[swift-evolution] Optionals and nil in Switch statement

[Kevin Nattinger]

Slight amendment for `let` case, which does evidently still require the explicit `.some`

switch str {
case "foo": print("foo")
case .some(let string): print(string) // `case let .some(string)` also works
case nil: print("nil")
}

> On Jun 28, 2016, at 11:10 AM, Kevin Nattinger via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I’ve always thought it’s a bit odd, but that’s the way it is. FWIW, if you define `T? ~= T?` (switch uses `~=` under the hood), you can use that syntax:
> 
> public func ~=<T : Equatable>(a: T?, b: T?) -> Bool {
> 	return a == b
> }
> 
> switch str {
> case "foo": print("foo")
> case "bar": print("bar")
> case nil: print("nil")
> default: print("other")
> }
> 
> For better or worse, this prevents you from using the `.some(x)` / `.none` version.
> 
> You could propose adding this to the standard library to the swift-evolution list, see how they react.
> 
>> On Jun 28, 2016, at 9:52 AM, Lucas Jordan <lucasjordan at gmail.com> wrote:
>> 
>> This is sort of weird right? because comparing nil to a non nil string is a reasonable thing to do:
>> 
>> var nilString:String? = nil
>> 
>> if nilString == "this always fails" {}
>> 
>> is totally reasonable.
>> 
>> On Tue, Jun 28, 2016 at 12:33 PM, Kevin Nattinger <swift at nattinger.net> wrote:
>> No
>> 
>>   7> switch str {
>>   8. case "foo": print("case foo")
>>   9. case .none: print("(nil)")
>>  10. }
>> error: repl.swift:8:6: error: value of optional type 'String?' not unwrapped; did you mean to use '!' or '?'?
>> case "foo": print("case foo")
>>      ^
>>           !
>> Odd error, but at least it suggests it’s an issue with optionaity.
>> 
>> 
>>> On Jun 28, 2016, at 9:27 AM, Nevin Brackett-Rozinsky <nevin.brackettrozinsky at gmail.com> wrote:
>>> 
>>> Does `case "text"?` work?
>>> 
>>> 
>>> On Tuesday, June 28, 2016, Kevin Nattinger via swift-evolution <swift-evolution at swift.org> wrote:
>>> Case .none:
>>> Case .some("string"):
>>> 
>>> 
>>> On Jun 28, 2016, at 06:40, Lucas Jordan via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>>> Forgive me if this was/is discussed already, I am new to the process here....
>>>> 
>>>> (code is attached as a playground too)
>>>> 
>>>> 
>>>> 
>>>> Sometimes when I am working with a String? nil can be a reasonable value, and what I want to do is something like the following:
>>>> 
>>>> import UIKit
>>>> 
>>>> 
>>>> 
>>>> var str:String? = "Hello, playground"
>>>> 
>>>> 
>>>> 
>>>> switch str{
>>>> 
>>>> case nil:
>>>> 
>>>>     print("Nil!")
>>>> 
>>>> case "Hello, playground":  //it would be super nice if this worked.
>>>> 
>>>>     print("Match")
>>>> 
>>>> default:
>>>> 
>>>>     print("Some other non nil value?")
>>>> 
>>>> }
>>>> 
>>>> 
>>>> 
>>>> But it does not work, the orange  text is a compile time error, "Expression pattern of type 'String' cannot match value of type 'String?'. I realize that this can be replaced with a let statement (case let s where s == "Hello, playground":), but that is verbose. 
>>>> 
>>>> Seems like the compiler could be OK with the orange text, since it is clearly not nil.
>>>> 
>>>> Thoughts?
>>>> 
>>>> -Lucas
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> <NilInSwitchStatements.playground.zip>
>>>> _______________________________________________
>>>> swift-evolution mailing list
>>>> swift-evolution at swift.org
>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>> 
>> 
> 
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