[swift-evolution] Optionals and nil in Switch statement

[Kevin Nattinger]

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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