[swift-evolution] Declaring a function that always returns nil

[Jeff Kelley]

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 at gmail.com | @SlaunchaMan <https://twitter.com/SlaunchaMan> | jeffkelley.org <http://jeffkelley.org/>
> On Jan 3, 2016, at 10:51 PM, Jeff Kelley <SlaunchaMan at 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 at gmail.com <mailto:SlaunchaMan at gmail.com> | @SlaunchaMan <https://twitter.com/SlaunchaMan> | jeffkelley.org <http://jeffkelley.org/>
>> On Jan 3, 2016, at 10:43 PM, Andrew Duncan via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at 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.)

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