Re: [swift-evolution] [Pitch] Never as a bottom type

[André Videla via swift-evolution]

> Joe also referred to the following model in a thread about Typed Throws.
> 
>() -> () == () throws Never -> ()
>() throws -> () == () throws Error -> ()
That makes sense! As a bottom type it would be perfectly fit for this.

> On 15 May 2017, at 03:01, Robert Widmann  wrote:
> 
> Though our type lattice doesn’t necessarily have a bottom in the way you’re 
> looking for, you can use Never-returning functions to inhabit a combinator 
> that will do this for you.
Very useful.
What do you think of [] as [Never], or nil as Optional. Can the current 
inference and subtyping rules deal with that? 
If it can there are very interesting things to be done in order to help the 
developper fill up those placeholders. Typically, the compiler could compute 
the expected type of a placeholder and IDEs could show this to the user. For 
example alt-click on undefined could display 
`func undefined() -> Never. Expected: Int`
(Of course this get horribly complex when the compiler needs to compute the 
lowest upper bound of an intersection of types)


> On 15 May 2017, at 02:04, Xiaodi Wu  wrote:
> 
> Clearly many uses for this; would like to see it some day, definitely. 
> However, IIRC, the last day for any _code_ to be incorporated in Swift 4 is 
> June 1. There's simply no chance that this can be considered for the current 
> phase of Swift evolution.
Not a problem, I do no think it is necessary to rush out this feature. But I 
really wanted to see if it is something that other people here consider useful. 
I am glad to see that it’s the case. What would be a good timing for a proposal 
on this matter?
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Re: [swift-evolution] Swift's Optional Int as NSNumber in Objective-C

[Kenny Leung via swift-evolution]

> On May 12, 2017, at 9:56 AM, John McCall via swift-evolution 
>  wrote:

> Exporting Int? as an optional NSNumber does not feel obvious and idiomatic 
> when we would export Int as NSInteger.  It feels like reaching for an 
> arbitrary solution.

I don’t understand this reasoning. I’ve had cause to distinguish 0 from null in 
both Objective-C and Java, and I would do exactly the same thing.

-Kenny

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Re: [swift-evolution] [Pitch] Never as a bottom type

[Yuta Koshizawa via swift-evolution]

Joe also referred to the following model in a thread about Typed Throws.

() -> () == () throws Never -> ()
() throws -> () == () throws Error -> ()

https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170213/032267.html

It means `Never` must be a subtype of `Error`. I think it is an
interesting use case of "Never as bottom type".


2017-05-15 7:27 GMT+09:00 André Videla via swift-evolution
:
> Hi Swift-Evolution,
>
> At the end of SE-0102
> (https://github.com/apple/swift-evolution/blob/master/proposals/0102-noreturn-bottom-type.md)
> it is mentioned that the Never type could be used as a universal bottom type
> but that is currently not the case.
>
> As of today the Never type is used as a return type of `fatalError` and as
> function which do not return by the virtue of calling `fatalError`.
> This allows for very useful behaviours for example:
>
> let anything: Any = …
>
> switch anything {
> case let view as UIView: …
> case let str as String: …
> default: fatalError("we got an unexpected type")
> }
>
> But it has its limits, most notably, it cannot be used as an expression
>
> let dunno: Int = fatalError("what should I put here?") // cannot convert
> value of type 'Never'
>
> It makes sense because Never is not a bottom type. If it were, this
> statement would be absolutely valid.
>
> Having a bottom type and a value for it has several advantages:
>
> - More informative error messages with forced unwrap:
>
> protocol WonkyAPI {
> func apiCall() -> Int? //this always return an Int, why is it optional?
> }
>
> func mycode() {
> let valueFromAPI = apiCall() ?? fatalError("The API used to always
> return a value, but now it does not!")
> …
> }
>
> It sometimes happen that some part of the code uses an optional but in your
> particular codepath, the optional is always containing a value (for example
> after an assignment).
> As of today, you can write
> guard let value = value else { fatalError("something terrible happened") }
> for the same effect with a more verbose syntax.
>
> - Use as a hole/placeholder during development
> During development it is very likely that you will want to write a piece of
> functionality but be stuck on an intermediate part of the computation.
> Assume we have an identifier `undefined` of type `Never` which would
> represent an impossible value as a bottom type. We would ben able to write:
>
> func transform(point: CGPoint) -> CGPoint {
> let translation =  Matrix3D(1, 0, 2,
> 0, 1, -2,
> 0, 0, 1)
> let rotation: Matrix3D = undefined //what was it? I forgot
> return (translation * rotation * point.homogenous).toPoint()
> }
>
> We can debate on the right naming for undefined. Haskell uses 'undefined',
> Scala uses `???`. `unimplemented`, `impossible`, `void`are all valid
> contenders.
>
> - Eliminate type annotations for generic covariant types
> As of today this is not valid
>
> struct Person {
> let name: String
> }
>
> var maybeSomeone = nil
> maybeSomeone = Person(name: "Doug”)
>
> Even though it is clear that maybeSomeone is of type Optional.
> That is because the compiler cannot guess which type the Optional wraps when
> `maybeSomeone` is declared. But with a bottom type, a naked nil can be
> mapped to `Optional` until the type inference figures out from the
> context what is the type of Optional. If it cannot because no use or no
> assignment is done, the compiler could emit an “unreachable” warning just
> like it does for
>
> func unreach() {
> fatalError("stop here")
> print("not printed”) // warning: will never be executed
> }
>
> Should I write a proposal?
>
> André Videla
>
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>
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Re: [swift-evolution] [Pitch] Never as a bottom type

[Robert Widmann via swift-evolution]

> 
> But it has its limits, most notably, it cannot be used as an expression
> 
> let dunno: Int = fatalError("what should I put here?") // cannot convert 
> value of type 'Never'
> 

Though our type lattice doesn’t necessarily have a bottom in the way you’re 
looking for, you can use Never-returning functions to inhabit a combinator that 
will do this for you.

func undefined() -> A {
fatalError("")
}

> It makes sense because Never is not a bottom type. If it were, this statement 
> would be absolutely valid.
> 
> Having a bottom type and a value for it has several advantages:
> 
> - More informative error messages with forced unwrap:
> 
> protocol WonkyAPI {
> func apiCall() -> Int? //this always return an Int, why is it optional?
> }
> 
> func mycode() {
> let valueFromAPI = apiCall() ?? fatalError("The API used to always return 
> a value, but now it does not!")
> …
> }
> 
> It sometimes happen that some part of the code uses an optional but in your 
> particular codepath, the optional is always containing a value (for example 
> after an assignment).
> As of today, you can write
> guard let value = value else { fatalError("something terrible happened") }
> for the same effect with a more verbose syntax.
> 
> - Use as a hole/placeholder during development
> During development it is very likely that you will want to write a piece of 
> functionality but be stuck on an intermediate part of the computation. Assume 
> we have an identifier `undefined` of type `Never` which would represent an 
> impossible value as a bottom type. We would ben able to write:
> 
> func transform(point: CGPoint) -> CGPoint {
> let translation =  Matrix3D(1, 0, 2,
> 0, 1, -2,
> 0, 0, 1)
> let rotation: Matrix3D = undefined //what was it? I forgot
> return (translation * rotation * point.homogenous).toPoint()
> }
> 
> We can debate on the right naming for undefined. Haskell uses 'undefined', 
> Scala uses `???`. `unimplemented`, `impossible`, `void`are all valid 
> contenders.
> 
> - Eliminate type annotations for generic covariant types
> As of today this is not valid 
> 
> struct Person {
> let name: String
> }
> 
> var maybeSomeone = nil
> maybeSomeone = Person(name: "Doug”)
> 
> Even though it is clear that maybeSomeone is of type Optional.
> That is because the compiler cannot guess which type the Optional wraps when 
> `maybeSomeone` is declared. But with a bottom type, a naked nil can be mapped 
> to `Optional` until the type inference figures out from the context 
> what is the type of Optional. If it cannot because no use or no assignment is 
> done, the compiler could emit an “unreachable” warning just like it does for 
> 
> func unreach() {
> fatalError("stop here")
> print("not printed”) // warning: will never be executed
> }
> 
> Should I write a proposal?
> 
> André Videla
> ___
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> swift-evolution@swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution

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Re: [swift-evolution] [Pitch] Never as a bottom type

[Xiaodi Wu via swift-evolution]

Clearly many uses for this; would like to see it some day, definitely.
However, IIRC, the last day for any _code_ to be incorporated in Swift 4 is
June 1. There's simply no chance that this can be considered for the
current phase of Swift evolution.

When it does come into scope (which will depend on the core team's
identified priorities), the one thing I would comment is that there is no
need for a name such as "undefined": we already have "fatalError()".


On Sun, May 14, 2017 at 17:27 André Videla via swift-evolution <
swift-evolution@swift.org> wrote:

> Hi Swift-Evolution,
>
> At the end of SE-0102 (
> https://github.com/apple/swift-evolution/blob/master/proposals/0102-noreturn-bottom-type.md)
> it is mentioned that the Never type could be used as a universal bottom
> type but that is currently not the case.
>
> As of today the Never type is used as a return type of `fatalError` and as
> function which do not return by the virtue of calling `fatalError`.
> This allows for very useful behaviours for example:
>
> let anything: Any = …
>
> switch anything {
> case let view as UIView: …
> case let str as String: …
> default: fatalError("we got an unexpected type")
> }
>
> But it has its limits, most notably, it cannot be used as an expression
>
> let dunno: Int = fatalError("what should I put here?") // cannot convert
> value of type 'Never'
>
> It makes sense because Never is not a bottom type. If it were, this
> statement would be absolutely valid.
>
> Having a bottom type and a value for it has several advantages:
>
> - More informative error messages with forced unwrap:
>
> protocol WonkyAPI {
> func apiCall() -> Int? //this always return an Int, why is it
> optional?
> }
>
> func mycode() {
> let valueFromAPI = apiCall() ?? fatalError("The API used to always
> return a value, but now it does not!")
> …
> }
>
> It sometimes happen that some part of the code uses an optional but in
> your particular codepath, the optional is always containing a value (for
> example after an assignment).
> As of today, you can write
> guard let value = value else { fatalError("something terrible happened") }
> for the same effect with a more verbose syntax.
>
> - Use as a hole/placeholder during development
> During development it is very likely that you will want to write a piece
> of functionality but be stuck on an intermediate part of the computation.
> Assume we have an identifier `undefined` of type `Never` which would
> represent an impossible value as a bottom type. We would ben able to write:
>
> func transform(point: CGPoint) -> CGPoint {
> let translation =  Matrix3D(1, 0, 2,
> 0, 1, -2,
> 0, 0, 1)
> let rotation: Matrix3D = undefined //what was it? I forgot
> return (translation * rotation * point.homogenous).toPoint()
> }
>
> We can debate on the right naming for undefined. Haskell uses 'undefined',
> Scala uses `???`. `unimplemented`, `impossible`, `void`are all valid
> contenders.
>
> - Eliminate type annotations for generic covariant types
> As of today this is not valid
>
> struct Person {
> let name: String
> }
>
> var maybeSomeone = nil
> maybeSomeone = Person(name: "Doug”)
>
> Even though it is clear that maybeSomeone is of type Optional.
> That is because the compiler cannot guess which type the Optional wraps
> when `maybeSomeone` is declared. But with a bottom type, a naked nil can be
> mapped to `Optional` until the type inference figures out from the
> context what is the type of Optional. If it cannot because no use or no
> assignment is done, the compiler could emit an “unreachable” warning just
> like it does for
>
> func unreach() {
> fatalError("stop here")
> print("not printed”) // warning: will never be executed
> }
>
> Should I write a proposal?
>
> André Videla
> ___
> swift-evolution mailing list
> swift-evolution@swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution
>
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[swift-evolution] [Pitch] Never as a bottom type

[André Videla via swift-evolution]

Hi Swift-Evolution,

At the end of SE-0102 
(https://github.com/apple/swift-evolution/blob/master/proposals/0102-noreturn-bottom-type.md)
 it is mentioned that the Never type could be used as a universal bottom type 
but that is currently not the case.

As of today the Never type is used as a return type of `fatalError` and as 
function which do not return by the virtue of calling `fatalError`.
This allows for very useful behaviours for example:

let anything: Any = …

switch anything {
case let view as UIView: …
case let str as String: …
default: fatalError("we got an unexpected type")
}

But it has its limits, most notably, it cannot be used as an expression

let dunno: Int = fatalError("what should I put here?") // cannot convert value 
of type 'Never'

It makes sense because Never is not a bottom type. If it were, this statement 
would be absolutely valid.

Having a bottom type and a value for it has several advantages:

- More informative error messages with forced unwrap:

protocol WonkyAPI {
func apiCall() -> Int? //this always return an Int, why is it optional?
}

func mycode() {
let valueFromAPI = apiCall() ?? fatalError("The API used to always return a 
value, but now it does not!")
…
}

It sometimes happen that some part of the code uses an optional but in your 
particular codepath, the optional is always containing a value (for example 
after an assignment).
As of today, you can write
guard let value = value else { fatalError("something terrible happened") }
for the same effect with a more verbose syntax.

- Use as a hole/placeholder during development
During development it is very likely that you will want to write a piece of 
functionality but be stuck on an intermediate part of the computation. Assume 
we have an identifier `undefined` of type `Never` which would represent an 
impossible value as a bottom type. We would ben able to write:

func transform(point: CGPoint) -> CGPoint {
let translation =  Matrix3D(1, 0, 2,
0, 1, -2,
0, 0, 1)
let rotation: Matrix3D = undefined //what was it? I forgot
return (translation * rotation * point.homogenous).toPoint()
}

We can debate on the right naming for undefined. Haskell uses 'undefined', 
Scala uses `???`. `unimplemented`, `impossible`, `void`are all valid contenders.

- Eliminate type annotations for generic covariant types
As of today this is not valid 

struct Person {
let name: String
}

var maybeSomeone = nil
maybeSomeone = Person(name: "Doug”)

Even though it is clear that maybeSomeone is of type Optional.
That is because the compiler cannot guess which type the Optional wraps when 
`maybeSomeone` is declared. But with a bottom type, a naked nil can be mapped 
to `Optional` until the type inference figures out from the context what 
is the type of Optional. If it cannot because no use or no assignment is done, 
the compiler could emit an “unreachable” warning just like it does for 

func unreach() {
fatalError("stop here")
print("not printed”) // warning: will never be executed
}

Should I write a proposal?

André Videla___
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