Re: [swift-evolution] Pitch: Limit typealias extensions to the typealias

[Dave Abrahams via swift-evolution]

on Fri Jun 09 2017, Matthew Johnson  wrote:

>> On Jun 9, 2017, at 12:09 PM, Xiaodi Wu via swift-evolution
>>  wrote:
>> 
>> On Fri, Jun 9, 2017 at 12:44 Robert Bennett via swift-evolution
>> 
>> >
>> wrote:
>> Somewhat related to this, shouldn’t it be possible to sub-struct a
>> struct as long as you only add functions and computed properties
>> (i.e., no stored properties)? Traditionally structs cannot be
>> subtyped because their size must be known at compile time. I don’t
>> know the implementation details of where functions and computed
>> properties live, but something tells me they belong to the type and
>> not the object (although I’ve never really made the effort to sit
>> down and fully understand Swift’s type model), in which case adding
>> them to a struct’s definition would not change the size of the
>> object on the stack. Thus it should be possible to make custom
>> substructs of String that add additional functionality but no new
>> stored properties. Thoughts?
>> 
>> Value subtyping is a large subject and, IIUC, newtype would be a
>> subset of that topic. Unlikely to be in scope for Swift 5, though,
>> but that’s up to the core team.
>
> I see newtype as being more related to forwarding than subtyping.
> Usually you want to hide significant parts of the interface to the
> wrapped type.

Yes, and whether you want an implicit conversion relationship between
the newtype and the old is an orthogonal issue.

-- 
-Dave

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Re: [swift-evolution] Pitch: Limit typealias extensions to the typealias

[David Sweeris via swift-evolution]

> On Jun 9, 2017, at 16:38, Daryle Walker via swift-evolution 
>  wrote:
> 
> 
>> On Jun 9, 2017, at 12:14 AM, Yvo van Beek via swift-evolution 
>>  wrote:
>> 
>> Typealiases can greatly reduce the complexity of code. But I think one 
>> change in how the compiler handles them could make them even more powerful.
> [SNIP]
>> Perhaps it would be better if the extension would only apply to the parts of 
>> my code where I use the HeaderKey typealias and not to all Strings. This 
>> could be a great tool to specialize classes by creating a typealias and 
>> adding functionality to it. Another example I can think of is typealiases 
>> for dictionaries or arrays with added business logic through extensions 
>> (especially since you can't inherit from structs).
>> 
>> If you want to create an extension that adds functionality to all Strings 
>> you could have created an extension for String instead of HeaderKey.
>> 
>> Please let me know what you think. I'm not sure how complex this change 
>> would be.
>> I could write a proposal if you're interested.
> 
> Isn’t the point of “typealias" is that it does NOT have any change in 
> semantics? The compiler doesn’t even have to acknowledge aliases in any 
> run-time type tables, it just references the existing row of what the alias 
> points to (based on a compile-time type table).
> 
> As others suggested, this new semantic could be moved to a new type concept 
> (with a new keyword).

Agreed... +1 for something like "newtype", -1 for hoisting that functionality 
onto "typealias".

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Re: [swift-evolution] Pitch: Limit typealias extensions to the typealias

[Daryle Walker via swift-evolution]

> On Jun 9, 2017, at 12:14 AM, Yvo van Beek via swift-evolution 
>  wrote:
> 
> Typealiases can greatly reduce the complexity of code. But I think one change 
> in how the compiler handles them could make them even more powerful.
[SNIP]
> Perhaps it would be better if the extension would only apply to the parts of 
> my code where I use the HeaderKey typealias and not to all Strings. This 
> could be a great tool to specialize classes by creating a typealias and 
> adding functionality to it. Another example I can think of is typealiases for 
> dictionaries or arrays with added business logic through extensions 
> (especially since you can't inherit from structs).
> 
> If you want to create an extension that adds functionality to all Strings you 
> could have created an extension for String instead of HeaderKey.
> 
> Please let me know what you think. I'm not sure how complex this change would 
> be.
> I could write a proposal if you're interested.

Isn’t the point of “typealias" is that it does NOT have any change in 
semantics? The compiler doesn’t even have to acknowledge aliases in any 
run-time type tables, it just references the existing row of what the alias 
points to (based on a compile-time type table).

As others suggested, this new semantic could be moved to a new type concept 
(with a new keyword).

— 
Daryle Walker
Mac, Internet, and Video Game Junkie
darylew AT mac DOT com 

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Re: [swift-evolution] Pitch: Limit typealias extensions to the typealias

[Yvo van Beek via swift-evolution]

@Doug:

It might indeed be confusing to use "typealias" for this.

Looking at the HeaderKey example, alternatives could be:
1) struct inheritance (HeaderKey would inherit from String), value
subtyping?
2) the ability to specify implicit conversion between types, C# has
the implicit
keyword

for this

@Will:

Your solution of creating a HeaderKey struct resolves most issues, but has
a few downsides:
1) the HeaderKey struct would contain quite some boilerplate code that
mimics String (Equality, CustomStringConvertible etc.)
2) the ExpressibleByStringLiteral enables the use of static strings, but a
dynamic string would need to be wrapped: HeaderKey(dynamicHeader) leading
to clunky code
3) the headers dictionary would become more difficult to process, the key
would no longer be an actual String, resulting in more code that distracts
from the actual code

Thanks everyone for chiming in on this.


On Fri, Jun 9, 2017 at 11:41 AM, Will Field-Thompson 
wrote:

> I feel like this might be better served by something like newtype which I
> know I've seen floating around on this list before. The idea is that
> something like:
>
> newtype HeaderKey = String
>
> would serve roughly as syntactic sugar for something like
>
> struct HeaderKey {
>let value: String
>init(_ value: String) { self.value = value }
>/* Possibly insert methods from String here, possibly do something
> else, whatever */
> }
>
> I've always thought this might be interesting, but there's a few reasons I
> suggest it in place of extending a typealias:
> 1) Backwards compatibility — this could very easily break existing code
> 2) Semantically it seems to me that an *alias* is a useful distinction
> from *"let's make a new type" *— sometimes you actually just want to call
> one type by another name in some context
>
> Regardless, I think you could solve your particular problem like this:
>
> public struct HeaderKey {
>let value: String
>public init(_ value: String) { self.value = value }
> }
>
> and conform HeaderKey to ExpressibleByStringLiteral. That way your
> framework's users can still use headers[.lastModified] and
> headers["X-MyHeader"] syntax, and it makes your library's code only very
> slightly more complicated.
>
> Not trying to tell you how to write your library, but this approach worked
> really well for me recently.
>
> Best,
>
> Will
>
> On Fri, Jun 9, 2017 at 12:14 AM Yvo van Beek via swift-evolution <
> swift-evolution@swift.org> wrote:
>
>> Typealiases can greatly reduce the complexity of code. But I think one
>> change in how the compiler handles them could make them even more powerful.
>>
>> Let's say I'm creating a web server framework and I've created a simple
>> dictionary to store HTTP headers (I know that headers are more complex than
>> that, but as an example). I could write something like this:
>>
>> typealias HeaderKey = String
>>
>>   var headers = [HeaderKey: String]()
>>   headers["Host"] = "domain.com"
>>
>> Now I can define a couple of default headers like this:
>>
>>   extension HeaderKey {
>> static var lastModified: String { return "Last-Modified" }
>> static var host: String { return "Host" }
>>   }
>>
>> After that I can do this:
>>
>>   var headers = [HeaderKey: String]()
>>   headers[.host] = "domain.com"
>>   headers[.lastModified] = "some date"
>>   headers["X-MyHeader"] = "This still works too"
>>
>> But unfortunately the extension is also applied to normal strings:
>>
>> var normalString: String = .host
>>
>> Perhaps it would be better if the extension would only apply to the parts
>> of my code where I use the HeaderKey typealias and not to all Strings. This
>> could be a great tool to specialize classes by creating a typealias and
>> adding functionality to it. Another example I can think of is typealiases
>> for dictionaries or arrays with added business logic through extensions
>> (especially since you can't inherit from structs).
>>
>> If you want to create an extension that adds functionality to all Strings
>> you could have created an extension for String instead of HeaderKey.
>>
>> Please let me know what you think. I'm not sure how complex this change
>> would be.
>> I could write a proposal if you're interested.
>>
>> Kind regards,
>> Yvo
>> ___
>> swift-evolution mailing list
>> swift-evolution@swift.org
>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>
>
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Re: [swift-evolution] Pitch: Limit typealias extensions to the typealias

[Karl Wagner via swift-evolution]

> On 9. Jun 2017, at 21:47, Matthew Johnson via swift-evolution 
>  wrote:
> 
> 
>> On Jun 9, 2017, at 2:39 PM, Xiaodi Wu > > wrote:
>> 
>> Interesting. So you’d want `newtype Foo = String` to start off with no 
>> members on Foo?
> 
> Yeah.  Previous discussions of newtype have usually led to discussion of ways 
> to forward using a protocol-oriented approach.  Nothing has gotten too far, 
> but it usually comes up that suppressing undesired members is important.
> 
> It is also important to have some way to distinguish between members with a 
> parameter of the underlying type from members that should be treated by 
> newtype as Self parameters.  The mechanism we have for doing that in Swift 
> happens to be a protocol.

It’s important to note that you can create powerful quasi-subtype relationships 
using composition. This is going to be a little divergent, but this is WWDC 
week and everybody’s here, and its related to the topic of architecture with 
value-types.

The only practical difference between composition and subclassing is that you 
can’t add storage and have it carried along with the original “instance”. I’m 
not even sure that really makes sense for value types, which don’t have 
identity — the alternative, that a value is be composed of sub-values, makes 
more sense to me.

I’ve recently converted an entire project from using class hierarchies and 
stacks of protocols down to a handful of protocols and value-types. I cut the 
number of files in half, reduced duplicated logic, increased testability, 
maintainability, all kinds of good stuff. I’m pretty happy with how it turned 
out, so let me very briefly outline how I did it, to give a concrete example of 
the kinds of things you can do with composition.

The new data subsystem of this App is now entirely stateless - ultimately, if 
you look through all of the wrappers, we’re literally just passing around a 
handful of “let” variables (api endpoint, api key, login key, and a cache 
identifier), and the entire framework boils down to isolated islands of 
business logic written in extensions on those structs.

There are only two really important protocols: an ObjectStore (i.e. a cache) 
and an ObjectSource (i.e. the API). They have very generic methods, like 
“fetch”, “put” and “delete”. All of the business logic about which queries to 
run on the cache, and where to put the data, or how to query the correct data 
out of the API, is written in a collection of wrapper structs which you access 
to via a computed property (a kind of namespaced protocol-extension, e.g. 
myObjectStore.userInformation). Above the source and store (i.e. wrapping them) 
sits a Repository struct, which coordinates getting data from the ObjectStore, 
querying for that data from the ObjectSource if it doesn’t have anything (or if 
its expired), and returns a future (actually it’s a Reactive Observable, but 
any kind of future-like-object will do) encapsulating the operation. 

There’s lots you can do with value-types. For example, I created a wrapper for 
the top-level “Session” type which dynamically checks if the session belongs to 
a logged-in user. There is a separate repository for public data (e.g. store 
locations) and private data (e.g. purchase history), and we can model all of 
this separation really easily in the type-system with no cognitive or 
computational overhead.

/// An API session, which may or may not belong to a logged-in user.
///
struct Session {
typealias Identity = (loginKey: String, cacheIdentifier: String)

let configuration: SessionConfiguration // creates repository on behalf of 
the session, for unit-testing.
let identity: Identity?
let publicData: PublicDataRepository

init(configuration: SessionConfiguration, identity: Identity) {
self.configuration = configuration
self.identity  = identity
self.publicData= configuration.makePublicRepository()
}
}

/// A session which is dynamically known to belong to a logged-in user.
///
struct AuthenticatedSession {

let base: Session // you can think of this like ‘super’
let privateData: PrivateDataRepository

init?(base: Session) {
guard let identity = base.identity else { return nil }
self.base   = base
privateData = base.configuration.makePrivateRepository(for: identity)
}
}

/* methods which do not require authentication */

extension Session { 
func getStoreLocations() -> Future<[StoreLocation]> { … }
 }

/* methods which require a logged-in user */

extension AuthenticatedSession {  
func buySomething(_: ThingToBuy) -> Future { … }
}

When it comes to storage, AuthenticatedSession not having the same memory 
layout as Session means you can’t store one variable that could be either — 
unless you box it. You can use a protocol to create a semantically-meaningful 
box (e.g. PublicDataProvider, with one computed 

Re: [swift-evolution] Pitch: Limit typealias extensions to the typealias

[Tony Allevato via swift-evolution]

+1 to this. My ideal imagining of this behavior is to:

1) Define a protocol containing the precise operations you want to support
via forwarding.
2) Internally/fileprivately extend the original type to conform to this
protocol.
3) In your new type, tell it to forward protocol members for a particular
property (the underlying String, or whatever) and the compiler will
synthesize the protocol member stubs on the new type.

There are definitely finer details to be worked out but that's a rough
sketch of how I've been imagining it could work. Nice advantages include
(1) if there already exists a protocol that defines the exact operations
you want to forward, you can skip step 2, and (2) it's explicit/opt-in
synthesis where you have to provide the members to forward (this is
expected, at a minimum), but you don't have to write all the boilerplate
implementations.


On Fri, Jun 9, 2017 at 12:47 PM Matthew Johnson via swift-evolution <
swift-evolution@swift.org> wrote:

> On Jun 9, 2017, at 2:39 PM, Xiaodi Wu  wrote:
>
> Interesting. So you’d want `newtype Foo = String` to start off with no
> members on Foo?
>
>
> Yeah.  Previous discussions of newtype have usually led to discussion of
> ways to forward using a protocol-oriented approach.  Nothing has gotten too
> far, but it usually comes up that suppressing undesired members is
> important.
>
> It is also important to have some way to distinguish between members with
> a parameter of the underlying type from members that should be treated by
> newtype as Self parameters.  The mechanism we have for doing that in Swift
> happens to be a protocol.
>
> On Fri, Jun 9, 2017 at 15:18 Matthew Johnson 
> wrote:
>
>> On Jun 9, 2017, at 12:09 PM, Xiaodi Wu via swift-evolution <
>> swift-evolution@swift.org> wrote:
>>
>> On Fri, Jun 9, 2017 at 12:44 Robert Bennett via swift-evolution <
>> swift-evolution@swift.org> wrote:
>>
>>> Somewhat related to this, shouldn’t it be possible to sub-struct a
>>> struct as long as you only add functions and computed properties (i.e., no
>>> stored properties)? Traditionally structs cannot be subtyped because their
>>> size must be known at compile time. I don’t know the implementation details
>>> of where functions and computed properties live, but something tells me
>>> they belong to the type and not the object (although I’ve never really made
>>> the effort to sit down and fully understand Swift’s type model), in which
>>> case adding them to a struct’s definition would not change the size of the
>>> object on the stack. Thus it should be possible to make custom substructs
>>> of String that add additional functionality but no new stored properties.
>>> Thoughts?
>>>
>>
>> Value subtyping is a large subject and, IIUC, newtype would be a subset
>> of that topic. Unlikely to be in scope for Swift 5, though, but that’s up
>> to the core team.
>>
>>
>> I see newtype as being more related to forwarding than subtyping.
>> Usually you want to hide significant parts of the interface to the wrapped
>> type.
>>
>>
>>
>> On Jun 9, 2017, at 12:12 PM, Jacob Williams via swift-evolution <
>>> swift-evolution@swift.org> wrote:
>>>
>>> On Jun 9, 2017, at 9:53 AM, Charlie Monroe 
>>> wrote:
>>>
>>> -1 - this would disallow e.g. to share UI code between iOS and macOS:
>>>
>>> #if os(iOS)
>>> typealias XUView = UIView
>>> #else
>>> typealias XUView = NSView
>>> #endif
>>>
>>> extension XUView {
>>> ...
>>> }
>>>
>>>
>>> I really don’t see how this disallows code sharing between the two
>>> systems? Could you explain further? Based on my understanding of the pitch,
>>> this is valid code still. (Although I do like the suggestion of a new
>>> keyword rather than just limiting type alias).
>>>
>>> Even if your example was invalid, you could also just do something like
>>> this:
>>>
>>> #if os(iOS)
>>> typealias XUView = UIView
>>> extension XUView {
>>> //extension code here
>>> }
>>> #if os(macOS)
>>> typealias XUView = UIView
>>> extension XUView {
>>> // extension code here
>>> }
>>> #endif
>>>
>>> While not as pretty, still just as effective if you have to deal with
>>> different types based on the system being compiled for and you could easily
>>> still make the type alias extensions for each type work the same.
>>>
>>> On Jun 9, 2017, at 9:53 AM, Charlie Monroe 
>>> wrote:
>>>
>>> -1 - this would disallow e.g. to share UI code between iOS and macOS:
>>>
>>> #if os(iOS)
>>> typealias XUView = UIView
>>> #else
>>> typealias XUView = NSView
>>> #endif
>>>
>>> extension XUView {
>>> ...
>>> }
>>>
>>> or with any similar compatibility typealiases.
>>>
>>> On Jun 9, 2017, at 5:38 PM, Jacob Williams via swift-evolution <
>>> swift-evolution@swift.org> wrote:
>>>
>>> +1 from me.
>>>
>>> There have been times I’ve wanted to subclass an object (such as String)
>>> but since it is a non-class, non-protocol type you can only extend Strings
>>> existing 

Re: [swift-evolution] Pitch: Limit typealias extensions to the typealias

[Matthew Johnson via swift-evolution]

> On Jun 9, 2017, at 2:53 PM, Tony Allevato  wrote:
> 
> +1 to this. My ideal imagining of this behavior is to:
> 
> 1) Define a protocol containing the precise operations you want to support 
> via forwarding.
> 2) Internally/fileprivately extend the original type to conform to this 
> protocol.
> 3) In your new type, tell it to forward protocol members for a particular 
> property (the underlying String, or whatever) and the compiler will 
> synthesize the protocol member stubs on the new type.
> 
> There are definitely finer details to be worked out but that's a rough sketch 
> of how I've been imagining it could work. Nice advantages include (1) if 
> there already exists a protocol that defines the exact operations you want to 
> forward, you can skip step 2, and (2) it's explicit/opt-in synthesis where 
> you have to provide the members to forward (this is expected, at a minimum), 
> but you don't have to write all the boilerplate implementations.

I have a mostly finished second draft of a protocol-oriented forwarding 
proposal which is pretty similar to what you describe here.  The most important 
difference is that while protocols are used to perform forwarding neither type 
is required to actually conform to the protocol.  

That turns out to be pretty important when you think about edge cases.  
Sometimes you don’t actually want the forwardee to conform (that’s probably why 
you said internal / fileprivate).  Sometimes you also don’t want the forwarder 
to conform.  IIRC, there are also cases where it cannot conform (I would have 
to dig up the proposal to recall the details).

> 
> 
> On Fri, Jun 9, 2017 at 12:47 PM Matthew Johnson via swift-evolution 
> > wrote:
>> On Jun 9, 2017, at 2:39 PM, Xiaodi Wu > > wrote:
>> 
>> Interesting. So you’d want `newtype Foo = String` to start off with no 
>> members on Foo?
> 
> Yeah.  Previous discussions of newtype have usually led to discussion of ways 
> to forward using a protocol-oriented approach.  Nothing has gotten too far, 
> but it usually comes up that suppressing undesired members is important.
> 
> It is also important to have some way to distinguish between members with a 
> parameter of the underlying type from members that should be treated by 
> newtype as Self parameters.  The mechanism we have for doing that in Swift 
> happens to be a protocol.
> 
>> On Fri, Jun 9, 2017 at 15:18 Matthew Johnson > > wrote:
>>> On Jun 9, 2017, at 12:09 PM, Xiaodi Wu via swift-evolution 
>>> > wrote:
>>> 
>>> On Fri, Jun 9, 2017 at 12:44 Robert Bennett via swift-evolution 
>>> > wrote:
>>> Somewhat related to this, shouldn’t it be possible to sub-struct a struct 
>>> as long as you only add functions and computed properties (i.e., no stored 
>>> properties)? Traditionally structs cannot be subtyped because their size 
>>> must be known at compile time. I don’t know the implementation details of 
>>> where functions and computed properties live, but something tells me they 
>>> belong to the type and not the object (although I’ve never really made the 
>>> effort to sit down and fully understand Swift’s type model), in which case 
>>> adding them to a struct’s definition would not change the size of the 
>>> object on the stack. Thus it should be possible to make custom substructs 
>>> of String that add additional functionality but no new stored properties. 
>>> Thoughts?
>>> 
>>> Value subtyping is a large subject and, IIUC, newtype would be a subset of 
>>> that topic. Unlikely to be in scope for Swift 5, though, but that’s up to 
>>> the core team.
>> 
>> I see newtype as being more related to forwarding than subtyping.  Usually 
>> you want to hide significant parts of the interface to the wrapped type.
>> 
>>> 
>>> 
>>> On Jun 9, 2017, at 12:12 PM, Jacob Williams via swift-evolution 
>>> > wrote:
>>> 
> On Jun 9, 2017, at 9:53 AM, Charlie Monroe  > wrote:
> 
> -1 - this would disallow e.g. to share UI code between iOS and macOS:
> 
> #if os(iOS)
>   typealias XUView = UIView
> #else
>   typealias XUView = NSView
> #endif
> 
> extension XUView {
>   ...
> }
 
 I really don’t see how this disallows code sharing between the two 
 systems? Could you explain further? Based on my understanding of the 
 pitch, this is valid code still. (Although I do like the suggestion of a 
 new keyword rather than just limiting type alias).
 
 Even if your example was invalid, you could also just do something like 
 this:
 
 #if os(iOS)

Re: [swift-evolution] Pitch: Limit typealias extensions to the typealias

[Xiaodi Wu via swift-evolution]

On Fri, Jun 9, 2017 at 15:47 Matthew Johnson  wrote:

> On Jun 9, 2017, at 2:39 PM, Xiaodi Wu  wrote:
>
> Interesting. So you’d want `newtype Foo = String` to start off with no
> members on Foo?
>
>
> Yeah.  Previous discussions of newtype have usually led to discussion of
> ways to forward using a protocol-oriented approach.  Nothing has gotten too
> far, but it usually comes up that suppressing undesired members is
> important.
>

That’s right. These conversations have slipped my mind but that does ring a
bell now.

I think the ask in this thread is for some facility to add members only (or
even, perhaps, to add nothing at all), which would be well served by value
subtyping.

In addition, you’re absolutely right that there’s been discussion of better
ways of forwarding where the starting point is a wrapped type with no
members. Interesting.

It is also important to have some way to distinguish between members with a
> parameter of the underlying type from members that should be treated by
> newtype as Self parameters.  The mechanism we have for doing that in Swift
> happens to be a protocol.
>
> On Fri, Jun 9, 2017 at 15:18 Matthew Johnson 
> wrote:
>
>> On Jun 9, 2017, at 12:09 PM, Xiaodi Wu via swift-evolution <
>> swift-evolution@swift.org> wrote:
>>
>> On Fri, Jun 9, 2017 at 12:44 Robert Bennett via swift-evolution <
>> swift-evolution@swift.org> wrote:
>>
>>> Somewhat related to this, shouldn’t it be possible to sub-struct a
>>> struct as long as you only add functions and computed properties (i.e., no
>>> stored properties)? Traditionally structs cannot be subtyped because their
>>> size must be known at compile time. I don’t know the implementation details
>>> of where functions and computed properties live, but something tells me
>>> they belong to the type and not the object (although I’ve never really made
>>> the effort to sit down and fully understand Swift’s type model), in which
>>> case adding them to a struct’s definition would not change the size of the
>>> object on the stack. Thus it should be possible to make custom substructs
>>> of String that add additional functionality but no new stored properties.
>>> Thoughts?
>>>
>>
>> Value subtyping is a large subject and, IIUC, newtype would be a subset
>> of that topic. Unlikely to be in scope for Swift 5, though, but that’s up
>> to the core team.
>>
>>
>> I see newtype as being more related to forwarding than subtyping.
>> Usually you want to hide significant parts of the interface to the wrapped
>> type.
>>
>>
>>
>> On Jun 9, 2017, at 12:12 PM, Jacob Williams via swift-evolution <
>>> swift-evolution@swift.org> wrote:
>>>
>>> On Jun 9, 2017, at 9:53 AM, Charlie Monroe 
>>> wrote:
>>>
>>> -1 - this would disallow e.g. to share UI code between iOS and macOS:
>>>
>>> #if os(iOS)
>>> typealias XUView = UIView
>>> #else
>>> typealias XUView = NSView
>>> #endif
>>>
>>> extension XUView {
>>> ...
>>> }
>>>
>>>
>>> I really don’t see how this disallows code sharing between the two
>>> systems? Could you explain further? Based on my understanding of the pitch,
>>> this is valid code still. (Although I do like the suggestion of a new
>>> keyword rather than just limiting type alias).
>>>
>>> Even if your example was invalid, you could also just do something like
>>> this:
>>>
>>> #if os(iOS)
>>> typealias XUView = UIView
>>> extension XUView {
>>> //extension code here
>>> }
>>> #if os(macOS)
>>> typealias XUView = UIView
>>> extension XUView {
>>> // extension code here
>>> }
>>> #endif
>>>
>>> While not as pretty, still just as effective if you have to deal with
>>> different types based on the system being compiled for and you could easily
>>> still make the type alias extensions for each type work the same.
>>>
>>> On Jun 9, 2017, at 9:53 AM, Charlie Monroe 
>>> wrote:
>>>
>>> -1 - this would disallow e.g. to share UI code between iOS and macOS:
>>>
>>> #if os(iOS)
>>> typealias XUView = UIView
>>> #else
>>> typealias XUView = NSView
>>> #endif
>>>
>>> extension XUView {
>>> ...
>>> }
>>>
>>> or with any similar compatibility typealiases.
>>>
>>> On Jun 9, 2017, at 5:38 PM, Jacob Williams via swift-evolution <
>>> swift-evolution@swift.org> wrote:
>>>
>>> +1 from me.
>>>
>>> There have been times I’ve wanted to subclass an object (such as String)
>>> but since it is a non-class, non-protocol type you can only extend Strings
>>> existing functionality which adds that same functionality to Strings
>>> everywhere. It would be nice if we could either extend type aliases (and
>>> only the type alias), or if it were possible to inherit from structs so
>>> that we could create a custom string type like so:
>>>
>>> struct HeaderKey: String {
>>> static var lastModified: String { return “Last-Modified” }
>>> static var host: String { return “Host” }
>>> }
>>>
>>> I realize that struct inheritance is far less likely, since that 

Re: [swift-evolution] Pitch: Limit typealias extensions to the typealias

[Matthew Johnson via swift-evolution]

> On Jun 9, 2017, at 3:16 PM, Tony Allevato  wrote:
> 
> Makes sense. I guess from an implementation point of view, the compiler only 
> needs to be able to see the signatures of the members to synthesize the calls 
> to them. My thinking was that conforming the original type would remove the 
> need for the compiler to verify separately that the members exist on the type 
> (because of it conforms, they must, so you get it for free), but I suppose in 
> this case the compiler would just do a separate validation pass.

Yeah, conformance could be used as a shortcut when it exists but requiring it 
is an unnecessary limitation that reduces flexibility and actually prevents 
some uses altogether.

> 
> I'm really looking forward to reading your draft!
> On Fri, Jun 9, 2017 at 12:58 PM Matthew Johnson  > wrote:
>> On Jun 9, 2017, at 2:53 PM, Tony Allevato > > wrote:
>> 
>> +1 to this. My ideal imagining of this behavior is to:
>> 
>> 1) Define a protocol containing the precise operations you want to support 
>> via forwarding.
>> 2) Internally/fileprivately extend the original type to conform to this 
>> protocol.
>> 3) In your new type, tell it to forward protocol members for a particular 
>> property (the underlying String, or whatever) and the compiler will 
>> synthesize the protocol member stubs on the new type.
>> 
>> There are definitely finer details to be worked out but that's a rough 
>> sketch of how I've been imagining it could work. Nice advantages include (1) 
>> if there already exists a protocol that defines the exact operations you 
>> want to forward, you can skip step 2, and (2) it's explicit/opt-in synthesis 
>> where you have to provide the members to forward (this is expected, at a 
>> minimum), but you don't have to write all the boilerplate implementations.
> 
> I have a mostly finished second draft of a protocol-oriented forwarding 
> proposal which is pretty similar to what you describe here.  The most 
> important difference is that while protocols are used to perform forwarding 
> neither type is required to actually conform to the protocol.  
> 
> That turns out to be pretty important when you think about edge cases.  
> Sometimes you don’t actually want the forwardee to conform (that’s probably 
> why you said internal / fileprivate).  Sometimes you also don’t want the 
> forwarder to conform.  IIRC, there are also cases where it cannot conform (I 
> would have to dig up the proposal to recall the details).
> 
>> 
>> 
>> On Fri, Jun 9, 2017 at 12:47 PM Matthew Johnson via swift-evolution 
>> > wrote:
>>> On Jun 9, 2017, at 2:39 PM, Xiaodi Wu >> > wrote:
>>> 
>>> Interesting. So you’d want `newtype Foo = String` to start off with no 
>>> members on Foo?
>> 
>> Yeah.  Previous discussions of newtype have usually led to discussion of 
>> ways to forward using a protocol-oriented approach.  Nothing has gotten too 
>> far, but it usually comes up that suppressing undesired members is important.
>> 
>> It is also important to have some way to distinguish between members with a 
>> parameter of the underlying type from members that should be treated by 
>> newtype as Self parameters.  The mechanism we have for doing that in Swift 
>> happens to be a protocol.
>> 
>>> On Fri, Jun 9, 2017 at 15:18 Matthew Johnson >> > wrote:
 On Jun 9, 2017, at 12:09 PM, Xiaodi Wu via swift-evolution 
 > wrote:
 
 On Fri, Jun 9, 2017 at 12:44 Robert Bennett via swift-evolution 
 > wrote:
 Somewhat related to this, shouldn’t it be possible to sub-struct a struct 
 as long as you only add functions and computed properties (i.e., no stored 
 properties)? Traditionally structs cannot be subtyped because their size 
 must be known at compile time. I don’t know the implementation details of 
 where functions and computed properties live, but something tells me they 
 belong to the type and not the object (although I’ve never really made the 
 effort to sit down and fully understand Swift’s type model), in which case 
 adding them to a struct’s definition would not change the size of the 
 object on the stack. Thus it should be possible to make custom substructs 
 of String that add additional functionality but no new stored properties. 
 Thoughts?
 
 Value subtyping is a large subject and, IIUC, newtype would be a subset of 
 that topic. Unlikely to be in scope for Swift 5, though, but that’s up to 
 the core team.
>>> 
>>> I see newtype as being more related to forwarding than subtyping.  Usually 
>>> you 

Re: [swift-evolution] Pitch: Limit typealias extensions to the typealias

[Tony Allevato via swift-evolution]

Makes sense. I guess from an implementation point of view, the compiler
only needs to be able to see the signatures of the members to synthesize
the calls to them. My thinking was that conforming the original type would
remove the need for the compiler to verify separately that the members
exist on the type (because of it conforms, they must, so you get it for
free), but I suppose in this case the compiler would just do a separate
validation pass.

I'm really looking forward to reading your draft!
On Fri, Jun 9, 2017 at 12:58 PM Matthew Johnson 
wrote:

> On Jun 9, 2017, at 2:53 PM, Tony Allevato  wrote:
>
> +1 to this. My ideal imagining of this behavior is to:
>
> 1) Define a protocol containing the precise operations you want to support
> via forwarding.
> 2) Internally/fileprivately extend the original type to conform to this
> protocol.
> 3) In your new type, tell it to forward protocol members for a particular
> property (the underlying String, or whatever) and the compiler will
> synthesize the protocol member stubs on the new type.
>
> There are definitely finer details to be worked out but that's a rough
> sketch of how I've been imagining it could work. Nice advantages include
> (1) if there already exists a protocol that defines the exact operations
> you want to forward, you can skip step 2, and (2) it's explicit/opt-in
> synthesis where you have to provide the members to forward (this is
> expected, at a minimum), but you don't have to write all the boilerplate
> implementations.
>
>
> I have a mostly finished second draft of a protocol-oriented forwarding
> proposal which is pretty similar to what you describe here.  The most
> important difference is that while protocols are used to perform forwarding
> neither type is required to actually conform to the protocol.
>
> That turns out to be pretty important when you think about edge cases.
> Sometimes you don’t actually want the forwardee to conform (that’s probably
> why you said internal / fileprivate).  Sometimes you also don’t want the
> forwarder to conform.  IIRC, there are also cases where it cannot conform
> (I would have to dig up the proposal to recall the details).
>
>
>
> On Fri, Jun 9, 2017 at 12:47 PM Matthew Johnson via swift-evolution <
> swift-evolution@swift.org> wrote:
>
>> On Jun 9, 2017, at 2:39 PM, Xiaodi Wu  wrote:
>>
>> Interesting. So you’d want `newtype Foo = String` to start off with no
>> members on Foo?
>>
>>
>> Yeah.  Previous discussions of newtype have usually led to discussion of
>> ways to forward using a protocol-oriented approach.  Nothing has gotten too
>> far, but it usually comes up that suppressing undesired members is
>> important.
>>
>> It is also important to have some way to distinguish between members with
>> a parameter of the underlying type from members that should be treated by
>> newtype as Self parameters.  The mechanism we have for doing that in Swift
>> happens to be a protocol.
>>
>> On Fri, Jun 9, 2017 at 15:18 Matthew Johnson 
>> wrote:
>>
>>> On Jun 9, 2017, at 12:09 PM, Xiaodi Wu via swift-evolution <
>>> swift-evolution@swift.org> wrote:
>>>
>>> On Fri, Jun 9, 2017 at 12:44 Robert Bennett via swift-evolution <
>>> swift-evolution@swift.org> wrote:
>>>
 Somewhat related to this, shouldn’t it be possible to sub-struct a
 struct as long as you only add functions and computed properties (i.e., no
 stored properties)? Traditionally structs cannot be subtyped because their
 size must be known at compile time. I don’t know the implementation details
 of where functions and computed properties live, but something tells me
 they belong to the type and not the object (although I’ve never really made
 the effort to sit down and fully understand Swift’s type model), in which
 case adding them to a struct’s definition would not change the size of the
 object on the stack. Thus it should be possible to make custom substructs
 of String that add additional functionality but no new stored properties.
 Thoughts?

>>>
>>> Value subtyping is a large subject and, IIUC, newtype would be a subset
>>> of that topic. Unlikely to be in scope for Swift 5, though, but that’s up
>>> to the core team.
>>>
>>>
>>> I see newtype as being more related to forwarding than subtyping.
>>> Usually you want to hide significant parts of the interface to the wrapped
>>> type.
>>>
>>>
>>>
>>> On Jun 9, 2017, at 12:12 PM, Jacob Williams via swift-evolution <
 swift-evolution@swift.org> wrote:

 On Jun 9, 2017, at 9:53 AM, Charlie Monroe 
 wrote:

 -1 - this would disallow e.g. to share UI code between iOS and macOS:

 #if os(iOS)
 typealias XUView = UIView
 #else
 typealias XUView = NSView
 #endif

 extension XUView {
 ...
 }


 I really don’t see how this disallows code sharing 

Re: [swift-evolution] Pitch: Limit typealias extensions to the typealias

[Matthew Johnson via swift-evolution]

> On Jun 9, 2017, at 2:39 PM, Xiaodi Wu  wrote:
> 
> Interesting. So you’d want `newtype Foo = String` to start off with no 
> members on Foo?

Yeah.  Previous discussions of newtype have usually led to discussion of ways 
to forward using a protocol-oriented approach.  Nothing has gotten too far, but 
it usually comes up that suppressing undesired members is important.

It is also important to have some way to distinguish between members with a 
parameter of the underlying type from members that should be treated by newtype 
as Self parameters.  The mechanism we have for doing that in Swift happens to 
be a protocol.

> On Fri, Jun 9, 2017 at 15:18 Matthew Johnson  > wrote:
>> On Jun 9, 2017, at 12:09 PM, Xiaodi Wu via swift-evolution 
>> > wrote:
>> 
>> On Fri, Jun 9, 2017 at 12:44 Robert Bennett via swift-evolution 
>> > wrote:
>> Somewhat related to this, shouldn’t it be possible to sub-struct a struct as 
>> long as you only add functions and computed properties (i.e., no stored 
>> properties)? Traditionally structs cannot be subtyped because their size 
>> must be known at compile time. I don’t know the implementation details of 
>> where functions and computed properties live, but something tells me they 
>> belong to the type and not the object (although I’ve never really made the 
>> effort to sit down and fully understand Swift’s type model), in which case 
>> adding them to a struct’s definition would not change the size of the object 
>> on the stack. Thus it should be possible to make custom substructs of String 
>> that add additional functionality but no new stored properties. Thoughts?
>> 
>> Value subtyping is a large subject and, IIUC, newtype would be a subset of 
>> that topic. Unlikely to be in scope for Swift 5, though, but that’s up to 
>> the core team.
> 
> I see newtype as being more related to forwarding than subtyping.  Usually 
> you want to hide significant parts of the interface to the wrapped type.
> 
>> 
>> 
>> On Jun 9, 2017, at 12:12 PM, Jacob Williams via swift-evolution 
>> > wrote:
>> 
 On Jun 9, 2017, at 9:53 AM, Charlie Monroe > wrote:
 
 -1 - this would disallow e.g. to share UI code between iOS and macOS:
 
 #if os(iOS)
typealias XUView = UIView
 #else
typealias XUView = NSView
 #endif
 
 extension XUView {
...
 }
>>> 
>>> I really don’t see how this disallows code sharing between the two systems? 
>>> Could you explain further? Based on my understanding of the pitch, this is 
>>> valid code still. (Although I do like the suggestion of a new keyword 
>>> rather than just limiting type alias).
>>> 
>>> Even if your example was invalid, you could also just do something like 
>>> this:
>>> 
>>> #if os(iOS)
>>> typealias XUView = UIView
>>> extension XUView {
>>> //extension code here
>>> }
>>> #if os(macOS)
>>> typealias XUView = UIView
>>> extension XUView {
>>> // extension code here
>>> }
>>> #endif
>>> 
>>> While not as pretty, still just as effective if you have to deal with 
>>> different types based on the system being compiled for and you could easily 
>>> still make the type alias extensions for each type work the same.
>>> 
 On Jun 9, 2017, at 9:53 AM, Charlie Monroe > wrote:
 
 -1 - this would disallow e.g. to share UI code between iOS and macOS:
 
 #if os(iOS)
typealias XUView = UIView
 #else
typealias XUView = NSView
 #endif
 
 extension XUView {
...
 }
 
 or with any similar compatibility typealiases.
 
> On Jun 9, 2017, at 5:38 PM, Jacob Williams via swift-evolution 
> > wrote:
> 
> +1 from me.
> 
> There have been times I’ve wanted to subclass an object (such as String) 
> but since it is a non-class, non-protocol type you can only extend 
> Strings existing functionality which adds that same functionality to 
> Strings everywhere. It would be nice if we could either extend type 
> aliases (and only the type alias), or if it were possible to inherit from 
> structs so that we could create a custom string type like so:
> 
> struct HeaderKey: String {
>   static var lastModified: String { return “Last-Modified” }
>   static var host: String { return “Host” }
> }
> 
> I realize that struct inheritance is far less likely, since that defeats 
> one of the main pieces of what makes a struct a struct. So I’m all for 
> this proposal of allowing 

Re: [swift-evolution] Pitch: Limit typealias extensions to the typealias

[Xiaodi Wu via swift-evolution]

Interesting. So you’d want `newtype Foo = String` to start off with no
members on Foo?
On Fri, Jun 9, 2017 at 15:18 Matthew Johnson  wrote:

> On Jun 9, 2017, at 12:09 PM, Xiaodi Wu via swift-evolution <
> swift-evolution@swift.org> wrote:
>
> On Fri, Jun 9, 2017 at 12:44 Robert Bennett via swift-evolution <
> swift-evolution@swift.org> wrote:
>
>> Somewhat related to this, shouldn’t it be possible to sub-struct a struct
>> as long as you only add functions and computed properties (i.e., no stored
>> properties)? Traditionally structs cannot be subtyped because their size
>> must be known at compile time. I don’t know the implementation details of
>> where functions and computed properties live, but something tells me they
>> belong to the type and not the object (although I’ve never really made the
>> effort to sit down and fully understand Swift’s type model), in which case
>> adding them to a struct’s definition would not change the size of the
>> object on the stack. Thus it should be possible to make custom substructs
>> of String that add additional functionality but no new stored properties.
>> Thoughts?
>>
>
> Value subtyping is a large subject and, IIUC, newtype would be a subset of
> that topic. Unlikely to be in scope for Swift 5, though, but that’s up to
> the core team.
>
>
> I see newtype as being more related to forwarding than subtyping.  Usually
> you want to hide significant parts of the interface to the wrapped type.
>
>
>
> On Jun 9, 2017, at 12:12 PM, Jacob Williams via swift-evolution <
>> swift-evolution@swift.org> wrote:
>>
>> On Jun 9, 2017, at 9:53 AM, Charlie Monroe 
>> wrote:
>>
>> -1 - this would disallow e.g. to share UI code between iOS and macOS:
>>
>> #if os(iOS)
>> typealias XUView = UIView
>> #else
>> typealias XUView = NSView
>> #endif
>>
>> extension XUView {
>> ...
>> }
>>
>>
>> I really don’t see how this disallows code sharing between the two
>> systems? Could you explain further? Based on my understanding of the pitch,
>> this is valid code still. (Although I do like the suggestion of a new
>> keyword rather than just limiting type alias).
>>
>> Even if your example was invalid, you could also just do something like
>> this:
>>
>> #if os(iOS)
>> typealias XUView = UIView
>> extension XUView {
>> //extension code here
>> }
>> #if os(macOS)
>> typealias XUView = UIView
>> extension XUView {
>> // extension code here
>> }
>> #endif
>>
>> While not as pretty, still just as effective if you have to deal with
>> different types based on the system being compiled for and you could easily
>> still make the type alias extensions for each type work the same.
>>
>> On Jun 9, 2017, at 9:53 AM, Charlie Monroe 
>> wrote:
>>
>> -1 - this would disallow e.g. to share UI code between iOS and macOS:
>>
>> #if os(iOS)
>> typealias XUView = UIView
>> #else
>> typealias XUView = NSView
>> #endif
>>
>> extension XUView {
>> ...
>> }
>>
>> or with any similar compatibility typealiases.
>>
>> On Jun 9, 2017, at 5:38 PM, Jacob Williams via swift-evolution <
>> swift-evolution@swift.org> wrote:
>>
>> +1 from me.
>>
>> There have been times I’ve wanted to subclass an object (such as String)
>> but since it is a non-class, non-protocol type you can only extend Strings
>> existing functionality which adds that same functionality to Strings
>> everywhere. It would be nice if we could either extend type aliases (and
>> only the type alias), or if it were possible to inherit from structs so
>> that we could create a custom string type like so:
>>
>> struct HeaderKey: String {
>> static var lastModified: String { return “Last-Modified” }
>> static var host: String { return “Host” }
>> }
>>
>> I realize that struct inheritance is far less likely, since that defeats
>> one of the main pieces of what makes a struct a struct. So I’m all for this
>> proposal of allowing type aliases to be extended as though they were their
>> own struct/class.
>>
>> Unfortunately, I’m not sure how feasible this kind of functionality would
>> actually be, but if it’s possible then I’m in favor of implementing it.
>>
>> On Jun 8, 2017, at 10:14 PM, Yvo van Beek via swift-evolution <
>> swift-evolution@swift.org> wrote:
>>
>> Typealiases can greatly reduce the complexity of code. But I think one
>> change in how the compiler handles them could make them even more powerful.
>>
>> Let's say I'm creating a web server framework and I've created a simple
>> dictionary to store HTTP headers (I know that headers are more complex than
>> that, but as an example). I could write something like this:
>>
>> typealias HeaderKey = String
>>
>>   var headers = [HeaderKey: String]()
>>   headers["Host"] = "domain.com"
>>
>> Now I can define a couple of default headers like this:
>>
>>   extension HeaderKey {
>> static var lastModified: String { return "Last-Modified" }
>> static var host: String { return "Host" }
>>   }
>>
>> After that I can do 

Re: [swift-evolution] Pitch: Limit typealias extensions to the typealias

[Matthew Johnson via swift-evolution]

> On Jun 9, 2017, at 12:09 PM, Xiaodi Wu via swift-evolution 
>  wrote:
> 
> On Fri, Jun 9, 2017 at 12:44 Robert Bennett via swift-evolution 
> > wrote:
> Somewhat related to this, shouldn’t it be possible to sub-struct a struct as 
> long as you only add functions and computed properties (i.e., no stored 
> properties)? Traditionally structs cannot be subtyped because their size must 
> be known at compile time. I don’t know the implementation details of where 
> functions and computed properties live, but something tells me they belong to 
> the type and not the object (although I’ve never really made the effort to 
> sit down and fully understand Swift’s type model), in which case adding them 
> to a struct’s definition would not change the size of the object on the 
> stack. Thus it should be possible to make custom substructs of String that 
> add additional functionality but no new stored properties. Thoughts?
> 
> Value subtyping is a large subject and, IIUC, newtype would be a subset of 
> that topic. Unlikely to be in scope for Swift 5, though, but that’s up to the 
> core team.

I see newtype as being more related to forwarding than subtyping.  Usually you 
want to hide significant parts of the interface to the wrapped type.

> 
> 
> On Jun 9, 2017, at 12:12 PM, Jacob Williams via swift-evolution 
> > wrote:
> 
>>> On Jun 9, 2017, at 9:53 AM, Charlie Monroe >> > wrote:
>>> 
>>> -1 - this would disallow e.g. to share UI code between iOS and macOS:
>>> 
>>> #if os(iOS)
>>> typealias XUView = UIView
>>> #else
>>> typealias XUView = NSView
>>> #endif
>>> 
>>> extension XUView {
>>> ...
>>> }
>> 
>> I really don’t see how this disallows code sharing between the two systems? 
>> Could you explain further? Based on my understanding of the pitch, this is 
>> valid code still. (Although I do like the suggestion of a new keyword rather 
>> than just limiting type alias).
>> 
>> Even if your example was invalid, you could also just do something like this:
>> 
>> #if os(iOS)
>>  typealias XUView = UIView
>>  extension XUView {
>>  //extension code here
>>  }
>> #if os(macOS)
>>  typealias XUView = UIView
>>  extension XUView {
>>  // extension code here
>>  }
>> #endif
>> 
>> While not as pretty, still just as effective if you have to deal with 
>> different types based on the system being compiled for and you could easily 
>> still make the type alias extensions for each type work the same.
>> 
>>> On Jun 9, 2017, at 9:53 AM, Charlie Monroe >> > wrote:
>>> 
>>> -1 - this would disallow e.g. to share UI code between iOS and macOS:
>>> 
>>> #if os(iOS)
>>> typealias XUView = UIView
>>> #else
>>> typealias XUView = NSView
>>> #endif
>>> 
>>> extension XUView {
>>> ...
>>> }
>>> 
>>> or with any similar compatibility typealiases.
>>> 
 On Jun 9, 2017, at 5:38 PM, Jacob Williams via swift-evolution 
 > wrote:
 
 +1 from me.
 
 There have been times I’ve wanted to subclass an object (such as String) 
 but since it is a non-class, non-protocol type you can only extend Strings 
 existing functionality which adds that same functionality to Strings 
 everywhere. It would be nice if we could either extend type aliases (and 
 only the type alias), or if it were possible to inherit from structs so 
 that we could create a custom string type like so:
 
 struct HeaderKey: String {
static var lastModified: String { return “Last-Modified” }
static var host: String { return “Host” }
 }
 
 I realize that struct inheritance is far less likely, since that defeats 
 one of the main pieces of what makes a struct a struct. So I’m all for 
 this proposal of allowing type aliases to be extended as though they were 
 their own struct/class.
 
 Unfortunately, I’m not sure how feasible this kind of functionality would 
 actually be, but if it’s possible then I’m in favor of implementing it.
 
> On Jun 8, 2017, at 10:14 PM, Yvo van Beek via swift-evolution 
> > wrote:
> 
> Typealiases can greatly reduce the complexity of code. But I think one 
> change in how the compiler handles them could make them even more 
> powerful.
> 
> Let's say I'm creating a web server framework and I've created a simple 
> dictionary to store HTTP headers (I know that headers are more complex 
> than that, but as an example). I could write something like this:
> 
> typealias HeaderKey = String
> 
>   var headers = [HeaderKey: 

Re: [swift-evolution] Pitch: Limit typealias extensions to the typealias

[Xiaodi Wu via swift-evolution]

On Fri, Jun 9, 2017 at 12:44 Robert Bennett via swift-evolution <
swift-evolution@swift.org> wrote:

> Somewhat related to this, shouldn’t it be possible to sub-struct a struct
> as long as you only add functions and computed properties (i.e., no stored
> properties)? Traditionally structs cannot be subtyped because their size
> must be known at compile time. I don’t know the implementation details of
> where functions and computed properties live, but something tells me they
> belong to the type and not the object (although I’ve never really made the
> effort to sit down and fully understand Swift’s type model), in which case
> adding them to a struct’s definition would not change the size of the
> object on the stack. Thus it should be possible to make custom substructs
> of String that add additional functionality but no new stored properties.
> Thoughts?
>

Value subtyping is a large subject and, IIUC, newtype would be a subset of
that topic. Unlikely to be in scope for Swift 5, though, but that’s up to
the core team.


On Jun 9, 2017, at 12:12 PM, Jacob Williams via swift-evolution <
> swift-evolution@swift.org> wrote:
>
> On Jun 9, 2017, at 9:53 AM, Charlie Monroe 
> wrote:
>
> -1 - this would disallow e.g. to share UI code between iOS and macOS:
>
> #if os(iOS)
> typealias XUView = UIView
> #else
> typealias XUView = NSView
> #endif
>
> extension XUView {
> ...
> }
>
>
> I really don’t see how this disallows code sharing between the two
> systems? Could you explain further? Based on my understanding of the pitch,
> this is valid code still. (Although I do like the suggestion of a new
> keyword rather than just limiting type alias).
>
> Even if your example was invalid, you could also just do something like
> this:
>
> #if os(iOS)
> typealias XUView = UIView
> extension XUView {
> //extension code here
> }
> #if os(macOS)
> typealias XUView = UIView
> extension XUView {
> // extension code here
> }
> #endif
>
> While not as pretty, still just as effective if you have to deal with
> different types based on the system being compiled for and you could easily
> still make the type alias extensions for each type work the same.
>
> On Jun 9, 2017, at 9:53 AM, Charlie Monroe 
> wrote:
>
> -1 - this would disallow e.g. to share UI code between iOS and macOS:
>
> #if os(iOS)
> typealias XUView = UIView
> #else
> typealias XUView = NSView
> #endif
>
> extension XUView {
> ...
> }
>
> or with any similar compatibility typealiases.
>
> On Jun 9, 2017, at 5:38 PM, Jacob Williams via swift-evolution <
> swift-evolution@swift.org> wrote:
>
> +1 from me.
>
> There have been times I’ve wanted to subclass an object (such as String)
> but since it is a non-class, non-protocol type you can only extend Strings
> existing functionality which adds that same functionality to Strings
> everywhere. It would be nice if we could either extend type aliases (and
> only the type alias), or if it were possible to inherit from structs so
> that we could create a custom string type like so:
>
> struct HeaderKey: String {
> static var lastModified: String { return “Last-Modified” }
> static var host: String { return “Host” }
> }
>
> I realize that struct inheritance is far less likely, since that defeats
> one of the main pieces of what makes a struct a struct. So I’m all for this
> proposal of allowing type aliases to be extended as though they were their
> own struct/class.
>
> Unfortunately, I’m not sure how feasible this kind of functionality would
> actually be, but if it’s possible then I’m in favor of implementing it.
>
> On Jun 8, 2017, at 10:14 PM, Yvo van Beek via swift-evolution <
> swift-evolution@swift.org> wrote:
>
> Typealiases can greatly reduce the complexity of code. But I think one
> change in how the compiler handles them could make them even more powerful.
>
> Let's say I'm creating a web server framework and I've created a simple
> dictionary to store HTTP headers (I know that headers are more complex than
> that, but as an example). I could write something like this:
>
> typealias HeaderKey = String
>
>   var headers = [HeaderKey: String]()
>   headers["Host"] = "domain.com"
>
> Now I can define a couple of default headers like this:
>
>   extension HeaderKey {
> static var lastModified: String { return "Last-Modified" }
> static var host: String { return "Host" }
>   }
>
> After that I can do this:
>
>   var headers = [HeaderKey: String]()
>   headers[.host] = "domain.com"
>   headers[.lastModified] = "some date"
>   headers["X-MyHeader"] = "This still works too"
>
> But unfortunately the extension is also applied to normal strings:
>
> var normalString: String = .host
>
> Perhaps it would be better if the extension would only apply to the parts
> of my code where I use the HeaderKey typealias and not to all Strings. This
> could be a great tool to specialize classes by creating a typealias and
> adding functionality to it. Another example 

Re: [swift-evolution] Pitch: Limit typealias extensions to the typealias

[Charlie Monroe via swift-evolution]

> On Jun 9, 2017, at 6:12 PM, Jacob Williams  wrote:
> 
>> On Jun 9, 2017, at 9:53 AM, Charlie Monroe > > wrote:
>> 
>> -1 - this would disallow e.g. to share UI code between iOS and macOS:
>> 
>> #if os(iOS)
>>  typealias XUView = UIView
>> #else
>>  typealias XUView = NSView
>> #endif
>> 
>> extension XUView {
>>  ...
>> }
> 
> I really don’t see how this disallows code sharing between the two systems? 
> Could you explain further? Based on my understanding of the pitch, this is 
> valid code still. (Although I do like the suggestion of a new keyword rather 
> than just limiting type alias).
> 
> Even if your example was invalid, you could also just do something like this:
> 
> #if os(iOS)
>   typealias XUView = UIView
>   extension XUView {
>   //extension code here
>   }
> #if os(macOS)
>   typealias XUView = UIView
>   extension XUView {
>   // extension code here
>   }
> #endif

Um... And you'd implement the same thing twice? The idea behind declaring the 
typealias and extension on it is that you can easily create additional 
functionality with the same code base on both UIView and NSView without 
duplicit code.

>From what I understand that is suggested by this proposal, given the following 
>example expanding my original:

/// Extension of NSView/UIView depending on the target.
extension XUView {

/// Creates some subview and returns it.
func createSubview() -> XUView {
...
}

}

I wouldn't then be able to:

> But unfortunately the extension is also applied to normal strings:
> 
> var normalString: String = .host

a) assign: let view: UIView = view.createSubview()

> Perhaps it would be better if the extension would only apply to the parts of 
> my code where I use the HeaderKey typealias and not to all Strings. 


b) use any of the extensions of views that are not declared as the typealias 
"XUView" (e.g. anything in the Cocoa/UIKit frameworks).






> 
> While not as pretty, still just as effective if you have to deal with 
> different types based on the system being compiled for and you could easily 
> still make the type alias extensions for each type work the same.
> 
>> On Jun 9, 2017, at 9:53 AM, Charlie Monroe > > wrote:
>> 
>> -1 - this would disallow e.g. to share UI code between iOS and macOS:
>> 
>> #if os(iOS)
>>  typealias XUView = UIView
>> #else
>>  typealias XUView = NSView
>> #endif
>> 
>> extension XUView {
>>  ...
>> }
>> 
>> or with any similar compatibility typealiases.
>> 
>>> On Jun 9, 2017, at 5:38 PM, Jacob Williams via swift-evolution 
>>> > wrote:
>>> 
>>> +1 from me.
>>> 
>>> There have been times I’ve wanted to subclass an object (such as String) 
>>> but since it is a non-class, non-protocol type you can only extend Strings 
>>> existing functionality which adds that same functionality to Strings 
>>> everywhere. It would be nice if we could either extend type aliases (and 
>>> only the type alias), or if it were possible to inherit from structs so 
>>> that we could create a custom string type like so:
>>> 
>>> struct HeaderKey: String {
>>> static var lastModified: String { return “Last-Modified” }
>>> static var host: String { return “Host” }
>>> }
>>> 
>>> I realize that struct inheritance is far less likely, since that defeats 
>>> one of the main pieces of what makes a struct a struct. So I’m all for this 
>>> proposal of allowing type aliases to be extended as though they were their 
>>> own struct/class.
>>> 
>>> Unfortunately, I’m not sure how feasible this kind of functionality would 
>>> actually be, but if it’s possible then I’m in favor of implementing it.
>>> 
 On Jun 8, 2017, at 10:14 PM, Yvo van Beek via swift-evolution 
 > wrote:
 
 Typealiases can greatly reduce the complexity of code. But I think one 
 change in how the compiler handles them could make them even more powerful.
 
 Let's say I'm creating a web server framework and I've created a simple 
 dictionary to store HTTP headers (I know that headers are more complex 
 than that, but as an example). I could write something like this:
 
 typealias HeaderKey = String
 
   var headers = [HeaderKey: String]()
   headers["Host"] = "domain.com "
 
 Now I can define a couple of default headers like this:
 
   extension HeaderKey {
 static var lastModified: String { return "Last-Modified" }
 static var host: String { return "Host" }
   }
 
 After that I can do this:
 
   var headers = [HeaderKey: String]()
   headers[.host] = "domain.com "
   

Re: [swift-evolution] Pitch: Limit typealias extensions to the typealias

[Robert Bennett via swift-evolution]

Somewhat related to this, shouldn’t it be possible to sub-struct a struct as 
long as you only add functions and computed properties (i.e., no stored 
properties)? Traditionally structs cannot be subtyped because their size must 
be known at compile time. I don’t know the implementation details of where 
functions and computed properties live, but something tells me they belong to 
the type and not the object (although I’ve never really made the effort to sit 
down and fully understand Swift’s type model), in which case adding them to a 
struct’s definition would not change the size of the object on the stack. Thus 
it should be possible to make custom substructs of String that add additional 
functionality but no new stored properties. Thoughts?

> On Jun 9, 2017, at 12:12 PM, Jacob Williams via swift-evolution 
>  wrote:
> 
>> On Jun 9, 2017, at 9:53 AM, Charlie Monroe  wrote:
>> 
>> -1 - this would disallow e.g. to share UI code between iOS and macOS:
>> 
>> #if os(iOS)
>>  typealias XUView = UIView
>> #else
>>  typealias XUView = NSView
>> #endif
>> 
>> extension XUView {
>>  ...
>> }
> 
> I really don’t see how this disallows code sharing between the two systems? 
> Could you explain further? Based on my understanding of the pitch, this is 
> valid code still. (Although I do like the suggestion of a new keyword rather 
> than just limiting type alias).
> 
> Even if your example was invalid, you could also just do something like this:
> 
> #if os(iOS)
>   typealias XUView = UIView
>   extension XUView {
>   //extension code here
>   }
> #if os(macOS)
>   typealias XUView = UIView
>   extension XUView {
>   // extension code here
>   }
> #endif
> 
> While not as pretty, still just as effective if you have to deal with 
> different types based on the system being compiled for and you could easily 
> still make the type alias extensions for each type work the same.
> 
>> On Jun 9, 2017, at 9:53 AM, Charlie Monroe  wrote:
>> 
>> -1 - this would disallow e.g. to share UI code between iOS and macOS:
>> 
>> #if os(iOS)
>>  typealias XUView = UIView
>> #else
>>  typealias XUView = NSView
>> #endif
>> 
>> extension XUView {
>>  ...
>> }
>> 
>> or with any similar compatibility typealiases.
>> 
>>> On Jun 9, 2017, at 5:38 PM, Jacob Williams via swift-evolution 
>>>  wrote:
>>> 
>>> +1 from me.
>>> 
>>> There have been times I’ve wanted to subclass an object (such as String) 
>>> but since it is a non-class, non-protocol type you can only extend Strings 
>>> existing functionality which adds that same functionality to Strings 
>>> everywhere. It would be nice if we could either extend type aliases (and 
>>> only the type alias), or if it were possible to inherit from structs so 
>>> that we could create a custom string type like so:
>>> 
>>> struct HeaderKey: String {
>>> static var lastModified: String { return “Last-Modified” }
>>> static var host: String { return “Host” }
>>> }
>>> 
>>> I realize that struct inheritance is far less likely, since that defeats 
>>> one of the main pieces of what makes a struct a struct. So I’m all for this 
>>> proposal of allowing type aliases to be extended as though they were their 
>>> own struct/class.
>>> 
>>> Unfortunately, I’m not sure how feasible this kind of functionality would 
>>> actually be, but if it’s possible then I’m in favor of implementing it.
>>> 
 On Jun 8, 2017, at 10:14 PM, Yvo van Beek via swift-evolution 
  wrote:
 
 Typealiases can greatly reduce the complexity of code. But I think one 
 change in how the compiler handles them could make them even more powerful.
 
 Let's say I'm creating a web server framework and I've created a simple 
 dictionary to store HTTP headers (I know that headers are more complex 
 than that, but as an example). I could write something like this:
 
 typealias HeaderKey = String
 
   var headers = [HeaderKey: String]()
   headers["Host"] = "domain.com"
 
 Now I can define a couple of default headers like this:
 
   extension HeaderKey {
 static var lastModified: String { return "Last-Modified" }
 static var host: String { return "Host" }
   }
 
 After that I can do this:
 
   var headers = [HeaderKey: String]()
   headers[.host] = "domain.com"
   headers[.lastModified] = "some date"
   headers["X-MyHeader"] = "This still works too"
 
 But unfortunately the extension is also applied to normal strings:
 
 var normalString: String = .host
 
 Perhaps it would be better if the extension would only apply to the parts 
 of my code where I use the HeaderKey typealias and not to all Strings. 
 This could be a great tool to specialize classes by creating a typealias 

Re: [swift-evolution] Pitch: Limit typealias extensions to the typealias

[Jacob Williams via swift-evolution]

> On Jun 9, 2017, at 9:53 AM, Charlie Monroe  wrote:
> 
> -1 - this would disallow e.g. to share UI code between iOS and macOS:
> 
> #if os(iOS)
>   typealias XUView = UIView
> #else
>   typealias XUView = NSView
> #endif
> 
> extension XUView {
>   ...
> }

I really don’t see how this disallows code sharing between the two systems? 
Could you explain further? Based on my understanding of the pitch, this is 
valid code still. (Although I do like the suggestion of a new keyword rather 
than just limiting type alias).

Even if your example was invalid, you could also just do something like this:

#if os(iOS)
typealias XUView = UIView
extension XUView {
//extension code here
}
#if os(macOS)
typealias XUView = UIView
extension XUView {
// extension code here
}
#endif

While not as pretty, still just as effective if you have to deal with different 
types based on the system being compiled for and you could easily still make 
the type alias extensions for each type work the same.

> On Jun 9, 2017, at 9:53 AM, Charlie Monroe  wrote:
> 
> -1 - this would disallow e.g. to share UI code between iOS and macOS:
> 
> #if os(iOS)
>   typealias XUView = UIView
> #else
>   typealias XUView = NSView
> #endif
> 
> extension XUView {
>   ...
> }
> 
> or with any similar compatibility typealiases.
> 
>> On Jun 9, 2017, at 5:38 PM, Jacob Williams via swift-evolution 
>> > wrote:
>> 
>> +1 from me.
>> 
>> There have been times I’ve wanted to subclass an object (such as String) but 
>> since it is a non-class, non-protocol type you can only extend Strings 
>> existing functionality which adds that same functionality to Strings 
>> everywhere. It would be nice if we could either extend type aliases (and 
>> only the type alias), or if it were possible to inherit from structs so that 
>> we could create a custom string type like so:
>> 
>> struct HeaderKey: String {
>>  static var lastModified: String { return “Last-Modified” }
>>  static var host: String { return “Host” }
>> }
>> 
>> I realize that struct inheritance is far less likely, since that defeats one 
>> of the main pieces of what makes a struct a struct. So I’m all for this 
>> proposal of allowing type aliases to be extended as though they were their 
>> own struct/class.
>> 
>> Unfortunately, I’m not sure how feasible this kind of functionality would 
>> actually be, but if it’s possible then I’m in favor of implementing it.
>> 
>>> On Jun 8, 2017, at 10:14 PM, Yvo van Beek via swift-evolution 
>>> > wrote:
>>> 
>>> Typealiases can greatly reduce the complexity of code. But I think one 
>>> change in how the compiler handles them could make them even more powerful.
>>> 
>>> Let's say I'm creating a web server framework and I've created a simple 
>>> dictionary to store HTTP headers (I know that headers are more complex than 
>>> that, but as an example). I could write something like this:
>>> 
>>> typealias HeaderKey = String
>>> 
>>>   var headers = [HeaderKey: String]()
>>>   headers["Host"] = "domain.com "
>>> 
>>> Now I can define a couple of default headers like this:
>>> 
>>>   extension HeaderKey {
>>> static var lastModified: String { return "Last-Modified" }
>>> static var host: String { return "Host" }
>>>   }
>>> 
>>> After that I can do this:
>>> 
>>>   var headers = [HeaderKey: String]()
>>>   headers[.host] = "domain.com "
>>>   headers[.lastModified] = "some date"
>>>   headers["X-MyHeader"] = "This still works too"
>>> 
>>> But unfortunately the extension is also applied to normal strings:
>>> 
>>> var normalString: String = .host
>>> 
>>> Perhaps it would be better if the extension would only apply to the parts 
>>> of my code where I use the HeaderKey typealias and not to all Strings. This 
>>> could be a great tool to specialize classes by creating a typealias and 
>>> adding functionality to it. Another example I can think of is typealiases 
>>> for dictionaries or arrays with added business logic through extensions 
>>> (especially since you can't inherit from structs).
>>> 
>>> If you want to create an extension that adds functionality to all Strings 
>>> you could have created an extension for String instead of HeaderKey.
>>> 
>>> Please let me know what you think. I'm not sure how complex this change 
>>> would be.
>>> I could write a proposal if you're interested.
>>> 
>>> Kind regards,
>>> Yvo
>>> ___
>>> swift-evolution mailing list
>>> swift-evolution@swift.org 
>>> https://lists.swift.org/mailman/listinfo/swift-evolution 
>>> 
>> 
>> 

Re: [swift-evolution] Pitch: Limit typealias extensions to the typealias

[Charlie Monroe via swift-evolution]

-1 - this would disallow e.g. to share UI code between iOS and macOS:

#if os(iOS)
typealias XUView = UIView
#else
typealias XUView = NSView
#endif

extension XUView {
...
}

or with any similar compatibility typealiases.

> On Jun 9, 2017, at 5:38 PM, Jacob Williams via swift-evolution 
>  wrote:
> 
> +1 from me.
> 
> There have been times I’ve wanted to subclass an object (such as String) but 
> since it is a non-class, non-protocol type you can only extend Strings 
> existing functionality which adds that same functionality to Strings 
> everywhere. It would be nice if we could either extend type aliases (and only 
> the type alias), or if it were possible to inherit from structs so that we 
> could create a custom string type like so:
> 
> struct HeaderKey: String {
>   static var lastModified: String { return “Last-Modified” }
>   static var host: String { return “Host” }
> }
> 
> I realize that struct inheritance is far less likely, since that defeats one 
> of the main pieces of what makes a struct a struct. So I’m all for this 
> proposal of allowing type aliases to be extended as though they were their 
> own struct/class.
> 
> Unfortunately, I’m not sure how feasible this kind of functionality would 
> actually be, but if it’s possible then I’m in favor of implementing it.
> 
>> On Jun 8, 2017, at 10:14 PM, Yvo van Beek via swift-evolution 
>> > wrote:
>> 
>> Typealiases can greatly reduce the complexity of code. But I think one 
>> change in how the compiler handles them could make them even more powerful.
>> 
>> Let's say I'm creating a web server framework and I've created a simple 
>> dictionary to store HTTP headers (I know that headers are more complex than 
>> that, but as an example). I could write something like this:
>> 
>> typealias HeaderKey = String
>> 
>>   var headers = [HeaderKey: String]()
>>   headers["Host"] = "domain.com "
>> 
>> Now I can define a couple of default headers like this:
>> 
>>   extension HeaderKey {
>> static var lastModified: String { return "Last-Modified" }
>> static var host: String { return "Host" }
>>   }
>> 
>> After that I can do this:
>> 
>>   var headers = [HeaderKey: String]()
>>   headers[.host] = "domain.com "
>>   headers[.lastModified] = "some date"
>>   headers["X-MyHeader"] = "This still works too"
>> 
>> But unfortunately the extension is also applied to normal strings:
>> 
>> var normalString: String = .host
>> 
>> Perhaps it would be better if the extension would only apply to the parts of 
>> my code where I use the HeaderKey typealias and not to all Strings. This 
>> could be a great tool to specialize classes by creating a typealias and 
>> adding functionality to it. Another example I can think of is typealiases 
>> for dictionaries or arrays with added business logic through extensions 
>> (especially since you can't inherit from structs).
>> 
>> If you want to create an extension that adds functionality to all Strings 
>> you could have created an extension for String instead of HeaderKey.
>> 
>> Please let me know what you think. I'm not sure how complex this change 
>> would be.
>> I could write a proposal if you're interested.
>> 
>> Kind regards,
>> Yvo
>> ___
>> swift-evolution mailing list
>> swift-evolution@swift.org 
>> https://lists.swift.org/mailman/listinfo/swift-evolution
> 
> ___
> swift-evolution mailing list
> swift-evolution@swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution

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

Re: [swift-evolution] Pitch: Limit typealias extensions to the typealias

[Will Field-Thompson via swift-evolution]

I feel like this might be better served by something like newtype which I
know I've seen floating around on this list before. The idea is that
something like:

newtype HeaderKey = String

would serve roughly as syntactic sugar for something like

struct HeaderKey {
   let value: String
   init(_ value: String) { self.value = value }
   /* Possibly insert methods from String here, possibly do something else,
whatever */
}

I've always thought this might be interesting, but there's a few reasons I
suggest it in place of extending a typealias:
1) Backwards compatibility — this could very easily break existing code
2) Semantically it seems to me that an *alias* is a useful distinction
from *"let's
make a new type" *— sometimes you actually just want to call one type by
another name in some context

Regardless, I think you could solve your particular problem like this:

public struct HeaderKey {
   let value: String
   public init(_ value: String) { self.value = value }
}

and conform HeaderKey to ExpressibleByStringLiteral. That way your
framework's users can still use headers[.lastModified] and
headers["X-MyHeader"] syntax, and it makes your library's code only very
slightly more complicated.

Not trying to tell you how to write your library, but this approach worked
really well for me recently.

Best,

Will

On Fri, Jun 9, 2017 at 12:14 AM Yvo van Beek via swift-evolution <
swift-evolution@swift.org> wrote:

> Typealiases can greatly reduce the complexity of code. But I think one
> change in how the compiler handles them could make them even more powerful.
>
> Let's say I'm creating a web server framework and I've created a simple
> dictionary to store HTTP headers (I know that headers are more complex than
> that, but as an example). I could write something like this:
>
> typealias HeaderKey = String
>
>   var headers = [HeaderKey: String]()
>   headers["Host"] = "domain.com"
>
> Now I can define a couple of default headers like this:
>
>   extension HeaderKey {
> static var lastModified: String { return "Last-Modified" }
> static var host: String { return "Host" }
>   }
>
> After that I can do this:
>
>   var headers = [HeaderKey: String]()
>   headers[.host] = "domain.com"
>   headers[.lastModified] = "some date"
>   headers["X-MyHeader"] = "This still works too"
>
> But unfortunately the extension is also applied to normal strings:
>
> var normalString: String = .host
>
> Perhaps it would be better if the extension would only apply to the parts
> of my code where I use the HeaderKey typealias and not to all Strings. This
> could be a great tool to specialize classes by creating a typealias and
> adding functionality to it. Another example I can think of is typealiases
> for dictionaries or arrays with added business logic through extensions
> (especially since you can't inherit from structs).
>
> If you want to create an extension that adds functionality to all Strings
> you could have created an extension for String instead of HeaderKey.
>
> Please let me know what you think. I'm not sure how complex this change
> would be.
> I could write a proposal if you're interested.
>
> Kind regards,
> Yvo
> ___
> swift-evolution mailing list
> swift-evolution@swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
>
___
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swift-evolution@swift.org
https://lists.swift.org/mailman/listinfo/swift-evolution

Re: [swift-evolution] Pitch: Limit typealias extensions to the typealias

[Jacob Williams via swift-evolution]

+1 from me.

There have been times I’ve wanted to subclass an object (such as String) but 
since it is a non-class, non-protocol type you can only extend Strings existing 
functionality which adds that same functionality to Strings everywhere. It 
would be nice if we could either extend type aliases (and only the type alias), 
or if it were possible to inherit from structs so that we could create a custom 
string type like so:

struct HeaderKey: String {
static var lastModified: String { return “Last-Modified” }
static var host: String { return “Host” }
}

I realize that struct inheritance is far less likely, since that defeats one of 
the main pieces of what makes a struct a struct. So I’m all for this proposal 
of allowing type aliases to be extended as though they were their own 
struct/class.

Unfortunately, I’m not sure how feasible this kind of functionality would 
actually be, but if it’s possible then I’m in favor of implementing it.

> On Jun 8, 2017, at 10:14 PM, Yvo van Beek via swift-evolution 
>  wrote:
> 
> Typealiases can greatly reduce the complexity of code. But I think one change 
> in how the compiler handles them could make them even more powerful.
> 
> Let's say I'm creating a web server framework and I've created a simple 
> dictionary to store HTTP headers (I know that headers are more complex than 
> that, but as an example). I could write something like this:
> 
> typealias HeaderKey = String
> 
>   var headers = [HeaderKey: String]()
>   headers["Host"] = "domain.com "
> 
> Now I can define a couple of default headers like this:
> 
>   extension HeaderKey {
> static var lastModified: String { return "Last-Modified" }
> static var host: String { return "Host" }
>   }
> 
> After that I can do this:
> 
>   var headers = [HeaderKey: String]()
>   headers[.host] = "domain.com "
>   headers[.lastModified] = "some date"
>   headers["X-MyHeader"] = "This still works too"
> 
> But unfortunately the extension is also applied to normal strings:
> 
> var normalString: String = .host
> 
> Perhaps it would be better if the extension would only apply to the parts of 
> my code where I use the HeaderKey typealias and not to all Strings. This 
> could be a great tool to specialize classes by creating a typealias and 
> adding functionality to it. Another example I can think of is typealiases for 
> dictionaries or arrays with added business logic through extensions 
> (especially since you can't inherit from structs).
> 
> If you want to create an extension that adds functionality to all Strings you 
> could have created an extension for String instead of HeaderKey.
> 
> Please let me know what you think. I'm not sure how complex this change would 
> be.
> I could write a proposal if you're interested.
> 
> Kind regards,
> Yvo
> ___
> swift-evolution mailing list
> swift-evolution@swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution

___
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