[swift-users] try? with a function that returns an optional

[zhaoxin肇鑫]

​I hope this will show something clearer to you. As you can see, y is
defined as OptionalInt?, which is Int??.

​typealias OptionInt = Int?


func someThrowingFunction() throws -> OptionInt {

    return OptionInt()

}


let y: OptionInt?
​​

do {

    y = try someThrowingFunction()

} catch {

    y = nil

}


z
​haoxin​


On Mon, Jan 25, 2016 at 11:54 PM, Jonathan Bailey via swift-users <
swift-users at swift.org> wrote:

> So it would be legal to change the type of y in the second example to
> `Int??`
>
> Does that mean when assigning optionals, it will unwrap, check if it's
> nill and assign nil, else assign the original value? This seems kind of
> pointless to just assigning the complete optional
>
>
> On 25 Jan 2016, at 15:49, Svein Halvor Halvorsen <
> svein.h at lvor.halvorsen.cc> wrote:
>
> This is exactly according to the documentation.
> In your first example `someThrowingFunction` returns an `Int`, so `y` is
> defined as an `Int?`.
> In the second example `someThrowingFunction` returns an `Int?`, so `y`
> should be an `Int??`
>
> However, since you didn't update the definition of `y` in your second
> example, the if branch either assigns an `Int?` to an `Int?`, which is
> legal, and may be nil, or it explicitly sets it to nil, which is also
> legal. Thus, effectively unwrapping the nested optionals.
>
> Yu could also apply a `flatMap` to the nested optional, like so:
>
> let x = (try? someThrowingFunction())?.flatMap({$0})
>
> I'm not sure if it's more readable, though.
>
>
> 2016-01-25 14:01 GMT+01:00 Jonathan Bailey via swift-users <
> swift-users at swift.org>:
>
>> In the language guide on the apple website,
>> https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/ErrorHandling.html#//apple_ref/doc/uid/TP40014097-CH42-ID542
>>
>> It says the following assignments to x and y are equivalent:
>>
>>    1. func someThrowingFunction() throws -> Int { ... }
>>
>>
>>    1. let x = try? someThrowingFunction()
>>    2. // x has type `Int?`
>>    3.
>>    4. let y: Int?
>>    5. do {
>>    6. y = try someThrowingFunction()
>>    7. } catch {
>>    8. y = nil
>>    9. }
>>
>>
>> However this isn’t the case if someThrowingFunction also returns an
>> optional, say:
>>
>>
>>    1. func someThrowingFunction() throws -> Int? { ... }
>>
>>
>> The type of x would be `Int??`, but the type of y is still `Int?`, is
>> there some way to make the `try?` return an `Int?` instead of a double
>> optional, which is not very helpful.
>>
>> Thanks,
>> Jonathan
>>
>>
>> _______________________________________________
>> swift-users mailing list
>> swift-users at swift.org
>> https://lists.swift.org/mailman/listinfo/swift-users
>>
>>
>
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>
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