[swift-evolution] Reconsidering SE-0003 Removing var from Function Parameters and Pattern Matching

[Tyler Cloutier]

On Jan 29, 2016, at 8:54 AM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
> on Thu Jan 28 2016, Thorsten Seitz <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
> 
>> This is called flow typing in Ceylon and they are using "exists" for
>> that case but testing for conformance with "is" works just the same
>> way there. The latter is of importance because the common use of union
>> types in Ceylon.
>> 
>> if exists foo {
>>        // foo is non-optional here
>> }
>> 
>> if foo is String {
>>        // foo is of type String here
>> }
> 
> FWIW, some of us are concerned that if the types of names change
> without being announced by a let or var binding, it will be confusing.
> That's one reason we haven't gone this way in the past.
> 


True enough, but is type narrowing (as in the above) really more confusing than

if let x = x {

}

Nothing there suggests that the type of x should change, yet it does. If eliminating confusion was the goal wouldn't

if let x = x? {

}

be more explicit?

Tyler 


>> -Thorsten
>> 
>>> Am 28.01.2016 um 19:08 schrieb Jacob Bandes-Storch via swift-evolution <swift-evolution at swift.org>:
>>> 
>>>   if bind foo {
>>>       // foo is non-optional in here
>>>   }
>>> 
>>>   somethingAsync { [weak self] in
>>>       guard bind self else { return }
>>>       // ...
>>>   }
>>> 
>>> Elegant when you want to rebind the same name!
>>> 
>>> Jacob
>>> 
>>>> On Thu, Jan 28, 2016 at 10:05 AM, Erica Sadun via swift-evolution
>>>> <swift-evolution at swift.org> wrote:
>>>> Do you realize how much confusion it would save if Swift just went with
>>>> 
>>>> if bind foo = bar {...}
>>>> 
>>>> with let semantics?
>>>> 
>>>> -- Erica
>>>> 
>>>>> On Jan 28, 2016, at 11:03 AM, Joe Groff via swift-evolution
>>>>> <swift-evolution at swift.org> wrote:
>>>>> 
>>>>> 
>>>>>> On Jan 28, 2016, at 12:43 AM, Brent Royal-Gordon <brent at architechies.com> wrote:
>>>>>> 
>>>>>>> We have a lot of evidence that 'if var' confuses people—a lot
>>>>>>> of users think that 'if var' and 'var' bindings in case
>>>>>>> patterns will write back to the original value when this isn't
>>>>>>> the case.
>>>>>> 
>>>>>> Can we address this with a diagnostic?
>>>>>> 
>>>>>>     var bar: Int? = 1
>>>>>>     if var foo = bar {
>>>>>>             foo += 1        // Warning: Value is never used after modification (foo is a copy, not an alias)
>>>>>>     }
>>>>>>     print(bar)
>>>>> 
>>>>> Good idea.
>>>>> 
>>>>> -Joe
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>>>>> swift-evolution at swift.org
>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>> 
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> 
> -- 
> -Dave
> 
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