Declaring a function that always returns nil

Evolution Discussion
1

It should be possible to declare a function that returns only nil, and have its return type be substitutable for any function that returns and Optional. This is something like having a bottom type but not really. What I mean is:

func returnsNil(errCode:Int) -> nil {
    logError(errCode) // A side-effect. Not FP, sosumi.
    return nil
}

func returnAOptional() -> A? {
    // Bla bla. We discover an error so we decide to bail and return nil.
    return returnsNil(errCode) // Would be legal.
}
func returnsBOptional() -> B? {
    // Bla bla. We discover an error so we decide to bail and return nil.
    return returnsNil(errCode) // Would also be legal.
}

I seek a return type that conforms to any Optional -- I think that implies it *must* (or correct me here) be nil.

Now perhaps this is already possible with wizardry from the Next Level. (There always is one.)

2

Just spit-balling here, but couldn’t you do this with a generic extension on Optional?

extension Optional {
    func returnsNil(errorCode: Int) -> Wrapped? {
        logError(errorCode)
        return nil
    }
}

Jeff Kelley

SlaunchaMan@gmail.com | @SlaunchaMan <https://twitter.com/SlaunchaMan&gt; | jeffkelley.org <http://jeffkelley.org/&gt;

···

On Jan 3, 2016, at 10:43 PM, Andrew Duncan via swift-evolution <swift-evolution@swift.org> wrote:

It should be possible to declare a function that returns only nil, and have its return type be substitutable for any function that returns and Optional. This is something like having a bottom type but not really. What I mean is:

func returnsNil(errCode:Int) -> nil {
   logError(errCode) // A side-effect. Not FP, sosumi.
   return nil
}

func returnAOptional() -> A? {
   // Bla bla. We discover an error so we decide to bail and return nil.
   return returnsNil(errCode) // Would be legal.
}
func returnsBOptional() -> B? {
   // Bla bla. We discover an error so we decide to bail and return nil.
   return returnsNil(errCode) // Would also be legal.
}

I seek a return type that conforms to any Optional -- I think that implies it *must* (or correct me here) be nil.

Now perhaps this is already possible with wizardry from the Next Level. (There always is one.)

3

You could also use type inference on the return type:

  1> func returnsNil<T>(errCode: Int) -> T? {
  2. print(errCode)
  3. return nil
  4. }
  5> func returnsAnOptional() -> Int? {
  6. return returnsNil(5)
  7. }

···

On Jan 3, 2016, at 7:51 PM, Jeff Kelley via swift-evolution <swift-evolution@swift.org> wrote:

Just spit-balling here, but couldn’t you do this with a generic extension on Optional?

extension Optional {
    func returnsNil(errorCode: Int) -> Wrapped? {
        logError(errorCode)
        return nil
    }
}

Jeff Kelley

SlaunchaMan@gmail.com <mailto:SlaunchaMan@gmail.com> | @SlaunchaMan <https://twitter.com/SlaunchaMan&gt; | jeffkelley.org <http://jeffkelley.org/&gt;

On Jan 3, 2016, at 10:43 PM, Andrew Duncan via swift-evolution <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote:

It should be possible to declare a function that returns only nil, and have its return type be substitutable for any function that returns and Optional. This is something like having a bottom type but not really. What I mean is:

func returnsNil(errCode:Int) -> nil {
   logError(errCode) // A side-effect. Not FP, sosumi.
   return nil
}

func returnAOptional() -> A? {
   // Bla bla. We discover an error so we decide to bail and return nil.
   return returnsNil(errCode) // Would be legal.
}
func returnsBOptional() -> B? {
   // Bla bla. We discover an error so we decide to bail and return nil.
   return returnsNil(errCode) // Would also be legal.
}

I seek a return type that conforms to any Optional -- I think that implies it *must* (or correct me here) be nil.

Now perhaps this is already possible with wizardry from the Next Level. (There always is one.)

_______________________________________________
swift-evolution mailing list
swift-evolution@swift.org <mailto:swift-evolution@swift.org>
https://lists.swift.org/mailman/listinfo/swift-evolution

4

Cleaned up a bit after I tried to actually use it:

extension Optional {
    static func returnsNil(errorCode: Int) -> Wrapped? {
        logError(errorCode)
        return nil
    }
}

extension Int {
    func foo() -> Int? {
        return .returnsNil(12)
    }
}

You could write it as Optional.returnsNil(12), but the compiler can infer the type.

Jeff Kelley

SlaunchaMan@gmail.com | @SlaunchaMan <https://twitter.com/SlaunchaMan&gt; | jeffkelley.org <http://jeffkelley.org/&gt;

···

On Jan 3, 2016, at 10:51 PM, Jeff Kelley <SlaunchaMan@gmail.com> wrote:

Just spit-balling here, but couldn’t you do this with a generic extension on Optional?

extension Optional {
    func returnsNil(errorCode: Int) -> Wrapped? {
        logError(errorCode)
        return nil
    }
}

Jeff Kelley

SlaunchaMan@gmail.com <mailto:SlaunchaMan@gmail.com> | @SlaunchaMan <https://twitter.com/SlaunchaMan&gt; | jeffkelley.org <http://jeffkelley.org/&gt;

On Jan 3, 2016, at 10:43 PM, Andrew Duncan via swift-evolution <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote:

It should be possible to declare a function that returns only nil, and have its return type be substitutable for any function that returns and Optional. This is something like having a bottom type but not really. What I mean is:

func returnsNil(errCode:Int) -> nil {
   logError(errCode) // A side-effect. Not FP, sosumi.
   return nil
}

func returnAOptional() -> A? {
   // Bla bla. We discover an error so we decide to bail and return nil.
   return returnsNil(errCode) // Would be legal.
}
func returnsBOptional() -> B? {
   // Bla bla. We discover an error so we decide to bail and return nil.
   return returnsNil(errCode) // Would also be legal.
}

I seek a return type that conforms to any Optional -- I think that implies it *must* (or correct me here) be nil.

Now perhaps this is already possible with wizardry from the Next Level. (There always is one.)

5

IMO, this is the best way to approach it.

Félix

···

Le 3 janv. 2016 à 22:54:17, Jack Lawrence via swift-evolution <swift-evolution@swift.org> a écrit :

You could also use type inference on the return type:

  1> func returnsNil<T>(errCode: Int) -> T? {
  2. print(errCode)
  3. return nil
  4. }
  5> func returnsAnOptional() -> Int? {
  6. return returnsNil(5)
  7. }

On Jan 3, 2016, at 7:51 PM, Jeff Kelley via swift-evolution <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote:

Just spit-balling here, but couldn’t you do this with a generic extension on Optional?

extension Optional {
    func returnsNil(errorCode: Int) -> Wrapped? {
        logError(errorCode)
        return nil
    }
}

Jeff Kelley

SlaunchaMan@gmail.com <mailto:SlaunchaMan@gmail.com> | @SlaunchaMan <https://twitter.com/SlaunchaMan&gt; | jeffkelley.org <http://jeffkelley.org/&gt;

On Jan 3, 2016, at 10:43 PM, Andrew Duncan via swift-evolution <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote:

It should be possible to declare a function that returns only nil, and have its return type be substitutable for any function that returns and Optional. This is something like having a bottom type but not really. What I mean is:

func returnsNil(errCode:Int) -> nil {
   logError(errCode) // A side-effect. Not FP, sosumi.
   return nil
}

func returnAOptional() -> A? {
   // Bla bla. We discover an error so we decide to bail and return nil.
   return returnsNil(errCode) // Would be legal.
}
func returnsBOptional() -> B? {
   // Bla bla. We discover an error so we decide to bail and return nil.
   return returnsNil(errCode) // Would also be legal.
}

I seek a return type that conforms to any Optional -- I think that implies it *must* (or correct me here) be nil.

Now perhaps this is already possible with wizardry from the Next Level. (There always is one.)

_______________________________________________
swift-evolution mailing list
swift-evolution@swift.org <mailto:swift-evolution@swift.org>
https://lists.swift.org/mailman/listinfo/swift-evolution

_______________________________________________
swift-evolution mailing list
swift-evolution@swift.org <mailto:swift-evolution@swift.org>
https://lists.swift.org/mailman/listinfo/swift-evolution

6

Yep, this has transformed from an evolution question to a how-to answer. exit(1).

Thanks Jack and Jaden for pointing out the retrospectively obvious. Just couldn’t see my own nose.

I liked Jeff’s Optional gambit though. Félix, je regrette that your name doesn’t start with ‘J’.

···

On 3 Jan, 2016, at 20:36, Félix Cloutier <felixcca@yahoo.ca> > wrote:

IMO, this is the best way to approach it.

Félix

Le 3 janv. 2016 à 22:54:17, Jack Lawrence via swift-evolution <swift-evolution@swift.org> a écrit :

You could also use type inference on the return type:

  1> func returnsNil<T>(errCode: Int) -> T? {
  2. print(errCode)
  3. return nil
  4. }
  5> func returnsAnOptional() -> Int? {
  6. return returnsNil(5)
  7. }

On Jan 3, 2016, at 7:51 PM, Jeff Kelley via swift-evolution <swift-evolution@swift.org> wrote:

Just spit-balling here, but couldn’t you do this with a generic extension on Optional?

extension Optional {
    func returnsNil(errorCode: Int) -> Wrapped? {
        logError(errorCode)
        return nil
    }
}

Jeff Kelley

SlaunchaMan@gmail.com | @SlaunchaMan | jeffkelley.org

On Jan 3, 2016, at 10:43 PM, Andrew Duncan via swift-evolution <swift-evolution@swift.org> wrote:

It should be possible to declare a function that returns only nil, and have its return type be substitutable for any function that returns and Optional. This is something like having a bottom type but not really. What I mean is:

func returnsNil(errCode:Int) -> nil {
   logError(errCode) // A side-effect. Not FP, sosumi.
   return nil
}

func returnAOptional() -> A? {
   // Bla bla. We discover an error so we decide to bail and return nil.
   return returnsNil(errCode) // Would be legal.
}
func returnsBOptional() -> B? {
   // Bla bla. We discover an error so we decide to bail and return nil.
   return returnsNil(errCode) // Would also be legal.
}

I seek a return type that conforms to any Optional -- I think that implies it *must* (or correct me here) be nil.

Now perhaps this is already possible with wizardry from the Next Level. (There always is one.)

_______________________________________________
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