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

[Andrew Duncan]

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 at 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 at 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 at 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 at gmail.com | @SlaunchaMan | jeffkelley.org
>>> 
>>>> On Jan 3, 2016, at 10:43 PM, Andrew Duncan via swift-evolution <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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