[swift-evolution] [swift-evolution-announce] [Returned for revision] SE-0089: Renaming String.init<T>(_: T)

Thorsten Seitz tseitz42 at icloud.com
Tue May 31 10:12:48 CDT 2016

I agree with Vladimir. Having a value preserving representation is orthogonal to a human readable representation.


> Am 31.05.2016 um 00:04 schrieb Vladimir.S via swift-evolution <swift-evolution at swift.org>:
> Thank you, Brent. But for me you just described a serialization to/from string ;)
> So, using your example, if I have
> struct A: CustomStringConvertible {
>    var a = 0, b = 0
>    var description: String { return "a:\(a) b:\(b)" }
> }
> and I want to use it in your code. Will I be able to do this?
> For example, I'm ok to give it 'lossless' representation as "\(a)/\(b)" (i.e. 1/2 for example) and provide init() for string of such format.
> I.e. it seems like I just should create extension and conform to LosslessStringConvertible, but as I understand I can't, as I need not only introduce an init(:String), but modify `description` property ?
> I.e. assume you have no rights or don't want to modify the A type itself.
> This is why I don't understand why we should have the same .description for this LosslessStringConvertible(i.e. if it will be .loslessDescription - no problems).
> Most likely I don't understand something.
> Also there is a question regarding your example: what to do with Double type? We can have some configuration items of Double type, but how to use this LosslessStringConvertible here?
> On 31.05.2016 0:22, Brent Royal-Gordon wrote:
>>> I can't understand this. For me ValuePreservingStringConvertible usually will be different than CustomStringConvertible. Can't I want to have some string view of my struct to present data(also in UI) *and* value preserving string value for the same struct?
>>> So my .description will show something like "12.345-6.78" and value preserving string will contain something like "19083749347923847293487293483" to encode the data of the struct. No?
>> Rather than thinking of LosslessStringConvertible as a protocol for serializing data into a string, think of it as a protocol for those cases where the human-readable description is also parseable and can be used to completely recreate the instance. It's something you would use for things like command-line arguments, environment variables, interactive command-line programs, and configuration files that you expect humans to read and write by hand.
>>    func prompt<T: LosslessStringConvertible>(for field: String, of type: T.Type) -> T {
>>        while true {
>>            print("What's your \(field)?")
>>            let line = readline()
>>            if    !line.isEmpty
>>                let value = T(line) {        // how the hell do you indent this stupid syntax?
>>                return value
>>            }
>>        }
>>    }
>>    let name = prompt(for: "name", of: String)
>>    let age = prompt(for: "age", of: Int)
>>    let answer = age < 13 ? " not" : ""
>>    print("\(name), you are\(answer) too old to have a favorite color.")
>> In other words, write the `description` first, and then decide if you can write a good `init(_ description:)` to match it.
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