[swift-evolution] Idea: Properties in Failable Initializers less verbose

[Taylor Swift]

Catching and throwing exceptions is not the same as a controlled guard
fail. It’s also quite verbose and requires extra types to be defined
outside the scope of the initializer.

On Tue, Jul 25, 2017 at 5:35 PM, Rob Mayoff via swift-evolution <
swift-evolution at swift.org> wrote:

> On Tue, Jul 25, 2017 at 4:44 AM, philohan95 via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>> As you can see we had to use the properties twice (this would also be the
>> case of `if let`) making the initializer twice as long as necessary and
>> becomes a pain to implement when having more than 1 property.
>>
>
>> My idea is extending the power of the `guard` statement
>>
>> Idea:
>>         init?(data: [String: Any]) {
>>                 guard
>>                         someProperty = data["some_key"], // Currently
>> fails because `self` us used before all stored properties are initialized
>>                         anotherProperty = data["another_key"]
>>                 else {
>>                         return nil
>>                 }
>>         }
>> }
>>
>
> I'm not convinced new syntax is necessary. You can get pretty close to
> this today. First, put this in your project somewhere:
>
> struct NilError: Error { }
>
> func throwNilError<Whatever>() throws -> Whatever {
>     throw NilError()
> }
>
> Then use do/try/catch to initialize your properties:
>
> class MyObject {
>     let someProperty: Any
>     let anotherProperty: Any
>
>     init?(data: [String: Any]) {
>         do {
>             someProperty = try data["some_key"] ?? throwNilError()
>             anotherProperty = try data["another_key"] ?? throwNilError()
>         } catch {
>             return nil
>         }
>     }
> }
>
> It also works for Charles Srstka's example:
>
> let bar: String
> if someCondition {
>     do { bar = mightReturnOptional() ?? throwNilError() }
>     catch { return }
> } else {
>     bar = wontReturnOptional()
> }
>
>
>
>
>
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>
>
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