
>> requirements) and default implementations specified on protocol extensions. 
>> Without associated type inference, we would have had to write:
>> 
>> 
>> extension MyCollection: RandomAccessCollection {
>> typealias Element = T
>> typealias Index = Int
>> typealias Indices = CountableRange
>> typealias Iterator = IndexingIterator
>> typealias SubSequence = RandomAccessSlice
>> 
>> var startIndex: Int { return contents.startIndex }
>> var endIndex: Int { return contents.endIndex }
>> subscript(index: Int) -> T { return contents[index] }
>> }
>> 
>> where the bolded typealiases are currently inferred. It was worse back when 
>> we reviewed SE-0108, because IIRC there were a few underscored associated 
>> types (e.g., _Element) that have since been removed. Still, that’s a bit of 
>> additional work to define a “minimal” collection, and requires quite a bit 
>> more understanding: how do I know to choose IndexingIterator, and 
>> CountableRange, and RandomAccessSlice?
>> 
>> The issue would get worse with, e.g., SE-0174 “Change filter to return an 
>> associated type” 
>> ,
>>  which adds an associated type Filtered that almost nobody will ever 
>> customize, and isn’t really fundamental to the way collections work. Adding 
>> Filtered to the standard library would be a source-breaking change, because 
>> users would have to write a typealias giving it its default.
>> 
>> Associated Type Defaults
>> One of the ways in which we avoid having to specify typealiases is to use 
>> associated type defaults. For example, the standard library contains 
>> associated type defaults for Indices, Iterator, and SubSequence. Let’s focus 
>> on Indices:
>> 
>> protocol Collection : Sequence {
>>   associatedtype Indices = DefaultIndices
>>   // ...
>> }
>> 
>> protocol BidirectionalCollection : Collection {
>>   associatedtype Indices = DefaultBidirectionalIndices
>>   // ...
>> }
>> 
>> protocol RandomAccessCollection : BidirectionalCollection {
>>   associatedtype Indices = DefaultRandomAccessIndices
>>   // ...
>> }
>> 
>> The basic idea here is that different protocols in the hierarchy provide 
>> different defaults, and you presumably want the default from the most 
>> specific protocol. If I define a type and make it conform to 
>> BidirectionalCollection, I’d expect to get DefaultBidirectionalIndices 
>> for Indices. If a define a type and make it conform to RandomAccessIterator, 
>> I’d expect to get DefaultRandomAccessIndices.
>> 
>> (Aside: DefaultRandomAccessIndices and DefaultBidirectionalIndices got 
>> collapsed into DefaultIndices now that we have conditional conformances for 
>> the standard library , but the 
>> issues I’m describing remain).
>> 
>> Associated type defaults 

Re: [swift-evolution] [RFC] Associated type inference

[Greg Titus via swift-evolution]

> On Dec 1, 2017, at 9:11 AM, Ben Langmuir via swift-evolution 
>  wrote:
> Hey Doug,
> 
> I'm very much in favour of reducing the scope of associated type inference.  
> Can you outline why you believe that (3) is necessary?  If I am following 
> correctly, if we had (1) and (2) the only thing you'd need to add to the 
> "minimal collection" implementation would be a typealias for `Element`, which 
> seems reasonable to me.
> 
> Ben

If nothing else, dropping (3) would be source breaking for 90%+ of current 
associated type uses. Whereas even the very minimal inference in (3) probably 
brings that figure down to 1% or so (outside of the stdlib, which would need to 
adopt a bunch of (2)). Obviously these percentages are just my guesses and not 
based on any real survey, but certainly would be the case for all Swift code 
I’ve seen.

- Greg___
swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution

Re: [swift-evolution] [RFC] Associated type inference

[Ben Langmuir via swift-evolution]

> On Nov 30, 2017, at 4:28 PM, Douglas Gregor via swift-evolution 
>  wrote:
> 
> Hello Swift community,
> 
> Associated type inference, which is the process by which the Swift compiler 
> attempts to infer typealiases to satisfy associated-type requirements based 
> on other requirements, has caused both implementation problems and user 
> confusion for a long time. Some of you might remember a previous (failed) 
> attempt to remove this feature from the Swift language, in SE-0108 “Remove 
> associated type inference”. 
> 
>  
> 
> I’m not sure we can remove this feature outright (i.e., the concerns that 
> sank that proposal are still absolutely valid), because it is so very 
> convenient and a lot of user code likely depends on it in some form or other. 
> So, here I’d like to describe the uses of the feature, its current (very 
> problematic) implementation approach, and a half-baked proposal to narrow the 
> scope of associated type inference to something that I think is more 
> tractable. I need help with the design of this feature, because I feel like 
> it’s going to be a delicate balance between implementability and 
> expressiveness.
> 
> A Motivating Example
> As a motivating example, let’s consider a “minimal” random access collection:
> 
> struct MyCollection {
> var contents: [T]
> }
> 
> extension MyCollection: RandomAccessCollection {
> var startIndex: Int { return contents.startIndex }
> var endIndex: Int { return contents.endIndex }
> subscript(index: Int) -> T { return contents[index] }
> }
> 
> This is actually pretty awesome: by providing just two properties and a 
> subscript, we get the full breadth of the random access collection API! This 
> is relying heavily on associated type inference (for associated type 
> requirements) and default implementations specified on protocol extensions. 
> Without associated type inference, we would have had to write:
> 
> 
> extension MyCollection: RandomAccessCollection {
> typealias Element = T
> typealias Index = Int
> typealias Indices = CountableRange
> typealias Iterator = IndexingIterator
> typealias SubSequence = RandomAccessSlice
> 
> var startIndex: Int { return contents.startIndex }
> var endIndex: Int { return contents.endIndex }
> subscript(index: Int) -> T { return contents[index] }
> }
> 
> where the bolded typealiases are currently inferred. It was worse back when 
> we reviewed SE-0108, because IIRC there were a few underscored associated 
> types (e.g., _Element) that have since been removed. Still, that’s a bit of 
> additional work to define a “minimal” collection, and requires quite a bit 
> more understanding: how do I know to choose IndexingIterator, and 
> CountableRange, and RandomAccessSlice?
> 
> The issue would get worse with, e.g., SE-0174 “Change filter to return an 
> associated type” 
> ,
>  which adds an associated type Filtered that almost nobody will ever 
> customize, and isn’t really fundamental to the way collections work. Adding 
> Filtered to the standard library would be a source-breaking change, because 
> users would have to write a typealias giving it its default.
> 
> Associated Type Defaults
> One of the ways in which we avoid having to specify typealiases is to use 
> associated type defaults. For example, the standard library contains 
> associated type defaults for Indices, Iterator, and SubSequence. Let’s focus 
> on Indices:
> 
> protocol Collection : Sequence {
>   associatedtype Indices = DefaultIndices
>   // ...
> }
> 
> protocol BidirectionalCollection : Collection {
>   associatedtype Indices = DefaultBidirectionalIndices
>   // ...
> }
> 
> protocol RandomAccessCollection : BidirectionalCollection {
>   associatedtype Indices = DefaultRandomAccessIndices
>   // ...
> }
> 
> The basic idea here is that different protocols in the hierarchy provide 
> different defaults, and you presumably want the default from the most 
> specific protocol. If I define a type and make it conform to 
> BidirectionalCollection, I’d expect to get DefaultBidirectionalIndices 
> for Indices. If a define a type and make it conform to RandomAccessIterator, 
> I’d expect to get DefaultRandomAccessIndices.
> 
> (Aside: DefaultRandomAccessIndices and DefaultBidirectionalIndices got 
> collapsed into DefaultIndices now that we have conditional conformances for 
> the standard library , but the 
> issues I’m describing remain).
> 
> Associated type defaults seem like a reasonable feature that fits well enough 
> into the design. However, it’s not the only thing in place with our 
> MyCollection example, for which Indices was inferred to CountableRange. How’s 
> that happen?
> 

Re: [swift-evolution] [RFC] Associated type inference

[Johannes Weiß via swift-evolution]

Hi Douglas,

First of all, thanks very much for looking into this seemingly dark corner of 
the compiler. This caused us a lot of trouble already.

Comments inline.

> On 1 Dec 2017, at 12:28 am, Douglas Gregor via swift-evolution 
>  wrote:
> 
> Hello Swift community,
> 
> Associated type inference, which is the process by which the Swift compiler 
> attempts to infer typealiases to satisfy associated-type requirements based 
> on other requirements, has caused both implementation problems and user 
> confusion for a long time. Some of you might remember a previous (failed) 
> attempt to remove this feature from the Swift language, in SE-0108 “Remove 
> associated type inference”. 
> 
> I’m not sure we can remove this feature outright (i.e., the concerns that 
> sank that proposal are still absolutely valid), because it is so very 
> convenient and a lot of user code likely depends on it in some form or other. 
> So, here I’d like to describe the uses of the feature, its current (very 
> problematic) implementation approach, and a half-baked proposal to narrow the 
> scope of associated type inference to something that I think is more 
> tractable. I need help with the design of this feature, because I feel like 
> it’s going to be a delicate balance between implementability and 
> expressiveness.
> 
> A Motivating Example
> As a motivating example, let’s consider a “minimal” random access collection:
> 
> struct MyCollection {
> var contents: [T]
> }
> 
> extension MyCollection: RandomAccessCollection {
> var startIndex: Int { return contents.startIndex }
> var endIndex: Int { return contents.endIndex }
> subscript(index: Int) -> T { return contents[index] }
> }
> 
> This is actually pretty awesome: by providing just two properties and a 
> subscript, we get the full breadth of the random access collection API! This 
> is relying heavily on associated type inference (for associated type 
> requirements) and default implementations specified on protocol extensions. 
> Without associated type inference, we would have had to write:
> 
> 
> extension MyCollection: RandomAccessCollection {
> typealias Element = T
> typealias Index = Int
> typealias Indices = CountableRange
> typealias Iterator = IndexingIterator
> typealias SubSequence = RandomAccessSlice
> 
> var startIndex: Int { return contents.startIndex }
> var endIndex: Int { return contents.endIndex }
> subscript(index: Int) -> T { return contents[index] }
> }
> 
> where the bolded typealiases are currently inferred. It was worse back when 
> we reviewed SE-0108, because IIRC there were a few underscored associated 
> types (e.g., _Element) that have since been removed. Still, that’s a bit of 
> additional work to define a “minimal” collection, and requires quite a bit 
> more understanding: how do I know to choose IndexingIterator, and 
> CountableRange, and RandomAccessSlice?
> 
> The issue would get worse with, e.g., SE-0174 “Change filter to return an 
> associated type”, which adds an associated type Filtered that almost nobody 
> will ever customize, and isn’t really fundamental to the way collections 
> work. Adding Filtered to the standard library would be a source-breaking 
> change, because users would have to write a typealias giving it its default.
> 
> Associated Type Defaults
> One of the ways in which we avoid having to specify typealiases is to use 
> associated type defaults. For example, the standard library contains 
> associated type defaults for Indices, Iterator, and SubSequence. Let’s focus 
> on Indices:
> 
> protocol Collection : Sequence {
>   associatedtype Indices = DefaultIndices
>   // ...
> }
> 
> protocol BidirectionalCollection : Collection {
>   associatedtype Indices = DefaultBidirectionalIndices
>   // ...
> }
> 
> protocol RandomAccessCollection : BidirectionalCollection {
>   associatedtype Indices = DefaultRandomAccessIndices
>   // ...
> }
> 
> The basic idea here is that different protocols in the hierarchy provide 
> different defaults, and you presumably want the default from the most 
> specific protocol. If I define a type and make it conform to 
> BidirectionalCollection, I’d expect to get DefaultBidirectionalIndices 
> for Indices. If a define a type and make it conform to RandomAccessIterator, 
> I’d expect to get DefaultRandomAccessIndices.
> 
> (Aside: DefaultRandomAccessIndices and DefaultBidirectionalIndices got 
> collapsed into DefaultIndices now that we have conditional conformances for 
> the standard library, but the issues I’m describing remain).
> 
> Associated type defaults seem like a reasonable feature that fits well enough 
> into the design. However, it’s not the only thing in place with our 
> MyCollection example, for which Indices was inferred to CountableRange. How’s 
> that happen?
> 
> Associated Type Inference
> Associated type inference attempts to look at the 

Re: [swift-evolution] [RFC] Associated type inference

[Greg Titus via swift-evolution]

Hi Doug,

> On Nov 30, 2017, at 4:28 PM, Douglas Gregor via swift-evolution 
>  wrote:
> 
> I think this approach is more predictable and more implementable than the 
> current model. I’m curious whether the above makes sense to someone other 
> than me, and whether it covers existing use cases well enough. Thoughts?

I don’t have a lot to add here, except to say: Yes, this makes sense to me, and 
is much easier to reason about. The three new rough rules covers all of the 
Swift protocol code I can think of that I’ve written, and seems complete for 
any reasonable set of thought experiments I’ve come up with so far.

Thanks!
- Greg___
swift-evolution mailing list
swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution

[swift-evolution] [RFC] Associated type inference

[Douglas Gregor via swift-evolution]

Hello Swift community,

Associated type inference, which is the process by which the Swift compiler 
attempts to infer typealiases to satisfy associated-type requirements based on 
other requirements, has caused both implementation problems and user confusion 
for a long time. Some of you might remember a previous (failed) attempt to 
remove this feature from the Swift language, in SE-0108 “Remove associated type 
inference”. 

 

I’m not sure we can remove this feature outright (i.e., the concerns that sank 
that proposal are still absolutely valid), because it is so very convenient and 
a lot of user code likely depends on it in some form or other. So, here I’d 
like to describe the uses of the feature, its current (very problematic) 
implementation approach, and a half-baked proposal to narrow the scope of 
associated type inference to something that I think is more tractable. I need 
help with the design of this feature, because I feel like it’s going to be a 
delicate balance between implementability and expressiveness.

A Motivating Example
As a motivating example, let’s consider a “minimal” random access collection:

struct MyCollection {
var contents: [T]
}

extension MyCollection: RandomAccessCollection {
var startIndex: Int { return contents.startIndex }
var endIndex: Int { return contents.endIndex }
subscript(index: Int) -> T { return contents[index] }
}

This is actually pretty awesome: by providing just two properties and a 
subscript, we get the full breadth of the random access collection API! This is 
relying heavily on associated type inference (for associated type requirements) 
and default implementations specified on protocol extensions. Without 
associated type inference, we would have had to write:


extension MyCollection: RandomAccessCollection {
typealias Element = T
typealias Index = Int
typealias Indices = CountableRange
typealias Iterator = IndexingIterator
typealias SubSequence = RandomAccessSlice

var startIndex: Int { return contents.startIndex }
var endIndex: Int { return contents.endIndex }
subscript(index: Int) -> T { return contents[index] }
}

where the bolded typealiases are currently inferred. It was worse back when we 
reviewed SE-0108, because IIRC there were a few underscored associated types 
(e.g., _Element) that have since been removed. Still, that’s a bit of 
additional work to define a “minimal” collection, and requires quite a bit more 
understanding: how do I know to choose IndexingIterator, and CountableRange, 
and RandomAccessSlice?

The issue would get worse with, e.g., SE-0174 “Change filter to return an 
associated type” 
,
 which adds an associated type Filtered that almost nobody will ever customize, 
and isn’t really fundamental to the way collections work. Adding Filtered to 
the standard library would be a source-breaking change, because users would 
have to write a typealias giving it its default.

Associated Type Defaults
One of the ways in which we avoid having to specify typealiases is to use 
associated type defaults. For example, the standard library contains associated 
type defaults for Indices, Iterator, and SubSequence. Let’s focus on Indices:

protocol Collection : Sequence {
  associatedtype Indices = DefaultIndices
  // ...
}

protocol BidirectionalCollection : Collection {
  associatedtype Indices = DefaultBidirectionalIndices
  // ...
}

protocol RandomAccessCollection : BidirectionalCollection {
  associatedtype Indices = DefaultRandomAccessIndices
  // ...
}

The basic idea here is that different protocols in the hierarchy provide 
different defaults, and you presumably want the default from the most specific 
protocol. If I define a type and make it conform to BidirectionalCollection, 
I’d expect to get DefaultBidirectionalIndices for Indices. If a define a 
type and make it conform to RandomAccessIterator, I’d expect to get 
DefaultRandomAccessIndices.

(Aside: DefaultRandomAccessIndices and DefaultBidirectionalIndices got 
collapsed into DefaultIndices now that we have conditional conformances for the 
standard library , but the issues 
I’m describing remain).

Associated type defaults seem like a reasonable feature that fits well enough 
into the design. However, it’s not the only thing in place with our 
MyCollection example, for which Indices was inferred to CountableRange. How’s 
that happen?

Associated Type Inference
Associated type inference attempts to look at the requirements of a protocol, 
and then looks into the conforming type for declarations that might satisfy 
those requirements, and infers associated types from the types of those 
declarations. Let’s look at some examples. RandomAccessCollection